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* Copyright 2018 Google LLC
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
#ifndef SkNoDestructor_DEFINED
#define SkNoDestructor_DEFINED
#include <cstddef>
#include <new>
#include <type_traits> // IWYU pragma: keep
#include <utility>
// Helper type to create a function-local static variable of type `T` when `T`
// has a non-trivial destructor. Storing a `T` in a `SkNoDestructor<T>` will
// prevent `~T()` from running, even when the variable goes out of scope. This
// code is adapted from `base::NoDestructor<T>` in Chromium.
// Useful when a variable has static storage duration but its type has a
// non-trivial destructor. Chromium (and transitively, Skia) bans global
// constructors and destructors: using a function-local static variable prevents
// the former, while using `SkNoDestructor<T>` prevents the latter.
// ## Caveats
// - Must not be used for locals or fields; by definition, this does not run
// destructors, and this will likely lead to memory leaks and other
// surprising and undesirable behaviour.
// - If `T` is not constexpr constructible, must be a function-local static
// variable, since a global `NoDestructor<T>` will still generate a static
// initializer.
// - If `T` is constinit constructible, may be used as a global, but mark the
// global `constinit` (once C++20 is available)
// - If the data is rarely used, consider creating it on demand rather than
// caching it for the lifetime of the program. Though `SkNoDestructor<T>`
// does not heap allocate, the compiler still reserves space in bss for
// storing `T`, which costs memory at runtime.
// - If `T` is trivially destructible, do not use `SkNoDestructor<T>`:
// const uint64_t GetUnstableSessionSeed() {
// // No need to use `SkNoDestructor<T>` as `uint64_t` is trivially
// // destructible and does not require a global destructor.
// static const uint64_t kSessionSeed = GetRandUint64();
// return kSessionSeed;
// }
// ## Example Usage
// const std::string& GetDefaultText() {
// // Required since `static const std::string` requires a global destructor.
// static const SkNoDestructor<std::string> s("Hello world!");
// return *s;
// }
// More complex initialization using a lambda:
// const std::string& GetRandomNonce() {
// // `nonce` is initialized with random data the first time this function is
// // called, but its value is fixed thereafter.
// static const SkNoDestructor<std::string> nonce([] {
// std::string s(16);
// GetRandString(, s.size());
// return s;
// }());
// return *nonce;
// }
// ## Thread safety
// Initialization of function-local static variables is thread-safe since C++11.
// The standard guarantees that:
// - function-local static variables will be initialised the first time
// execution passes through the declaration.
// - if another thread's execution concurrently passes through the declaration
// in the middle of initialisation, that thread will wait for the in-progress
// initialisation to complete.
template <typename T> class SkNoDestructor {
static_assert(!(std::is_trivially_constructible_v<T> && std::is_trivially_destructible_v<T>),
"T is trivially constructible and destructible; please use a constinit object of "
"type T directly instead");
"T is trivially destructible; please use a function-local static of type T "
"directly instead");
// Not constexpr; just write static constexpr T x = ...; if the value should be a constexpr.
template <typename... Args> explicit SkNoDestructor(Args&&... args) {
new (fStorage) T(std::forward<Args>(args)...);
// Allows copy and move construction of the contained type, to allow construction from an
// initializer list, e.g. for std::vector.
explicit SkNoDestructor(const T& x) { new (fStorage) T(x); }
explicit SkNoDestructor(T&& x) { new (fStorage) T(std::move(x)); }
SkNoDestructor(const SkNoDestructor&) = delete;
SkNoDestructor& operator=(const SkNoDestructor&) = delete;
~SkNoDestructor() = default;
const T& operator*() const { return *get(); }
T& operator*() { return *get(); }
const T* operator->() const { return get(); }
T* operator->() { return get(); }
const T* get() const { return reinterpret_cast<const T*>(fStorage); }
T* get() { return reinterpret_cast<T*>(fStorage); }
alignas(T) std::byte fStorage[sizeof(T)];
#if defined(__clang__) && defined(__has_feature)
#if __has_feature(leak_sanitizer) || __has_feature(address_sanitizer)
// TODO( This is a hack to work around the fact that LSan doesn't seem
// to treat SkNoDestructor as a root for reachability analysis. This means that code like this:
// static SkNoDestructor<std::vector<int>> v({1, 2, 3});
// is considered a leak. Using the standard leak sanitizer annotations to suppress leaks doesn't
// work: std::vector is implicitly constructed before calling the SkNoDestructor constructor.
// Unfortunately, I haven't been able to demonstrate this issue in simpler reproductions: until
// that's resolved, hold an explicit pointer to the placement-new'd object in leak sanitizer
// mode to help LSan realize that objects allocated by the contained type are still reachable.
T* fStoragePtr = reinterpret_cast<T*>(fStorage);
#endif // leak_sanitizer/address_sanitizer
#endif // __has_feature
#endif // SkNoDestructor_DEFINED