layout: default title: Coding Guidelines nav_order: 1 parent: Misc

Coding Guidelines

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This section provides the guidelines for developing C and C++ code, based on the coding conventions used by ICU programmers in the creation of the ICU library.

Details about ICU Error Codes

When calling an ICU API function and an error code pointer (C) or reference (C++), a UErrorCode variable is often passed in. This variable is allocated by the caller and must pass the test U_SUCCESS() before the function call. Otherwise, the function will not work. Normally, an error code variable is initialized by U_ZERO_ERROR.

UErrorCode is passed around and used this way, instead of using C++ exceptions for the following reasons:

  • It is useful in the same form for C also
  • Some C++ compilers do not support exceptions

:point_right: Note: This error code mechanism, in fact, works similar to exceptions. If users call several ICU functions in a sequence, as soon as one sets a failure code, the functions in the following example will not work. This procedure prevents the API function from processing data that is not valid in the sequence of function calls and relieves the caller from checking the error code after each call. It is somewhat similar to how an exception terminates a function block or try block early.

The following code shows the inside of an ICU function implementation:

U_CAPI const UBiDiLevel * U_EXPORT2
ubidi_getLevels(UBiDi *pBiDi, UErrorCode *pErrorCode) {
    int32_t start, length;

    if(U_FAILURE(*pErrorCode)) {
        return NULL;
    } else if(pBiDi==NULL || (length=pBiDi->length)<=0) {
        return NULL;

    return result;

Note: We have decided that we do not want to test for pErrorCode==NULL. Some existing code does this, but new code should not.

Note: Callers (as opposed to implementers) of ICU APIs can simplify their code by defining and using a subclass of icu::ErrorCode. ICU implementers can use the IcuTestErrorCode class in intltest code.

It is not necessary to check for U_FAILURE() immediately before calling a function that takes a UErrorCode parameter, because that function is supposed to check for failure. Exception: If the failure comes from objection allocation or creation, then you probably have a NULL object pointer and must not call any method on that object, not even one with a UErrorCode parameter.

Sample Function with Error Checking

    U_CAPI int32_t U_EXPORT2
    uplrules_select(const UPluralRules *uplrules,   // Do not check
                                                    // "this"/uplrules vs. NULL.
                    double number,
                    UChar *keyword, int32_t capacity,
                    UErrorCode *status)             // Do not check status!=NULL.
        if (U_FAILURE(*status)) {                   // Do check for U_FAILURE()
                                                    // before setting *status
            return 0;                               // or calling UErrorCode-less
                                                    // select(number).
        if (keyword == NULL ? capacity != 0 : capacity < 0) {
                                                    // Standard destination buffer
                                                    // checks.
            *status = U_ILLEGAL_ARGUMENT_ERROR;
            return 0;
        UnicodeString result = ((PluralRules*)uplrules)->select(number);
        return result.extract(keyword, capacity, *status);

New API Functions

If the API function is non-const, then it should have a UErrorCode parameter. (Not the other way around: Some const functions may need a UErrorCode as well.)

Default C++ assignment operators and copy constructors should not be used (they should be declared private and not implemented). Instead, define an assign(Class &other, UErrorCode &errorCode) function. Normal constructors are fine, and should have a UErrorCode parameter.

Warning Codes

Some UErrorCode values do not indicate a failure but an additional informational return value. Their enum constants have the _WARNING suffix and they pass the U_SUCCESS() test.

However, experience has shown that they are problematic: They can get lost easily because subsequent function calls may set their own “warning” codes or may reset a UErrorCode to U_ZERO_ERROR.

The source of the problem is that the UErrorCode mechanism is designed to mimic C++/Java exceptions. It prevents ICU function execution after a failure code is set, but like exceptions it does not work well for non-failure information passing.

Therefore, we recommend to use warning codes very carefully:

  • Try not to rely on any warning codes.
  • Use real APIs to get the same information if possible. For example, when a string is completely written but cannot be NUL-terminated, then U_STRING_NOT_TERMINATED_WARNING indicates this, but so does the returned destination string length (which will have the same value as the destination capacity in this case). Checking the string length is safer than checking the warning code. (It is even safer to not rely on NUL-terminated strings but to use the length.)
  • If warning codes must be used, then the best is to set the UErrorCode to U_ZERO_ERROR immediately before calling the function in question, and to check for the expected warning code immediately after the function returns.

Future versions of ICU will not introduce new warning codes, and will provide real API replacements for all existing warning codes.

Bogus Objects

Some objects, for example UnicodeString and UnicodeSet, can become “bogus”. This is used when methods that create or modify the object fail (mostly due to an out-of-memory condition) but do not take a UErrorCode parameter and can therefore not otherwise report the failure.

  • A bogus object appears as empty.
  • A bogus object cannot be modified except with assignment-like functions.
  • The bogus state of one object does not transfer to another. For example, adding a bogus UnicodeString to a UnicodeSet does not make the set bogus. (It would be hard to make propagation consistent and test it well. Also, propagation among bogus states and error codes would be messy.)
  • If a bogus object is passed into a function that does have a UErrorCode parameter, then the function should set the U_ILLEGAL_ARGUMENT_ERROR code.

API Documentation

“API” means any public class, function, or constant.

API status tag

Aside from documenting an API's functionality, parameters, return values etc. we also mark every API with whether it is @draft, @stable, @deprecated or @internal. (Where @internal is used when something is not actually supported API but needs to be physically public anyway.) A new API is usually marked with “@draft ICU 4.8”. For details of how we mark APIs see the “ICU API compatibility” section of the ICU Architectural Design page. In Java, also see existing @draft APIs for complete examples.

Functions that override a base class or interface definition take the API status of the base class function. For C++, use the @copydoc base::function() tag to copy both the description and the API status from the base function definition. For Java methods the status tags must be added by hand; use the {@inheritDoc} JavaDoc tag to pick up the rest of the base function documentation. Documentation should not be manually replicated in overriding functions; it is too hard to keep multiple copies synchronized.

The policy for the treatment of status tags in overriding functions was introduced with ICU 64 for C++, and with ICU 59 for Java. Earlier code may deviate.

Coding Example

Coding examples help users to understand the usage of each API. Whenever possible, it is encouraged to embed a code snippet illustrating the usage of an API along with the functional specification.

Embedding Coding Examples in ICU4J - JCite

Since ICU4J 49M2, the ICU4J ant build target “doc” utilizes an external tool called JCite. The tool allows us to cite a fragment of existing source code into JavaDoc comment using a tag. To embed a code snippet with the tag. For example, {@.jcite} will be replaced a fragment of code marked by comment lines // ---getNextTransisionExample in in package When embedding code snippet using JCite, we recommend to follow next guidelines

  • A sample code should be placed in <icu4j_root>/samples/src directory, although you can cite any source fragment from source files in <icu4j_root>/demos/src, <icu4j_root\>/main/core/*/src, <icu4j_root>/main/test/*/src.
  • A sample code should use package name -<subpackage>.<facility>. <subpackage> is corresponding to the target ICU API class‘s package, that is, one of lang/math/text/util. <facility> is a name of facility, which is usually the base class of the service. For example, use package for samples related to ICU’s date format service, for samples related to time zone service.
  • A sample code should be self-contained as much as possible (use only JDK and ICU public APIs if possible). This allows readers to cut & paste a code snippet to try it out easily.
  • The citing comment should start with three consecutive hyphen followed by lower camel case token - for example, “// ---compareToExample
  • Keep in mind that the JCite tag {@.jcite ...} is not resolved without JCite. It is encouraged to avoid placing code snippet within a sentence. Instead, you should place a code snippet using JCite in an independent paragraph.

Embedding Coding Examples in ICU4C

Also since ICU4C 49M2, ICU4C docs (using the \snippet command which is new in Doxygen 1.7.5) can cite a fragment of existing sample or test code.

Example in ucnv.h:

  * \snippet samples/ucnv/convsamp.cpp ucnv_open
 ucnv_open( ... ) ...

This cites code in icu4c/source/samples/ucnv/convsamp.cpp as follows:

  //! [ucnv_open]
  conv = ucnv_open("koi8-r", &status);
  //! [ucnv_open]

Notice the tag “ucnv_open” which must be the same in all three places (in the header file, and twice in the cited file).

C and C++ Coding Conventions Overview

The ICU group uses the following coding guidelines to create software using the ICU C++ classes and methods as well as the ICU C methods.

C/C++ Hiding Un-@stable APIs

In C/C++, we enclose @draft and such APIs with #ifndef U_HIDE_DRAFT_API or similar as appropriate. When a draft API becomes stable, we need to remove the surrounding #ifndef.

