std::call_once
From cppreference.com
Defined in header <mutex>
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template< class Function, class... Args > void call_once( std::once_flag& flag, Function&& f, Args&& args... ); |
(since C++11) | |
Executes the function f exactly once, even if called from several threads.
Each group of call_once invocations, which receives the same std::once_flag object, follows the following requirements:
- Exactly one execution of exactly one of the functions, passed as f to the invocations in the group, is performed. It is undefined which function will be selected for execution. The selected function runs in the same thread as call_once invocation it was passed to.
- No invocation in the group returns before the abovementioned execution of the selected function is completed successfully, that is, doesn't exit via an exception.
- If the selected function exits via exception, it is propagated to the caller. Another function is selected and executed.
Contents |
[edit] Parameters
flag | - | an object, for which exactly one function gets executed |
f | - | function to call |
args... | - | arguments to pass to the function |
[edit] Return value
(none)
[edit] Exceptions
This section is incomplete |
[edit] Example
#include <iostream> #include <thread> #include <mutex> std::once_flag flag; void do_once() { std::call_once(flag, [](){ std::cout << "Called once" << std::endl; }); } int main() { std::thread t1(do_once); std::thread t2(do_once); std::thread t3(do_once); std::thread t4(do_once); t1.join(); t2.join(); t3.join(); t4.join(); }
Output:
Called once
[edit] See also
(C++11) |
helper object to ensure that call_once invokes the function only once (class) |