std::call_once
Defined in header <mutex>
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template< class Callable, class... Args > void call_once( std::once_flag& flag, Callable&& f, Args&&... args ); |
(since C++11) | |
Executes the Callable
object f
exactly once, even if called from several threads.
Each group of call_once
invocations that receives the same std::once_flag object will meet 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 thecall_once
invocation it was passed to.
- No invocation in the group returns before the above-mentioned 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 then selected and executed.
Parameters
flag | - | an object, for which exactly one function gets executed |
f | - | Callable object to invoke
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args... | - | arguments to pass to the function |
Return value
(none)
Exceptions
- std::system_error if any condition prevents calls to
call_once
from executing as specified - any exception thrown by
f
Notes
The arguments to the |
(until C++17) |
The arguments to the |
(since C++17) |
Initialization of function-local statics is guaranteed to occur only once even when called from multiple threads, and may be more efficient than the equivalent code using std::call_once
.
Example
#include <iostream> #include <thread> #include <mutex> std::once_flag flag1, flag2; void simple_do_once() { std::call_once(flag1, [](){ std::cout << "Simple example: called once\n"; }); } void may_throw_function(bool do_throw) { if (do_throw) { std::cout << "throw: call_once will retry\n"; // this may appear more than once throw std::exception(); } std::cout << "Didn't throw, call_once will not attempt again\n"; // guaranteed once } void do_once(bool do_throw) { try { std::call_once(flag2, may_throw_function, do_throw); } catch (...) { } } int main() { std::thread st1(simple_do_once); std::thread st2(simple_do_once); std::thread st3(simple_do_once); std::thread st4(simple_do_once); st1.join(); st2.join(); st3.join(); st4.join(); std::thread t1(do_once, true); std::thread t2(do_once, true); std::thread t3(do_once, false); std::thread t4(do_once, true); t1.join(); t2.join(); t3.join(); t4.join(); }
Possible output:
Simple example: called once throw: call_once will retry throw: call_once will retry Didn't throw, call_once will not attempt again
See also
(C++11) |
helper object to ensure that call_once invokes the function only once (class) |
C documentation for call_once
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