Note: The @system tag is in addition to the @draft/@stable/@deprecated/@obsolete status tag.

Copy/paste the appropriate #ifndef..#endif pair from the following:

#endif  // U_HIDE_DRAFT_API



#endif  // U_HIDE_SYSTEM_API


We #ifndef @draft/@deprecated/... APIs as much as possible, including C functions, many C++ class methods (see exceptions below), enum constants (see exceptions below), whole enums, whole classes, etc.

We do not #ifndef APIs where that would be problematic:

  • struct/class members where that would modify the object layout (non-static struct/class fields, virtual methods)
  • enum constants where that would modify the numeric values of following constants
    • actually, best to use #ifndef together with explicitly defining the numeric value of the next constant
  • C++ class boilerplate (e.g., default/copy constructors), if the compiler would auto-create public functions to replace #ifndef’ed ones
    • For example, the compiler automatically creates a default constructor if the class does not specify any other constructors.
  • private class members
  • definitions in internal/test/tools header files (that would be pointless; they should probably not have API tags in the first place)
  • forward or friend declarations
  • definitions that are needed for other definitions that would not be #ifndef'ed (e.g., for public macros or private methods)
  • platform macros (mostly in platform.h/umachine.h & similar) and user-configurable settings (mostly in uconfig.h)

More handy copy-paste text:

    // Do not enclose the protected default constructor with #ifndef U_HIDE_INTERNAL_API
    // or else the compiler will create a public default constructor.

    // Do not enclose protected default/copy constructors with #ifndef U_HIDE_INTERNAL_API
    // or else the compiler will create public ones.

C and C++ Type and Format Convention Guidelines

The following C and C++ type and format conventions are used to maximize portability across platforms and to provide consistency in the code:

Constants (#define, enum items, const)

Use uppercase letters for constants. For example, use UBREAKITERATOR_DONE, UBIDI_DEFAULT_LTR, ULESS.

For new enum types (as opposed to new values added to existing types), do not define enum types in C++ style. Instead, define C-style enums with U... type prefix and U_/UMODULE_ constants. Define such enum types outside the ICU namespace and outside any C++ class. Define them in C header files if there are appropriate ones.

Variables and Functions

Use mixed-case letters that start with a lowercase letter for variables and functions. For example, use getLength().

Types (class, struct, enum, union)

Use mixed-case that start with an uppercase letter for types. For example, use class DateFormatSymbols.

Function Style

Use the getProperty() and setProperty() style for functions where a lowercase letter begins the first word and the second word is capitalized without a space between it and the first word. For example, UnicodeString getSymbol(ENumberFormatSymbol symbol), void setSymbol(ENumberFormatSymbol symbol, UnicodeString value) and getLength(), getSomethingAt(index/offset).

Common Parameter Names

In order to keep function parameter names consistent, the following are recommendations for names or suffixes (usual “Camel case” applies):

  • “start”: the index (of the first of several code units) in a string or array
  • “limit”: the index (of the first code unit after a specified range) in a string or array (the number of units are (limit-start))
  • name the length (for the number of code units in a (range of a) string or array) either “length” or “somePrefixLength”
  • name the capacity (for the number of code units available in an output buffer) either “capacity” or “somePrefixCapacity”

Order of Source/Destination Arguments

Many ICU function signatures list source arguments before destination arguments, as is common in C++ and Java APIs. This is the preferred order for new APIs. (Example: ucol_getSortKey(const UCollator *coll, const UChar *source, int32_t sourceLength, uint8_t *result, int32_t resultLength))

Some ICU function signatures list destination arguments before source arguments, as is common in C standard library functions. This should be limited to functions that closely resemble such C standard library functions or closely related ICU functions. (Example: u_strcpy(UChar *dst, const UChar *src))

Order of Include File Includes

Include system header files (like <stdio.h>) before ICU headers followed by application-specific ones. This assures that ICU headers can use existing definitions from system headers if both happen to define the same symbols. In ICU files, all used headers should be explicitly included, even if some of them already include others.

Within a group of headers, place them in alphabetical order.

Style for ICU Includes

All ICU headers should be included using ""-style includes (like "unicode/utypes.h" or "cmemory.h") in source files for the ICU library, tools, and tests.

Pointer Conversions

Do not cast pointers to integers or integers to pointers. Also, do not cast between data pointers and function pointers. This will not work on some compilers, especially with different sizes of such types. Exceptions are only possible in platform-specific code where the behavior is known.

Please use C++-style casts, at least for pointers, for example const_cast.

  • For conversion between related types, for example from a base class to a subclass (when you know that the object is of that type), use static_cast. (When you are not sure if the object has the subclass type, then use a dynamic_cast; see a later section about that.)
  • Also use static_cast, not reinterpret_cast, for conversion from void * to a specific pointer type. (This is accepted and recommended because there is an implicit conversion available for the opposite conversion.) See ICU-9434 for details.
  • For conversion between unrelated types, for example between char * and uint8_t *, or between Collator * and UCollator *, use a reinterpret_cast.

Returning a Number of Items

To return a number of items, use countItems(), not getItemCount(), even if there is no need to actually count using that member function.

Ranges of Indexes

Specify a range of indexes by having start and limit parameters with names or suffix conventions that represent the index. A range should contain indexes from start to limit-1 such as an interval that is left-closed and right-open. Using mathematical notation, this is represented as: [start..limit[.

Functions with Buffers

Set the default value to -1 for functions that take a buffer (pointer) and a length argument with a default value so that the function determines the length of the input itself (for text, calling u_strlen()). Any other negative or undefined value constitutes an error.

Primitive Types

Primitive types are defined by the unicode/utypes.h file or a header file that includes other header files. The most common types are uint8_t, uint16_t, uint32_t, int8_t, int16_t, int32_t, char16_t, UChar (same as char16_t), UChar32 (signed, 32-bit), and UErrorCode.

The language built-in type bool and constants true and false may be used internally, for local variables and parameters of internal functions. The ICU type UBool must be used in public APIs and in the definition of any persistent data structures. UBool is guaranteed to be one byte in size and signed; bool is not.

Traditionally, ICU4C has defined its own FALSE=0 / TRUE=1 macros for use with UBool. Starting with ICU 68 (2020q4), we no longer define these in public header files (unless U_DEFINE_FALSE_AND_TRUE=1), in order to avoid name collisions with code outside ICU defining enum constants and similar with these names.

Instead, the versions of the C and C++ standards we require now do define type bool and values false & true, and we and our users can use these values.

As of ICU 68, we are not changing ICU4C API from UBool to bool. Doing so in C API, or in structs that cross the library boundary, would break binary compatibility. Doing so only in other places in C++ could be confusingly inconsistent. We may revisit this.

Note that the details of type bool (e.g., sizeof) depend on the compiler and may differ between C and C++.

File Names (.h, .c, .cpp, data files if possible, etc.)

Limit file names to 31 lowercase ASCII characters. (Older versions of MacOS have that length limit.)

Exception: The layout engine uses mixed-case file names.

(We have abandoned the 8.3 naming standard although we do not change the names of old header files.)

Language Extensions and Standards

Proprietary features, language extensions, or library functions, must not be used because they will not work on all C or C++ compilers. In Microsoft Visual C++, go to Project Settings(alt-f7)->All Configurations-> C/C++->Customize and check Disable Language Extensions.

Exception: some Microsoft headers will not compile without language extensions being enabled, which in turn requires some ICU files be built with language extensions.

Tabs and Indentation

Save files with spaces instead of tab characters (\x09). The indentation size is 4.


Use Java doc-style in-file documentation created with doxygen .

Multiple Statements

Place multiple statements in multiple lines. if() or loop heads must not be followed by their bodies on the same line.

Placements of {} Curly Braces

Place curly braces {} in reasonable and consistent locations. Each of us subscribes to different philosophies. It is recommended to use the style of a file, instead of mixing different styles. It is requested, however, to not have if() and loop bodies without curly braces.

if() {...} and Loop Bodies

Use curly braces for if() and else as well as loop bodies, etc., even if there is only one statement.

Function Declarations

Have one line that has the return type and place all the import declarations, extern declarations, export declarations, the function name, and function signature at the beginning of the next line.

Function declarations need to be in the form U_CAPI return-type U_EXPORT2 to satisfy all the compilers' requirements.

For example, use the following convention:

U_CAPI int32_t U_EXPORT2

:point_right: Note: The U_CAPI/U_DEPRECATED and U_EXPORT2 qualifiers are required for both the declaration and the definiton of exported C and static C++ functions. Use U_CAPI (or U_DEPRECATED) before and U_EXPORT2 after the return type of exported C and static C++ functions.

