std::call_once
| 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:
- If, by the time
std::call_onceis called, flag indicates that f was already called,std::call_oncereturns right away (such a call tostd::call_onceis known as passive).
- Otherwise,
std::call_oncecalls INVOKE(std::forward<Callable>(f), std::forward<Args>(args)...). Unlike the std::thread constructor or std::async, the arguments are not moved or copied because they do not need to be transferred to another thread of execution (such a call tostd::call_onceis known as active).
- If that invocation throws an exception, it is propagated to the caller of
std::call_once, and flag is not flipped so that another call will be attempted (such a call tostd::call_onceis known as exceptional ). - If that invocation returns normally (such a call to
std::call_onceis known as returning), flag is flipped, and all other calls tostd::call_oncewith the same flag are guaranteed to be passive.
- If that invocation throws an exception, it is propagated to the caller of
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 std::call_once are guaranteed to observe any side-effects made by the active call, with no additional synchronization.
Contents |
[edit] Parameters
| flag | - | an object, for which exactly one function gets executed |
| f | - | Callable object to invoke |
| args... | - | arguments to pass to the function |
[edit] Return value
(none)
[edit] Exceptions
- std::system_error if any condition prevents calls to
std::call_oncefrom executing as specified. - Any exception thrown by f.
[edit] Notes
If concurrent calls to std::call_once pass different functions f, it is unspecified which f will be called. The selected function runs in the same thread as the std::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 POSIX equivalent of this function is pthread_once.
[edit] Example
#include <iostream> #include <mutex> #include <thread> 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 << "Did not 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 Throw: call_once will retry Did not throw, call_once will not attempt again
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2080 | C++11 | std::invalid_argument would be thrown if f is invalid, but the scenario where f is invalidated is not specified |
removed this error condition |
| LWG 2442 | C++11 | the arguments were copied and/or moved before invocation | no copying/moving is performed |
[edit] See also
| (C++11) |
helper object to ensure that call_once invokes the function only once (class) |
| C documentation for call_once
| |
