Defined in header <mutex> | ||
---|---|---|
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 concurrently, from several threads.
In detail:
call_once
is called, flag
indicates that f
was already called, call_once
returns right away (such a call to call_once
is known as passive). std::forward<Callable>(f)
with the arguments std::forward<Args>(args)...
(as if by std::invoke
). Unlike the std::thread
constructor or std::async
, the arguments are not moved or copied because they don't need to be transferred to another thread of execution. (such a call to call_once
is known as active). call_once
, and the flag is not flipped so that another call will be attempted (such call to call_once
is known as exceptional). call_once
is known as returning), the flag is flipped, and all other calls to call_once
with the same flag are guaranteed to be passive. All active calls on the same flag
form a single total order consisting of zero or more exceptional calls, followed by one returning call. The end of each active call synchronizes-with the next active call in that order.
The return from the returning call synchronizes-with the returns from all passive calls on the same flag
: this means that all concurrent calls to call_once
are guaranteed to observe any side-effects made by the active call, with no additional synchronization.
flag | - | an object, for which exactly one function gets executed |
f | - | Callable object to invoke |
args... | - | arguments to pass to the function |
(none).
std::system_error
if any condition prevents calls to call_once
from executing as specified f
If concurrent calls to call_once pass different functions f
, it is unspecified which f
will be called. The selected function runs in the same thread as the call_once
invocation it was passed to.
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
.
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
LWG 2442 | C++11 | the arguments are copied and/or moved before invocation | no copying/moving is performed |
#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
(C++11) | helper object to ensure that call_once invokes the function only once (class) |
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
http://en.cppreference.com/w/cpp/thread/call_once