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| Member functions | ||||
| Getting the result | ||||
future::get | ||||
| State | ||||
Main template | ||
T get(); | (1) | (since C++11) |
std::future<T&> specializations | ||
T& get(); | (2) | (since C++11) |
std::future<void> specialization | ||
void get(); | (3) | (since C++11) |
Theget member function waits (by callingwait()) until the shared state is ready, then retrieves the value stored in the shared state (if any). Right after calling this function,valid() isfalse.
Ifvalid() isfalse before the call to this function, the behavior is undefined.
Contents |
If an exception was stored in the shared state referenced by the future (e.g. via a call tostd::promise::set_exception()) then that exception will be thrown.
The C++ standard recommends the implementations to detect the case whenvalid() isfalse before the call and throw astd::future_error with an error condition ofstd::future_errc::no_state.
#include <chrono>#include <future>#include <iostream>#include <string>#include <thread> std::string time(){staticauto start=std::chrono::steady_clock::now();std::chrono::duration<double> d=std::chrono::steady_clock::now()- start;return"["+std::to_string(d.count())+"s]";} int main(){usingnamespace std::chrono_literals; {std::cout<< time()<<" launching thread\n";std::future<int> f=std::async(std::launch::async,[]{std::this_thread::sleep_for(1s);return7;});std::cout<< time()<<" waiting for the future, f.valid() = "<< f.valid()<<'\n';int n= f.get();std::cout<< time()<<" f.get() returned "<< n<<", f.valid() = "<< f.valid()<<'\n';} {std::cout<< time()<<" launching thread\n";std::future<int> f=std::async(std::launch::async,[]{std::this_thread::sleep_for(1s);returntrue?throwstd::runtime_error("7"):7;});std::cout<< time()<<" waiting for the future, f.valid() = "<< f.valid()<<'\n'; try{int n= f.get();std::cout<< time()<<" f.get() returned "<< n<<", f.valid() = "<< f.valid()<<'\n';}catch(conststd::exception& e){std::cout<< time()<<" caught exception "<< e.what()<<", f.valid() = "<< f.valid()<<'\n';}}}
Possible output:
[0.000004s] launching thread[0.000461s] waiting for the future, f.valid() = 1[1.001156s] f.get() returned with 7, f.valid() = 0[1.001192s] launching thread[1.001275s] waiting for the future, f.valid() = 1[2.002356s] caught exception 7, f.valid() = 0
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2096 | C++11 | overload(1) needed to check whetherT isMoveAssignable | not required |
| checks if the future has a shared state (public member function)[edit] |
