Overview

template<typename T>
class pw::NoDestructor< T >

Helper type to create a global or function-local static variable of type T when T has a non-trivial destructor. Storing a T in a pw::NoDestructor<T> will prevent ~T() from running, even when the variable goes out of scope. pw::NoDestructor<T> provides an API similar to std::optional. Use * or -> to access the wrapped type.

This class is useful when a variable has static storage duration but its type has a non-trivial destructor. Destructor ordering is not defined and can cause issues in multithreaded environments. Additionally, removing destructor calls can save code size.

Even though pw::NoDestructor does not call the destructor, classes with a private destructor must friend class pw::NoDestructor<CrashInDestructor> to be used with pw::NoDestructor.

Except in generic code, do not use pw::NoDestructor<T> with trivially destructible types. Use the type directly instead. If the variable can be constexpr, make it constexpr.

NoDestructor instances can be constinit if T has a constexpr constructor. In C++20, NoDestructor instances may be constexpr if T has a constexpr destructor. NoDestructor is unnecessary for literal types.

Note: NoDestructor<T> instances may be constant initialized, whether they are constinit or not. This may be undesirable for large objects, since moving them from .bss to .data increases binary size. To prevent this, use pw::RuntimeInitGlobal, which prevents constant initialization and removes the destructor.

Example usage:

pw::sync::Mutex& GetMutex() {
  // Use NoDestructor to avoid running the mutex destructor when exit-time
  // destructors run.
  static const pw::NoDestructor<pw::sync::Mutex> global_mutex;
  return *global_mutex;
}

In Clang, pw::NoDestructor can be replaced with the [[clang::no_destroy]] attribute. pw::NoDestructor<T> is similar to Chromium’s base::NoDestructor<T> in src/base/no_destructor.h.

Warning: Misuse of NoDestructor can cause memory leaks and other problems. Only skip destructors when you know it is safe to do so.

Public Types
using	value_type = T

Public Member Functions
template<typename... Args, typename std::enable_if<!std::is_same< void(std::decay_t< Args > &...), void(NoDestructor &)>::value , int , ::type = 0>
constexpr	NoDestructor (Args &&... args)

constexpr	NoDestructor (const T &x)

constexpr	NoDestructor (T &&x)

	NoDestructor (const NoDestructor &)=delete

NoDestructor &	operator= (const NoDestructor &)=delete

constexpr const T &	operator* () const

constexpr T &	operator* ()

constexpr const T *	operator-> () const

constexpr T *	operator-> ()

The documentation for this class was generated from the following file:

pw_toolchain/public/pw_toolchain/no_destructor.h

Overview

Public Types

Public Member Functions