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std::future<T>::wait

void wait() const;

(since C++11)

Blocks until the result becomes available. valid() == true after the call.

The behavior is undefined if valid() == false before the call to this function.

Parameters

(none).

Return value

(none).

Exceptions

May throw implementation-defined exceptions.

Notes

The implementations are encouraged to detect the case when valid() == false before the call and throw a std::future_error with an error condition of std::future_errc::no_state.

Example

#include <chrono>
#include <iostream>
#include <future>
#include <thread>
 
int fib(int n)
{
  if (n < 3) return 1;
  else return fib(n-1) + fib(n-2);
}
 
int main()
{
    std::future<int> f1 = std::async(std::launch::async, [](){
        return fib(40);
    });
    std::future<int> f2 = std::async(std::launch::async, [](){
        return fib(43);
    });
 
    std::cout << "waiting... " << std::flush;
    const auto start = std::chrono::system_clock::now();
 
    f1.wait();
    f2.wait();
 
    const auto diff = std::chrono::system_clock::now() - start;
    std::cout << std::chrono::duration<double>(diff).count() << " seconds\n";
 
    std::cout << "f1: " << f1.get() << '\n';
    std::cout << "f2: " << f2.get() << '\n';
}

Possible output:

waiting... 1.61803 seconds
f1: 102334155
f2: 433494437