Internal functions that are visible outside a compilation unit need a U_CFUNC before the return type.

Non-static C++ class member functions do not get U_CAPI/U_EXPORT2 because they are exported and declared together with their class exports.

:point_right: Note: Before ICU 68 (2020q4) we used to use alternate qualifiers like U_DRAFT, U_STABLE etc. rather than U_CAPI, but keeping these in sync with API doc tags @draft and guard switches like U_HIDE_DRAFT_API was tedious and error-prone and added no value. Since ICU 68 (ICU-9961) we only use U_CAPI and U_DEPRECATED.

Use Anonymous Namesapces or Static For File Scope

Use anonymous namespaces or static for variables, functions, and constants that are not exported explicitly by a header file. Some platforms are confused if non-static symbols are not explicitly declared extern. These platforms will not be able to build ICU nor link to it.

Using C Callbacks From C++ Code

z/OS and Windows COM wrappers around ICU need __cdecl for callback functions. The reason is that C++ can have a different function calling convention from C. These callback functions also usually need to be private. So the following code

isAcceptable(void * /* context */,
             const char * /* type */, const char * /* name */,
             const UDataInfo *pInfo)
    // Do something here.

should be changed to look like the following by adding U_CDECL_BEGIN, static, U_CALLCONV and U_CDECL_END.

static UBool U_CALLCONV
isAcceptable(void * /* context */,
             const char * /* type */, const char * /* name */,
             const UDataInfo *pInfo)
    // Do something here.

Same Module and Functionality in C and in C++

Determine if two headers are needed. If the same functionality is provided with both a C and a C++ API, then there can be two headers, one for each language, even if one uses the other. For example, there can be umsg.h for C and msgfmt.h for C++.

Not all functionality has or needs both kinds of API. More and more functionality is available only via C APIs to avoid duplication of API, documentation, and maintenance. C APIs are perfectly usable from C++ code, especially with UnicodeString methods that alias or expose C-style string buffers.

Platform Dependencies

Use the platform dependencies that are within the header files that utypes.h files include. They are platform.h (which is generated by the configuration script from and its more specific cousins like pwin32.h for Windows, which define basic types, and putil.h, which defines platform utilities. Important: Outside of these files, and a small number of implementation files that depend on platform differences (like umutex.c), no ICU source code may have any #ifdef OperatingSystemName instructions.

Short, Unnested Mutex Blocks

Do not use function calls within a mutex block for mutual-exclusion (mutex) blocks. This can prevent deadlocks from occurring later. There should be as little code inside a mutex block as possible to minimize the performance degradation from blocked threads. Also, it is not guaranteed that mutex blocks are re-entrant; therefore, they must not be nested.

Names of Internal Functions

Internal functions that are not declared static (regardless of inlining) must follow the naming conventions for exported functions because many compilers and linkers do not distinguish between library exports and intra-library visible functions.

Which Language for the Implementation

Write implementation code in C++. Use objects very carefully, as always: Implicit constructors, assignments etc. can make simple-looking code surprisingly slow.

For every C API, make sure that there is at least one call from a pure C file in the cintltst test suite.

Background: We used to prefer C or C-style C++ for implementation code because we used to have users ask for pure C. However, there was never a large, usable subset of ICU that was usable without any C++ dependencies, and C++ can(!) make for much shorter, simpler, less error-prone and easier-to-maintain code, for example via use of “smart pointers” (unicode/localpointer.h and cmemory.h).

We still try to expose most functionality via C APIs because of the difficulties of binary compatible C++ APIs exported from DLLs/shared libraries.

No Compiler Warnings

ICU must compile without compiler warnings unless such warnings are verified to be harmless or bogus. Often times a warning on one compiler indicates a breaking error on another.

Enum Values

When casting an integer value to an enum type, the enum type should have a constant with this integer value, or at least it must have a constant whose value is at least as large as the integer value being cast, with the same signedness. For example, do not cast a -1 to an enum type that only has non-negative constants. Some compilers choose the internal representation very tightly for the defined enum constants, which may result in the equivalent of a uint8_t representation for an enum type with only small, non-negative constants. Casting a -1 to such a type may result in an actual value of 255. (This has happened!)

When casting an enum value to an integer type, make sure that the enum value's numeric value is within range of the integer type.

Do not check for this!=NULL, do not check for NULL references

In public APIs, assume this!=0 and assume that references are not 0. In C code, "this" is the “service object” pointer, such as set in uset_add(USet* set, UChar32 c) — don't check for set!=NULL.

We do usually check all other (non-this) pointers for NULL, in those cases when NULL is not valid. (Many functions allow a NULL string or buffer pointer if the length or capacity is 0.)

Rationale: "this" is not really an argument, and checking it costs a little bit of code size and runtime. Other libraries also commonly do not check for valid "this", and resulting failures are fairly obvious.

Memory Usage

Dynamically Allocated Memory

ICU4C APIs are designed to allow separate heaps for its libraries vs. the application. This is achieved by providing factory methods and matching destructors for all allocated objects. The C++ API uses a common base class with overridden new/delete operators and/or forms an equivalent pair with createXyz() factory methods and the delete operator. The C API provides pairs of open/close functions for each service. See the C++ and C guideline sections below for details.

Exception: Most C++ API functions that return a StringEnumeration (by pointer which the caller must delete) are named getXyz() rather than createXyz() because "get" is much more natural. (These are not factory methods in the sense of NumberFormat::createScientificInstance().) For example, static StringEnumeration *Collator::``get``Keywords(UErrorCode &). We should document clearly in the API comments that the caller must delete the returned StringEnumeration.

Declaring Static Data

All unmodifiable data should be declared const. This includes the pointers and the data itself. Also if you do not need a pointer to a string, declare the string as an array. This reduces the time to load the library and all its pointers. This should be done so that the same library data can be shared across processes automatically. Here is an example:

#define MY_MACRO_DEFINED_STR "macro string"
const char *myCString = "myCString";
int16_t myNumbers[] = {1, 2, 3};

This should be changed to the following:

static const char MY_MACRO_DEFINED_STR[] = "macro string";
static const char myCString[] = "myCString";
static const int16_t myNumbers[] = {1, 2, 3};

No Static Initialization

The most common reason to have static initialization is to declare a static const UnicodeString, for example (see utypes.h about invariant characters):

static const UnicodeString myStr("myStr", "");

The most portable and most efficient way to declare ASCII text as a Unicode string is to do the following instead:

static const UChar myStr[] = { 0x6D, 0x79, 0x53, 0x74, 0x72, 0}; /* "myStr" */

We do not use character literals for Unicode characters and strings because the execution character set of C/C++ compilers is almost never Unicode and may not be ASCII-compatible (especially on EBCDIC platforms). Depending on the API where the string is to be used, a terminating NUL (0) may or may not be required. The length of the string (number of UChars in the array) can be determined with sizeof(myStr)/U_SIZEOF_UCHAR, (subtract 1 for the NUL if present). Always remember to put in a comment at the end of the declaration what the Unicode string says.

Static initialization of C++ objects must not be used in ICU libraries because of the following reasons:

  1. It leads to intractable order-of-initialization dependencies.
  2. It makes it difficult or impossible to release all of the libraries resources. See u_cleanup().
  3. It takes time to initialize the library.
  4. Dependency checking is not completely done in C or C++. For instance, if an ICU user creates an ICU object or calls an ICU function statically that depends on static data, it is not guaranteed that the statically declared data is initialized.
  5. Certain users like to manage their own memory. They can not manage ICU's memory properly because of item #2.
  6. It is easier to debug code that does not use static initialization.
  7. Memory allocated at static initialization time is not guaranteed to be deallocated with a C++ destructor when the library is unloaded. This is a problem when ICU is unloaded and reloaded into memory and when you are using a heap debugging tool. It would also not work with the u_cleanup() function.
  8. Some platforms cannot handle static initialization or static destruction properly. Several compilers have this random bug (even in the year 2001).

ICU users can use the U_STRING_DECL and U_STRING_INIT macros for C strings. Note that on some platforms this will incur a small initialization cost (simple conversion). Also, ICU users need to make sure that they properly and consistently declare the strings with both macros. See ustring.h for details.

C++ Coding Guidelines

This section describes the C++ specific guidelines or conventions to use.

Portable Subset of C++

ICU uses only a portable subset of C++ for maximum portability. Also, it does not use features of C++ that are not implemented well in all compilers or are cumbersome. In particular, ICU does not use exceptions, or the Standard Template Library (STL).

We have started to use templates in ICU 4.2 (e.g., StringByteSink) and ICU 4.4 (LocalPointer and some internal uses). We try to limit templates to where they provide a lot of benefit (robust code, avoid duplication) without much or any code bloat.

