std::future<T>::get

 

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std::future

future::future

future::~future

future::operator=

future::share

获取结果

future::get

状态

future::valid

future::wait

future::wait_for

future::wait_until

 

T get();	(1)	(仅为泛型 future 模板的成员) (C++11 起)
T& get();	(2)	(仅为 future<T&> 模板特化的成员) (C++11 起)
void get();	(3)	(仅为 future<void> 模板特化的成员) (C++11 起)

get 方法等待直至 future 拥有合法结果并（依赖于使用哪个模板）获取它。它等效地调用 wait() 等待结果。

泛型模板和二个模板特化各含单个 get 版本。 get 的三个版本仅在返回类型有别。

若调用此函数前 valid() 为 false 则行为未定义。

释放任何共享状态。调用此方法后 valid() 为 false 。

参数

（无）

返回值

1) 存储于共享状态的值 v ，如同 std::move(v) 。

2) 存储于共享状态的值的引用。

3) 无。

异常

若 future 所引用的共享状态中存储异常（例如，通过调用 std::promise::set_exception() ），则抛出该异常。

注意

鼓励实现在调用前检测 valid() 为 false 的情况，并抛出以 std::future_errc::no_state 为 error_condition 的 std::future_error 。

示例

运行此代码

#include <thread>
#include <future>
#include <iostream>
#include <string>
#include <chrono>
 
std::string time() {
    static auto 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() {
    using namespace std::chrono_literals;
    {
        std::cout << time() << " launching thread\n";
        std::future<int> f = std::async(std::launch::async, []{
            std::this_thread::sleep_for(1s);
            return 7;
        });
        std::cout << time() << " waiting for the future, f.valid() == "
                  << f.valid() << "\n";
        int n = f.get();
        std::cout << time() << " future.get() returned with " << 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);
            return true ? throw std::runtime_error("7") : 7;
        });
        std::cout << time() << " waiting for the future, f.valid() == "
                  << f.valid() << "\n";
        try {
            int n = f.get();
            std::cout << time() << " future.get() returned with " << n
                      << " f.valid() = " << f.valid() << '\n';
        } catch(const std::exception& e) {
            std::cout << time() << " caught exception " << e.what()
                      << ", f.valid() == " << f.valid() << "\n";
        }
    }
}

可能的输出：

[0.000004s] launching thread
[0.000461s] waiting for the future, f.valid() == 1
[1.001156s] future.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

参阅

valid

检查 future 是否拥有共享状态 (公开成员函数)