32 Concurrency support library [thread] (original) (raw)

32.6.7.1 Struct once_flag [thread.once.onceflag]

namespace std { struct once_flag { constexpr once_flag() noexcept; once_flag(const once_flag&) = delete; once_flag& operator=(const once_flag&) = delete;};}

The class once_flag is an opaque data structure that call_once uses to initialize data without causing a data race or deadlock.

constexpr once_flag() noexcept;

Synchronization: The construction of a once_flag object is not synchronized.

Postconditions: The object's internal state is set to indicate to an invocation ofcall_once with the object as its initial argument that no function has been called.

32.6.7.2 Function call_once [thread.once.callonce]

template<class Callable, class... Args> void call_once(once_flag& flag, Callable&& func, Args&&... args);

Mandates: is_invocable_v<Callable, Args...> is true.

Effects: An execution of call_once that does not call its func is apassive execution.

An execution of call_once that calls its funcis an active execution.

An active execution evaluates_INVOKE_(​std​::​forward<Callable>(func), std​::​forward<Args>(args)...) ([func.require]).

If such a call to functhrows an exception the execution is exceptional, otherwise it is returning.

An exceptional execution propagates the exception to the caller ofcall_once.

Among all executions of call_once for any givenonce_flag: at most one is a returning execution; if there is a returning execution, it is the last active execution; and there are passive executions only if there is a returning execution.

[Note 1:

Passive executions allow other threads to reliably observe the results produced by the earlier returning execution.

— _end note_]

Synchronization: For any given once_flag: all active executions occur in a total order; completion of an active execution synchronizes withthe start of the next one in this total order; and the returning execution synchronizes with the return from all passive executions.

[Example 1: void init(); std::once_flag flag;void f() { std::call_once(flag, init);} struct initializer { void operator()();};void g() { static std::once_flag flag2; std::call_once(flag2, initializer());} class information { std::once_flag verified;void verifier();public: void verify() { std::call_once(verified, &information::verifier, *this); } }; — _end example_]