We continue to not use the Standard Template Library (STL) in ICU library code because its design causes a lot of code bloat. More importantly:

  • Exceptions: STL classes and algorithms throw exceptions. ICU does not throw exceptions, and ICU code is not exception-safe.
  • Memory management: STL uses default new/delete, or Allocator parameters which create different types; they throw out-of-memory exceptions. ICU memory allocation is customizable and must not throw exceptions.
  • Non-polymorphic: For APIs, STL classes are also problematic because different template specializations create different types. For example, some systems use custom string classes (different allocators, different strategies for buffer sharing vs. copying), and ICU should be able to interface with most of them.

We have started to use compiler-provided Run-Time Type Information (RTTI) in ICU 4.6. It is now required for building ICU, and encouraged for using ICU where RTTI is needed. For example, use dynamic_cast<DecimalFormat*> on a NumberFormat pointer that is usually but not always a DecimalFormat instance. Do not use dynamic_cast<> on a reference, because that throws a bad_cast exception on failure.

ICU uses a limited form of multiple inheritance equivalent to Java's interface mechanism: All but one base classes must be interface/mixin classes, i.e., they must contain only pure virtual member functions. For details see the ‘boilerplate’ discussion below. This restriction to at most one base class with non-virtual members eliminates problems with the use and implementation of multiple inheritance in C++. ICU does not use virtual base classes.

:point_right: Note: Every additional base class, even an interface/mixin class, adds another vtable pointer to each subclass object, that is, it increases the object/instance size by 8 bytes on most platforms.

Classes and Members

C++ classes and their members do not need a ‘U’ or any other prefix.

Global Operators

Global operators (operators that are not class members) can be problematic for library entry point versioning, may confuse users and cannot be easily ported to Java (ICU4J). They should be avoided if possible.

The issue with library entry point versioning is that on platforms that do not support namespaces, users must rename all classes and global functions via urename.h. This renaming process is not possible with operators. Starting with ICU 49, we require C++ namespace support. However, a global operator can be used in ICU4C (when necessary) if its function signature contains an ICU C++ class that is versioned. This will result in a mangled linker name that does contain the ICU version number via the versioned name of the class parameter. For example, ICU4C 2.8 added an operator + for UnicodeString, with two UnicodeString reference parameters.

Virtual Destructors

In classes with virtual methods, destructors must be explicitly declared, and must be defined (implemented) outside the class definition in a .cpp file.

More precisely:

  1. All classes with any virtual members or any bases with any virtual members should have an explicitly declared virtual destructor.
  2. Constructors and destructors should be declared and/or defined prior to any other methods, public or private, within the class definition.
  3. All virtual destructors should be defined out-of-line, and in a .cpp file rather than a header file.

This is so that the destructors serve as “key functions” so that the compiler emits the vtable in only and exactly the desired files. It can help make binaries smaller that use statically-linked ICU libraries, because the compiler and linker can prove more easily that some code is not used.

The Itanium C++ ABI (which is used on all x86 Linux) says: “The virtual table for a class is emitted in the same object containing the definition of its key function, i.e. the first non-pure virtual function that is not inline at the point of class definition. If there is no key function, it is emitted everywhere used.”

(This was first done in ICU 49; see ticket #8454


Beginning with ICU version 2.0, ICU uses namespaces. The actual namespace is icu_M_N with M being the major ICU release number and N being the minor ICU release number. For convenience, the namespace icu is an alias to the current release-specific one. (The actual namespace name is icu itself if renaming is turned off.)

Starting with ICU 49, we require C++ namespace support.

Class declarations, even forward declarations, must be scoped to the ICU namespace. For example:


class Locale;


extern void fn(icu::UnicodeString&);

// automatically set by utypes.h
// but recommended to be not set automatically
Locale loc("fi");

U_NAMESPACE_USE (expands to using namespace icu_M_N; when available) is automatically done when utypes.h is included, so that all ICU classes are immediately usable. However, we recommend that you turn this off via CXXFLAGS="-DU_USING_ICU_NAMESPACE=0".

Declare Class APIs

Class APIs need to be declared like either of the following:

Inline-Implemented Member Functions

Class member functions are usually declared but not inline-implemented in the class declaration. A long function implementation in the class declaration makes it hard to read the class declaration.

It is ok to inline-implement trivial functions in the class declaration. Pretty much everyone agrees that inline implementations are ok if they fit on the same line as the function signature, even if that means bending the single-statement-per-line rule slightly:

T *orphan() { T *p=ptr; ptr=NULL; return p; }

Most people also agree that very short multi-line implementations are ok inline in the class declaration. Something like the following is probably the maximum:

Value *getValue(int index) {
    if(index>=0 && index<fLimit) {
        return fArray[index];
    return NULL;

If the inline implementation is longer than that, then just declare the function inline and put the actual inline implementations after the class declaration in the same file. (See unicode/unistr.h for many examples.)

If it‘s significantly longer than that, then it’s probably not a good candidate for inlining anyway.

C++ class layout and ‘boilerplate’

There are different sets of requirements for different kinds of C++ classes. In general, all instantiable classes (i.e., all classes except for interface/mixin classes and ones with only static member functions) inherit the UMemory base class. UMemory provides new/delete operators, which allows to keep the ICU heap separate from the application heap, or to customize ICU's memory allocation consistently.

:point_right: Note: Public ICU APIs must return or orphan only C++ objects that are to be released with delete. They must not return allocated simple types (including pointers, and arrays of simple types or pointers) that would have to be released with a free() function call using the ICU library's heap. Simple types and pointers must be returned using fill-in parameters (instead of allocation), or cached and owned by the returning API.

Public ICU C++ classes must inherit either the UMemory or the UObject base class for proper memory management, and implement the following common set of ‘boilerplate’ functions:

  • default constructor
  • copy constructor
  • assignment operator
  • operator==
  • operator!=

:point_right: Note: Each of the above either must be implemented, verified that the default implementation according to the C++ standard will work (typically not if any pointers are used), or declared private without implementation.

  • If public subclassing is intended, then the public class must inherit UObject and should implement
    • clone()
  • RTTI:
    • If a class is a subclass of a parent (e.g., Format) with ICU‘s "poor man’s RTTI" (Run-Time Type Information) mechanism (via getDynamicClassID() and getStaticClassID()) then add that to the new subclass as well (copy implementations from existing C++ APIs).
    • If a class is a new, immediate subclass of UObject (e.g., Normalizer2), creating a whole new class hierarchy, then declare a private getDynamicClassID() and define it to return NULL (to override the pure virtual version in UObject); copy the relevant lines from normalizer2.h and normalizer2.cpp (UOBJECT_DEFINE_NO_RTTI_IMPLEMENTATION(className)). Do not add any “poor man's RTTI” at all to subclasses of this class.

Interface/mixin classes are equivalent to Java interfaces. They are as much multiple inheritance as ICU uses — they do not decrease performance, and they do not cause problems associated with multiple base classes having data members. Interface/mixin classes contain only pure virtual member functions, and must contain an empty virtual destructor. See for example the UnicodeMatcher class. Interface/mixin classes must not inherit any non-interface/mixin class, especially not UMemory or UObject. Instead, implementation classes must inherit one of these two (or a subclass of them) in addition to the interface/mixin classes they implement. See for example the UnicodeSet class.

Static classes contain only static member functions and are therefore never instantiated. They must not inherit UMemory or UObject. Instead, they must declare a private default constructor (without any implementation) to prevent instantiation. See for example the LESwaps layout engine class.

C++ classes internal to ICU need not (but may) implement the boilerplate functions as mentioned above. They must inherit at least UMemory if they are instantiable.

Make Sure The Compiler Uses C++

The __cplusplus macro being defined ensures that the compiler uses C++. Starting with ICU 49, we use this standard predefined macro.

Up until ICU 4.8 we used to define and use XP_CPLUSPLUS but that was redundant and did not add any value because it was defined if-and-only-if __cplusplus was defined.

Adoption of Objects

Some constructors and factory functions take pointers to objects that they adopt. The newly created object contains a pointer to the adoptee and takes over ownership and lifecycle control. If an error occurs while creating the new object (and thus in the code that adopts an object), then the semantics used within ICU must be adopt-on-call (as opposed to, for example, adopt-on-success):

  • General: A constructor or factory function that adopts an object does so in all cases, even if an error occurs and a UErrorCode is set. This means that either the adoptee is deleted immediately or its pointer is stored in the new object. The former case is most common when the constructor or factory function is called and the UErrorCode already indicates a failure. In the latter case, the new object must take care of deleting the adoptee once it is deleted itself regardless of whether or not the constructor was successful.

