在之前我们了解了class std::future,但是通过class std::future获得的结果只能get()一次,第二次调用就会产生不可预期的结果，通常会抛出一个std::future_error.
但是当多个其他线程也想处理这份结果，也就是想多次get(),基于这个目的C++ standard library提供了class std::shared_future,于是我们就可以多次调用get(),导致相同的结果，或者抛出(throw)同一个异常.
相对于class std::future来说只是少了一个share()操作.别的操作都是与class std::future一致的.
#include <iostream> #include <future> #include <thread> #include <exception> #include <stdexcept> int queryNumber() { std::cout << "enter a number: "; int number; std::cin >> number; if (!std::cin) { throw std::runtime_error(std::string("no number read!")); } return number; } void doSomething(const char& c, const std::shared_future<int>& f) { try { int number = f.get(); for (int i = 0; i < number; ++i) { std::this_thread::sleep_for(std::chrono::duration<int, std::ratio<1, 1000>>(1)); std::cout.put(c).flush(); } }catch (const std::exception& e) { std::cerr << "EXCEPTION in thread" << std::this_thread::get_id() << ": " << e.what() << std::endl; } } int main() { try { std::shared_future<int> result = std::async(std::launch::async, queryNumber); std::future<void> result1 = std::async(std::launch::async, doSomething, '.', result); //注意我们在std::async()中传递的参数result其实是对result的拷贝. //如果我们写成std::async(std::launch::async, '.', std::ref(result));这种情况下才是对result的引用. //这样一来就是对一个std::shared_future多次调用get()了.于是就带来了风险. std::future<void> result2 = std::async(std::launch::async, doSomething, '+', result); std::future<void> result3 = std::async(std::launch::async, doSomething, '*', result); result1.get(); result2.get(); result3.get(); }catch (const std::exception& e) { std::cout << "\nEXCEPTION: " << e.what() << std::endl; } std::cout << "\ndone" << std::endl; return 0; }
