用到 C++ future 庫，程式碼如下：

#include <iostream>
#include <future>
#include <unistd.h>
using namespace std;

double f(double a,double b){
    double c = a+b;
    sleep(3);
    return c;
}

int main(){
    double a=1.0,b=2.1;
    future<double> fu = async(f,a,b);
    cout<<"..."<<endl;
    cout<<"result:"<<fu.get()<<endl;
    return 0;
}
 

編譯方式：

g++ -std=c++11 async.cpp -lpthread -o demo

future 無法實現多個執行緒等待同一個非同步執行緒結果，future 物件呼叫 get 後會轉移控制權，導致只有一個執行緒可以獲取非同步執行緒的返回值，要解決這個問題可以用 shared_future,程式碼如下：

#include <iostream>
#include <future>
#include <unistd.h>
using namespace std;

double f(double a,double b){
    double c = a+b;
    sleep(3);
    return c;
}

int main(){
    double a=1.0,b=2.1;
    shared_future<double> fu = async(f,a,b);
    shared_future<double> c1 = async(f,a,fu.get());
    shared_future<double> c2 = async(f,a,fu.get());
    cout<<"..."<<endl;
    cout<<"result:"<<c1.get()<<endl;
    cout<<"result:"<<c2.get()<<endl;
    cout<<"result:"<<fu.get()<<endl;
    return 0;
}