  • Constructors: The code that creates the object with the new operator must check the resulting pointer returned by new and delete any adoptees if it is 0 because the constructor was not called. (Typically, a UErrorCode must be set to U_MEMORY_ALLOCATION_ERROR.)

    Pitfall: If you allocate/construct via “ClassName *p = new ClassName(adoptee);” and the memory allocation failed (p==NULL), then the constructor has not been called, the adoptee has not been adopted, and you are still responsible for deleting it!

  • Factory functions (createInstance()): The factory function must set a U_MEMORY_ALLOCATION_ERROR and delete any adoptees if it cannot allocate the new object. If the construction of the object fails otherwise, then the factory function must delete it and the factory function must delete its adoptees. As a result, a factory function always returns either a valid object and a successful UErrorCode, or a 0 pointer and a failure UErrorCode. A factory function returns a pointer to an object that must be deleted by the user/owner.

Example: (This is a best-practice example. It does not reflect current Calendar code.)

Calendar::createInstance(TimeZone* zone, UErrorCode& errorCode) {
    LocalPointer<TimeZone> adoptedZone(zone);
    if(U_FAILURE(errorCode)) {
        // The adoptedZone destructor deletes the zone.
        return NULL;
    // since the Locale isn't specified, use the default locale
    LocalPointer<Calendar> c(new GregorianCalendar(zone, Locale::getDefault(), errorCode));
    if(c.isNull()) {
        errorCode = U_MEMORY_ALLOCATION_ERROR;
        // The adoptedZone destructor deletes the zone. return NULL;
    } else if(U_FAILURE(errorCode)) {
        // The c destructor deletes the Calendar.
        return NULL;
    } // c adopted the zone. adoptedZone.orphan();
    return c.orphan();

Memory Allocation

All ICU C++ class objects directly or indirectly inherit UMemory (see ‘boilerplate’ discussion above) which provides new/delete operators, which in turn call the internal functions in cmemory.c. Creating and releasing ICU C++ objects with new/delete automatically uses the ICU allocation functions.

:point_right: Note: Remember that (in absence of explicit :: scoping) C++ determines which new/delete operator to use from which type is allocated or deleted, not from the context of where the statement is. Since non-class data types (like int) cannot define their own new/delete operators, C++ always uses the global ones for them by default.

When global new/delete operators are to be used in the application (never inside ICU!), then they should be properly scoped as e.g. ::new, and the application must ensure that matching new/delete operators are used. In some cases where such scoping is missing in non-ICU code, it may be simpler to compile ICU without its own new/delete operators. See source/common/unicode/uobject.h for details.

In ICU library code, allocation of non-class data types — simple integer types as well as pointers — must use the functions in cmemory.h/.c (uprv_malloc(), uprv_free(), uprv_realloc()). Such memory objects must be released inside ICU, never by the user; this is achieved either by providing a “close” function for a service or by avoiding to pass ownership of these objects to the user (and instead filling user-provided buffers or returning constant pointers without passing ownership).

The cmemory.h/.c functions can be overridden at ICU compile time for custom memory management. By default, UMemory's new/delete operators are implemented by calling these common functions. Overriding the cmemory.h/.c functions changes the memory management for both C and C++.

C++ objects that were either allocated with new or returned from a createXYZ() factory method must be deleted by the user/owner.

Memory Allocation Failures

All memory allocations and object creations should be checked for success. In the event of a failure (a NULL returned), a U_MEMORY_ALLOCATION_ERROR status should be returned by the ICU function in question. If the allocation failure leaves the ICU service in an invalid state, such that subsequent ICU operations could also fail, the situation should be flagged so that the subsequent operations will fail cleanly. Under no circumstances should a memory allocation failure result in a crash in ICU code, or cause incorrect results rather than a clean error return from an ICU function.

Some functions, such as the C++ assignment operator, are unable to return an ICU error status to their caller. In the event of an allocation failure, these functions should mark the object as being in an invalid or bogus state so that subsequent attempts to use the object will fail. Deletion of an invalid object should always succeed.

Memory Management

C++ memory management is error-prone, and memory leaks are hard to avoid, but the following helps a lot.

First, if you can stack-allocate an object (for example, a UnicodeString or UnicodeSet), do so. It is the easiest way to manage object lifetime.

Inside functions, avoid raw pointers to owned objects. Instead, use LocalPointer<UnicodeString> or LocalUResouceBundlePointer etc., which is ICU's “smart pointer” implementation. This is the “Resource Acquisition Is Initialization(RAII)” idiom. The “smart pointer” auto-deletes the object when it goes out of scope, which means that you can just return from the function when an error occurs and all auto-managed objects are deleted. You do not need to remember to write an increasing number of “delete xyz;” at every function exit point.

In fact, you should almost never need to write “delete” in any function.

  • Except in a destructor where you delete all of the objects which the class instance owns.
  • Also, in static “cleanup” functions you still need to delete cached objects.

When you pass on ownership of an object, for example to return the pointer of a newly built object, or when you call a function which adopts your object, use LocalPointer's .orphan().

  • Careful: When you return an object or pass it into an adopting factory method, you can use .orphan() directly.
  • However, when you pass it into an adopting constructor, you need to pass in the .getAlias(), and only if the allocation of the new owner succeeded (you got a non-NULL pointer for that) do you .orphan() your LocalPointer.
  • See the Calendar::createInstance() example above.
  • See the AlphabeticIndex implementation for live examples. Search for other uses of LocalPointer/LocalArray.

Every object must always be deletable/destructable. That is, at a minimum, all pointers to owned memory must always be either NULL or point to owned objects.


cmemory.h defines the LocalMemory class for chunks of memory of primitive types which will be uprv_free()'ed.

cmemory.h also defines MaybeStackArray and MaybeStackHeaderAndArray which automate management of arrays.

Use CharString (charstr.h) for char * strings that you build and modify.

Global Inline Functions

Global functions (non-class member functions) that are declared inline must be made static inline. Some compilers will export symbols that are declared inline but not static.

No Declarations in the for() Loop Head

Iterations through for() loops must not use declarations in the first part of the loop. There have been two revisions for the scoping of these declarations and some compilers do not comply to the latest scoping. Declarations of loop variables should be outside these loops.

Common or I18N

Decide whether or not the module is part of the common or the i18n API collection. Use the appropriate macros. For example, use U_COMMON_IMPLEMENTATION, U_I18N_IMPLEMENTATION, U_COMMON_API, U_I18N_API. See utypes.h.

Constructor Failure

If there is a reasonable chance that a constructor fails (For example, if the constructor relies on loading data), then either it must use and set a UErrorCode or the class needs to support an isBogus()/setToBogus() mechanism like UnicodeString and UnicodeSet, and the constructor needs to set the object to bogus if it fails.

UVector, UVector32, or UVector64

Use UVector to store arrays of void *; use UVector32 to store arrays of int32_t; use UVector64 to store arrays of int64_t. Historically, UVector has stored either int32_t or void *, but now storing int32_t in a UVector is deprecated in favor of UVector32.

C Coding Guidelines

This section describes the C-specific guidelines or conventions to use.

Declare and define C APIs with both U_CAPI and U_EXPORT2

All C APIs need to be both declared and defined using the U_CAPI and U_EXPORT2 qualifiers.

U_CAPI int32_t U_EXPORT2

:point_right: Note: Use U_CAPI before and U_EXPORT2 after the return type of exported C functions. Internal functions that are visible outside a compilation unit need a U_CFUNC before the return type.

Subdivide the Name Space

Use prefixes to avoid name collisions. Some of those prefixes contain a 3- (or sometimes 4-) letter module identifier. Very general names like u_charDirection() do not have a module identifier in their prefix.

  • For POSIX replacements, the (all lowercase) POSIX function names start with “u_”: u_strlen().
  • For other API functions, a ‘u’ is appended to the beginning with the module identifier (if appropriate), and an underscore ‘_’, followed by the mixed-case function name. For example, use u_charDirection(), ubidi_setPara().
  • For types (struct, enum, union), a “U” is appended to the beginning, often “U<module identifier>” directly to the typename, without an underscore. For example, use UComparisonResult.
  • For #defined constants and macros, a “U_” is appended to the beginning, often “U<module identifier>_” with an underscore to the uppercase macro name. For example, use U_ZERO_ERROR, U_SUCCESS(). For example, UNORM_NFC

Functions for Constructors and Destructors

Functions that roughly compare to constructors and destructors are called umod_open() and umod_close(). See the following example:


ubidi_openSized(UTextOffset maxLength, UTextOffset maxRunCount);

ubidi_close(UBiDi *pBiDi);

Each successful call to a umod_open() returns a pointer to an object that must be released by the user/owner by calling the matching umod_close().

C “Service Object” Types and LocalPointer Equivalents

For every C “service object” type (equivalent to C++ class), we want to have a LocalPointer equivalent, so that C++ code calling the C API can use the specific “smart pointer” to implement the “Resource Acquisition Is Initialization (RAII)” idiom.

For example, in ubidi.h we define the UBiDi “service object” type and also have the following “smart pointer” definition which will call ubidi_close() on destruction:

// Use config switches like this only after including unicode/utypes.h
// or another ICU header.


 * class LocalUBiDiPointer
 * "Smart pointer" class, closes a UBiDi via ubidi_close().
 * For most methods see the LocalPointerBase base class.
 * @see LocalPointerBase
 * @see LocalPointer
 * @stable ICU 4.4
U_DEFINE_LOCAL_OPEN_POINTER(LocalUBiDiPointer, UBiDi, ubidi_close);



Inline Implementation Functions

Some, but not all, C compilers allow ICU users to declare functions inline (which is a C++ language feature) with various keywords. This has advantages for implementations because inline functions are much safer and more easily debugged than macros.

ICU used to use a portable U_INLINE declaration macro that can be used for inline functions in C. However, this was an unnecessary platform dependency.

We have changed all code that used U_INLINE to C++ (.cpp) using “inline”, and removed the U_INLINE definition.

If you find yourself constrained by .c, change it to .cpp.

All functions that are declared inline, or are small enough that an optimizing compiler might inline them even without the inline declaration, should be defined (implemented) – not just declared – before they are first used. This is to enable as much inlining as possible, and also to prevent compiler warnings for functions that are declared inline but whose definition is not available when they are called.

C Equivalents for Classes with Multiple Constructors

In cases like BreakIterator and NumberFormat, instead of having several different ‘open’ APIs for each kind of instances, use an enum selector.

Source File Names

Source file names for C begin with a ‘u’.

Memory APIs Inside ICU

For memory allocation in C implementation files for ICU, use the functions and macros in cmemory.h. When allocated memory is returned from a C API function, there must be a corresponding function (like a ucnv_close()) that deallocates that memory.

All memory allocations in ICU should be checked for success. In the event of a failure (a NULL returned from uprv_malloc()), a U_MEMORY_ALLOCATION_ERROR status should be returned by the ICU function in question. If the allocation failure leaves the ICU service in an invalid state, such that subsequent ICU operations could also fail, the situation should be flagged so that the subsequent operations will fail cleanly. Under no circumstances should a memory allocation failure result in a crash in ICU code, or cause incorrect results rather than a clean error return from an ICU function.

// Comments

C++ style // comments may be used in plain C files and in headers that will be included in C files.

Source Code Strings with Unicode Characters

char * strings in ICU

Declared typeencodingexampleUsed with
char *varies with platform"Hello"Most ICU API functions taking char * parameters. Unless otherwise noted, characters are restricted to the “Invariant” set, described below
char *UTF-8u8"¡Hola!"Only functions that are explicitly documented as expecting UTF-8. No restrictions on the characters used.
UChar *UTF-16u"¡Hola!"All ICU functions with UChar * parameters
UChar32Code Point valueU'😁'UChar32 single code point constant.
wchar_tunknownL"Hello"Not used with ICU. Unknown encoding, unknown size, not portable.

ICU source files are UTF-8 encoded, allowing any Unicode character to appear in Unicode string or character literals, without the need for escaping. But, for clarity, use escapes when plain text would be confusing, e.g. for invisible characters.

For convenience, ICU4C tends to use char * strings in places where only “invariant characters” (a portable subset of the 7-bit ASCII repertoire) are used. This allows locale IDs, charset names, resource bundle item keys and similar items to be easily specified as string literals in the source code. The same types of strings are also stored as “invariant character” char * strings in the ICU data files.

ICU has hard coded mapping tables in source/common/putil.c to convert invariant characters to and from Unicode without using a full ICU converter. These tables must match the encoding of string literals in the ICU code as well as in the ICU data files.

:point_right: Note: Important: ICU assumes that at least the invariant characters always have the same codes as is common on platforms with the same charset family (ASCII vs. EBCDIC). ICU has not been tested on platforms where this is not the case.

Some usage of char * strings in ICU assumes the system charset instead of invariant characters. Such strings are only handled with the default converter (See the following section). The system charset is usually a superset of the invariant characters.

The following are the ASCII and EBCDIC byte values for all of the invariant characters (see also unicode/utypes.h):


Rules Strings with Unicode Characters

In order to include characters in source code strings that are not part of the invariant subset of ASCII, one has to use character escapes. In addition, rules strings for collation, etc. need to follow service-specific syntax, which means that spaces and ASCII punctuation must be quoted using the following rules:

  • Single quotes delineate literal text: a'>'b => a>b
  • Two single quotes, either between or outside of single quoted text, indicate a literal single quote:
    • a''b => a'b
    • a'>''<'b => a>'<b
  • A backslash precedes a single literal character:
  • Several standard mechanisms are handled by u_unescape() and its variants.

:point_right: Note: All of these quoting mechanisms are supported by the RuleBasedTransliterator. The single quote mechanisms (not backslash, not u_unescape()) are supported by the format classes. In its infancy, ResourceBundle supported the \uXXXX mechanism and nothing else. This quoting method is the current policy. However, there are modules within the ICU services that are being updated and this quoting method might not have been applied to all of the modules.

Java Coding Conventions Overview

The ICU group uses the following coding guidelines to create software using the ICU Java classes and methods.

Code style

The standard order for modifier keywords on APIs is:

  • public static final synchronized strictfp
  • public abstract

Do not use wild card import, such as “import java.util.*”. The sort order of import statements is java / javax / org / com. Within each top level package category, sub packages and classes are sorted by alphabetical order. We recommend ICU developers to use the Eclipse IDE feature [Source] - [Organize Imports] (Ctrl+Shift+O) to organize import statements.

All if/else/for/while/do loops use braces, even if the controlled statement is a single line. This is for clarity and to avoid mistakes due to bad nesting of control statements, especially during maintenance.

Tabs should not be present in source files.

Indentation is 4 spaces.

Make sure the code is formatted cleanly with regular indentation. Follow Java style code conventions, e.g., don't put multiple statements on a single line, use mixed-case identifiers for classes and methods and upper case for constants, and so on.

Java source formatting rules described above is coming with the Eclipse project file. It is recommended to run [Source] - [Format] (Ctrl+Shift+F) on Eclipse IDE to clean up source files if necessary.

Use UTF-8 encoding (without BOM) for java source files.

Javadoc should be complete and correct when code is checked in, to avoid playing catch-up later during the throes of the release. Please javadoc all methods, not just external APIs, since this helps with maintenance.

Code organization

Avoid putting more than one top-level class in a single file. Either use separate files or nested classes.

Always define at least one constructor in a public API class. The Java compiler automatically generates no-arg constructor when a class has no explicit constructors. We cannot provide proper API documentations for such default constructors.

Do not mix test, tool, and runtime code in the same file. If you need some access to private or package methods or data, provide public accessors for them and mark them @internal. Test code should be placed in package, and tools (e.g., code that generates data, source code, or computes constants) in package. Occasionally for very simple cases you can leave a few lines of tool code in the main source and comment it out, but maintenance is easier if you just comment the location of the tools in the source and put the actual code elsewhere.

Avoid creating new interfaces unless you know you need to mix the interface into two or more classes that have separate inheritance. Interfaces are impossible to modify later in a backwards-compatible way. Abstract classes, on the other hand, can add new methods with default behavior. Use interfaces only if it is required by the architecture, not just for expediency.

Current releases of ICU4J (since ICU 63) are restricted to use Java SE 7 APIs and language features.

ICU Packages

Public APIs should be placed in,, and For historical reasons and for easier migration from JDK classes, there are also APIs in but new APIs should not be added there.

APIs used only during development, testing, or tools work should be placed in

A class or method which is used by public APIs (listed above) but which is not itself public can be placed in different places:

  1. If it is only used by one class, make it private in that class.
  2. If it is only used by one class and its subclasses, make it protected in that class. In general, also tag it @internal unless you are working on a class that supports user-subclassing (rare).
  3. If it is used by multiple classes in one package, make it package private (also known as default access) and mark it @internal.
  4. If it is used by multiple packages, make it public and place the class in the package.

Error Handling and Exceptions

Errors should be indicated by throwing exceptions, not by returning “bogus” values.

If an input parameter is in error, then a new IllegalArgumentException("description") should be thrown.

Exceptions should be caught only when something must be done, for example special cleanup or rethrowing a different exception. If the error “should never occur”, then throw a new RuntimeException("description") (rare). In this case, a comment should be added with a justification.

Use exception chaining: When an exception is caught and a new one created and thrown (usually with additional information), the original exception should be chained to the new one.

A catch expression should not catch Throwable. Catch expressions should specify the most specific subclass of Throwable that applies. If there are two concrete subclasses, both should be specified in separate catch statements.

Binary Data Files

ICU4J uses the same binary data files as ICU4C, in the big-endian/ASCII form. The ICUBinary class should be used to read them.

Some data sources (for example, compressed Jar files) do not allow the use of several InputStream and related APIs:

  • Memory mapping is efficient, but not available for all data sources.
  • Do not depend on InputStream.available(): It does not provide reliable information for some data sources. Instead, the length of the data needs to be determined from the data itself.
  • Do not call mark() and reset() methods on InputStream without wrapping the InputStream object in a new BufferedInputStream object. These methods are not implemented by the ZipInputStream class, and their use may result in an IOException.

Compiler Warnings

There should be no compiler warnings when building ICU4J. It is recommended to develop using Eclipse, and to fix any problems that are shown in the Eclipse Problems panel (below the main window).

When a warning is not avoidable, you should add @SuppressWarnings annotations with minimum scope.


Objects should not be cast to a class in the sun.* packages because this would cause a SecurityException when run under a SecurityManager. The exception needs to be caught and default action taken, instead of propagating the exception.

Adding .c, .cpp and .h files to ICU

In order to add compilable files to ICU, add them to the source code control system in the appropriate folder and also to the build environment.

To add these files, use the following steps:

  1. Choose one of the ICU libraries:
    • The common library provides mostly low-level utilities and basic APIs that often do not make use of Locales. Examples are APIs that deal with character properties, the Locale APIs themselves, and ResourceBundle APIs.
    • The i18n library provides Locale-dependent and -using APIs, such as for collation and formatting, that are most useful for internationalized user input and output.
  2. Put the source code files into the folder icu/source/library-name, then add them to the build system:
    • For most platforms, add the expected .o files to icu/source/library-name/, to the OBJECTS variable. Add the public header files to the HEADERS variable.
    • For Microsoft Visual C++ 6.0, add all the source code files to icu/source/library-name/library-name.dsp. If you don't have Visual C++, add the filenames to the project file manually.
  3. Add test code to icu/source/test/cintltest for C APIs and to icu/source/test/intltest for C++ APIs.
  4. Make sure that the API functions are called by the test code (100% API coverage) and that at least 85% of the implementation code is exercised by the tests (>=85% code coverage).
  5. Create test code for C using the log_err(), log_info(), and log_verbose() APIs from cintltst.h (which uses ctest.h) and check it into the appropriate folder.
  6. In order to get your C test code called, add its top level function and a descriptive test module path to the test system by calling addTest(). The function that makes the call to addTest() ultimately must be called by addAllTests() in calltest.c. Groups of tests typically have a common addGroup() function that calls addTest() for the test functions in its group, according to the common part of the test module path.
  7. Add that test code to the build system also. Modify and the appropriate .dsp file (For example, the file for the library code).

C Test Suite Notes

The cintltst Test Suite contains all the tests for the International Components for Unicode C API. These tests may be automatically run by typing “cintltst” or “cintltst -all” at the command line. This depends on the C Test Services: cintltst or cintltst -all.

C Test Services

The purpose of the test services is to enable the writing of tests entirely in C. The services have been designed to make creating tests or converting old ones as simple as possible with a minimum of services overhead. A sample test file, “demo.c”, is included at the end of this document. For more information regarding C test services, please see the icu4c/source/tools/ctestfw directory.

Writing Test Functions

The following shows the possible format of test functions:

void some_test()

Output from the test is accomplished with three printf-like functions:

void log_err ( const char *fmt, ... );
void log_info ( const char *fmt, ... );
void log_verbose ( const char *fmt, ... );
  • log_info() writes to the console for informational messages.
  • log_verbose() writes to the console ONLY if the VERBOSE flag is turned on (or the -v option to the command line). This option is useful for debugging. By default, the VERBOSE flag is turned OFF.
  • log_error() can be called when a test failure is detected. The error is then logged and error count is incremented by one.

To use the tests, link them into a hierarchical structure. The root of the structure will be allocated by default.

TestNode *root = NULL; /* empty */
addTest( &root, &some_test, "/test");

Provide addTest() with the function pointer for the function that performs the test as well as the absolute ‘path’ to the test. Paths may be up to 127 chars in length and may be used to group tests.

The calls to addTest must be placed in a function or a hierarchy of functions (perhaps mirroring the paths). See the existing cintltst for more details.

Running the Tests

A subtree may be extracted from another tree of tests for the programmatic running of subtests.

TestNode* sub;
sub = getTest(root, "/mytests");

And a tree of tests may be run simply by:

runTests( root ); /* or 'sub' */

Similarly, showTests() lists out the tests. However, it is easier to use the command prompt with the Usage specified below.


The command line parser resets the error count and prints a summary of the failed tests. But if runTest is called directly, for instance, it needs to be managed manually. ERROR_COUNT contains the number of times log_err was called. runTests resets the count to zero before running the tests. VERBOSITY must be 1 to display log_verbose() data. Otherwise, VERBOSITY must be set to 0 (default).

Building cintltst

To compile this test suite using Microsoft Visual C++ (MSVC), follow the instructions in icu4c/source/readme.html#HowToInstall for building the allC workspace. This builds the libraries as well as the cintltst executable.

Executing cintltst

To run the test suite from the command line, change the directories to icu4c/source/test/cintltst/Debug for the debug build (or icu4c/source/test/cintltst/Release for the release build) and then type cintltst.

cintltst Usage

Type cintltst -h to view its command line parameters.

### Syntax:
### Usage: [ -l ] [ -v ] [ -verbose] [-a] [ -all] [-n]
 [-no_err_msg] [ -h] [ /path/to/test ]
### -l To get a list of test names
### -all To run all the test
### -a To run all the test(same as -all)
### -verbose To turn ON verbosity
### -v To turn ON verbosity(same as -verbose)
### -h To print this message
### -n To turn OFF printing error messages
### -no_err_msg (same as -n)
### -[/subtest] To run a subtest
### For example to run just the utility tests type: cintltest /tsutil)
### To run just the locale test type: cintltst /tsutil/loctst

/******************** sample ctestfw test ********************
********* Simply link this with libctestfw or ctestfw.dll ****
************************* demo.c *****************************/

#include "stdlib.h"
#include "ctest.h"
#include "stdio.h"
#include "string.h"

* Some sample dummy tests.
* the statics simply show how often the test is called.
void mytest()
    static i = 0;
    log_info("I am a test[%d]\n", i++);

void mytest_err()
    static i = 0;
    log_err("I am a test containing an error[%d]\n", i++);
    log_err("I am a test containing an error[%d]\n", i++);

void mytest_verbose()
    /* will only show if verbose is on (-v) */
    log_verbose("I am a verbose test, blabbing about nothing at

* Add your tests from this function

void add_tests( TestNode** root )
    addTest(root, &mytest, "/apple/bravo" );
    addTest(root, &mytest, "/a/b/c/d/mytest");
    addTest(root, &mytest_err, "/d/e/f/h/junk");
    addTest(root, &mytest, "/a/b/c/d/another");
    addTest(root, &mytest, "/a/b/c/etest");
    addTest(root, &mytest_err, "/a/b/c");
    addTest(root, &mytest, "/bertrand/andre/damiba");
    addTest(root, &mytest_err, "/bertrand/andre/OJSimpson");
    addTest(root, &mytest, "/bertrand/andre/juice/oj");
    addTest(root, &mytest, "/bertrand/andre/juice/prune");
    addTest(root, &mytest_verbose, "/verbose");


int main(int argc, const char *argv[])
    TestNode *root = NULL;

    add_tests(&root); /* address of root ptr- will be filled in */

    /* Run the tests. An int is returned suitable for the OS status code.
    (0 for success, neg for parameter errors, positive for the # of
    failed tests) */
    return processArgs( root, argc, argv );

C++ IntlTest Test Suite Documentation

The IntlTest suite contains all of the tests for the C++ API of International Components for Unicode. These tests may be automatically run by typing intltest at the command line. Since the verbose option prints out a considerable amount of information, it is recommended that the output be redirected to a file: intltest -v > testOutput.

Building IntlTest

To compile this test suite using MSVC, follow the instructions for building the alCPP (All C++ interfaces) workspace. This builds the libraries as well as the intltest executable.

Executing IntelTest

To run the test suite from the command line, change the directories to icu4c/source/test/intltest/Debug, then type: intltest -v >testOutput. For the release build, the executable will reside in the icu4c/source/test/intltest/Release directory.

IntelTest Usage

Type just intltest -h to see the usage:

### Syntax:
### IntlTest [-option1 -option2 ...] [testname1 testname2 ...]
### where options are: verbose (v), all (a), noerrormsg (n),
### exhaustive (e) and leaks (l).
### (Specify either -all (shortcut -a) or a test name).
### -all will run all of the tests.
### To get a list of the test names type: intltest LIST
### To run just the utility tests type: intltest utility
### Test names can be nested using slashes ("testA/subtest1")
### For example to list the utility tests type: intltest utility/LIST
### To run just the Locale test type: intltest utility/LocaleTest
### A parameter can be specified for a test by appending '@' and the value
### to the testname.

C: Testing with Fake Time

The “Fake Time” capability allows ICU4C to be tested as if the hardware clock is set to a specific time. This section documents how to use this facility. Note that this facility requires the POSIX 'gettimeofday' function to be operable.

This facility affects all ICU ‘current time’ calculations, including date, calendar, time zone formats, and relative formats. It doesn't affect any calls directly to the underlying operating system.

  1. Build ICU with the U_DEBUG_FAKETIME preprocessor macro set. This can be accomplished with the following line in a file icu/source/icudefs.local :

  2. Determine the UDate value (the time value in milliseconds ± Midnight, Jan 1, 1970 GMT) which you want to use as the target. For this sample we will use the value 28800000, which is Midnight, Pacific Standard Time 1/1/1970.

  3. Set the environment variable U_FAKETIME_START=28800000

  4. Now, the first time ICU checks the current time, it will start at midnight 1/1/1970 (pacific time) and roll forward. So, at the end of 10 seconds of program runtime, the clock will appear to be at 12:00:10.

  5. You can test this by running the utility ‘icuinfo -m’ which will print out the ‘Milliseconds since Epoch’.

  6. You can also test this by running the cintltest test /tsformat/ccaltst/TestCalendar in verbose mode which will print out the current time:

    $ make check ICUINFO_OPTS=-m U_FAKETIME_START=28800000 CINTLTST_OPTS=-v
    U_DEBUG_FAKETIME was set at compile time, so the ICU clock will start at a
    preset value
    env variable U_FAKETIME_START=28800000 (28800000) for an offset of
    -1281957858861 ms from the current time 1281986658861
    PASS: The current date and time fetched is Thursday, January 1, 1970 12:00:00

C: Threading Tests

Threading tests for ICU4C functions should be placed in under utility / MultithreadTest, in the file intltest/tsmthred.h and .cpp. See the existing tests in this file for examples.

Tests from this location are automatically run under the Thread Sanitizer (TSAN) in the ICU continuous build system. TSAN will reliably detect race conditions that could possibly occur, however improbable that occurrence might be normally.

Data races are one of the most common and hardest to debug types of bugs in concurrent systems. A data race occurs when two threads access the same variable concurrently and at least one of the accesses is write. The C++11 standard officially bans data races as undefined behavior.

Binary Data Formats

ICU services rely heavily on data to perform their functions. Such data is available in various more or less structured text file formats, which make it easy to update and maintain. For high runtime performance, most data items are pre-built into binary formats, i.e., they are parsed and processed once and then stored in a format that is used directly during processing.

Most of the data items are pre-built into binary files that are then installed on a user's machine. Some data can also be built at runtime but is not persistent. In the latter case, a primary object should be built once and then cloned to avoid the multiple parsing, processing, and building of the same data.

Binary data formats for ICU must be portable across platforms that share the same endianness and the same charset family (ASCII vs. EBCDIC). It would be possible to handle data from other platform types, but that would require load-time or even runtime conversion.

Data Types

Binary data items are memory-mapped, i.e., they are used as readonly, constant data. Their structures must be portable according to the criteria above and should be efficiently usable at runtime without building additional runtime data structures.

Most native C/C++ data types cannot be used as part of binary data formats because their sizes are not fixed across compilers. For example, an int could be 16/32/64 or even any other number of bits wide. Only types with absolutely known widths and semantics must be used.

Use for example:

  • uint8_t, uint16_t, int32_t etc.
  • UBool: same as int8_t
  • UChar: for 16-bit Unicode strings
  • UChar32: for Unicode code points
  • char: for “invariant characters”, see utypes.h

:point_right: Note: ICU assumes that char is an 8-bit byte but makes no assumption about its signedness.

Do not use for example:

  • short, int, long, unsigned int etc.: undefined widths
  • float, double: undefined formats
  • bool: undefined width and signedness
  • enum: undefined width and signedness
  • wchar_t: undefined width, signedness and encoding/charset

Each field in a binary/mappable data format must be aligned naturally. This means that a field with a primitive type of size n bytes must be at an n-aligned offset from the start of the data block. UChar must be 2-aligned, int32_t must be 4-aligned, etc.

It is possible to use struct types, but one must make sure that each field is naturally aligned, without possible implicit field padding by the compiler — assuming a reasonable compiler.

// bad because i will be preceded by compiler-dependent padding
// for proper alignment
struct BadExample {
    UBool flag;
    int32_t i;

// ok with explicitly added padding or generally conscious
// sequence of types
struct OKExample {
    UBool flag;
    uint8_t pad[3];
    int32_t i;

Within the binary data, a struct type field must be aligned according to its widest member field. The struct OKExample must be 4-aligned because it contains an int32_t field. Make padding explicit via additional fields, rather than letting the compiler choose optional padding.

Another potential problem with struct types, especially in C++, is that some compilers provide RTTI for all classes and structs, which inserts a _vtable pointer before the first declared field. When using struct types with binary/mappable data in C++, assert in some place in the code that offsetof the first field is 0. For an example see the genpname tool.


ICU data files have a UDataHeader structure preceding the actual data. Among other fields, it contains a formatVersion field with four parts (one uint8_t each). It is best to use only the first (major) or first and second (major/minor) fields in the runtime code to determine binary compatibility, i.e., reject a data item only if its formatVersion contains an unrecognized major (or major/minor) version number. The following parts of the version should be used to indicate variations in the format that are backward compatible, or carry other information.

For example, the current file's formatVersion (see the genprops tool and uchar.c/uprops.c) is set to indicate backward-incompatible changes with the major version number, backward-compatible additions with the minor version number, and shift width constants for the UTrie data structure in the third and fourth version numbers (these could change independently of the format).

C/C++ Debugging Hints and Tips

Makefile-based platforms

  • use Makefile.local files (override of Makefile), or icudefs.local (at the top level, override of to avoid the need to modify change-controlled source files with debugging information.
    • Example: CPPFLAGS+=-DUDATA_DEBUG in common to enable data debugging
    • Example: CINTLTST_OPTS=/tscoll in the cintltst directory provides arguments to the cintltest test upon make check, to only run collation tests.
      • intltest: INTLTEST_OPTS
      • cintltst: CINTLTST_OPTS
      • iotest: IOTEST_OPTS
      • icuinfo: ICUINFO_OPTS
      • (letest does not have an OPTS variable as of ICU 4.6.)

Windows/Microsoft Visual Studio

The following addition to autoexp.dat will cause **UnicodeString**s to be visible as strings in the debugger without expanding sub-items:

;; Copyright (C) 2010 IBM Corporation and Others. All Rights Reserved.
;; ICU Additions
;; Add to {VISUAL STUDIO} \Common7\Packages\Debugger\autoexp.dat
;;   in the [autoexpand] section just before the final [hresult] section.
;; Need to change 'icu_##' to the current major+minor (so icu_46 for 4.6.1 etc)

icu_46::UnicodeString {
    preview        (
              #if($e.fFlags & 2)   ; stackbuffer
                "U= '",
                [$e.fUnion.fStackBuffer, su],
                "', len=",
                [$e.fShortLength, u]
                ;[$e.fFields.fArray, su]
                "U* '",
                [$e.fUnion.fFields.fArray, su],
                "', len=",
                [$e.fShortLength, u]
                ;[$e.fFields.fArray, su]

    stringview    (
              #if($e.fFlags & 2)   ; stackbuffer
                "U= '",
                [$e.fUnion.fStackBuffer, su],
                "', len=",
                [$e.fShortLength, u]
                ;[$e.fFields.fArray, su]
                "U* '",
                [$e.fUnion.fFields.fArray, su],
                "', len=",
                [$e.fShortLength, u]
                ;[$e.fFields.fArray, su]

;;; End ICU Additions