async的用法

有以下几种形式

//从c++11到c++17有
template< class Function, class... Args >
std::future<typename std::result_of<typename std::decay<Function>::type(typename std::decay<Args>::type...)>::type>
    async( Function&& f, Args&&... args ); 
template< class Function, class... Args >
std::future<typename std::result_of<typename std::decay<Function>::type(
        typename std::decay<Args>::type...)>::type>
    async( std::launch policy, Function&& f, Args&&... args );

//从c++17到c++20有
template< class Function, class... Args >
std::future<std::invoke_result_t<std::decay_t<Function>,
                                 std::decay_t<Args>...>>
    async( Function&& f, Args&&... args ); 
template< class Function, class... Args >
std::future<std::invoke_result_t<std::decay_t<Function>,
                                 std::decay_t<Args>...>>
    async( std::launch policy, Function&& f, Args&&... args );
//从c++20有
template< class Function, class... Args >
[[nodiscard]] std::future<std::invoke_result_t<std::decay_t<Function>,                                        std::decay_t<Args>...>>
    async( Function&& f, Args&&... args ); 
template< class Function, class... Args >
[[nodiscard]] std::future<std::invoke_result_t<std::decay_t<Function>,                                            std::decay_t<Args>...>>
    async( std::launch policy, Function&& f, Args&&... args );

上面policy参数支持的值有
std::launch::async：异步调用，与调用线程是不同的线程
std::launch::deferred：延时计算，在调用的线程内
在不带launch类型参数时，默认是异步调用 的
代码样例

#include <algorithm>
#include <future>
#include <iostream>
#include <mutex>
#include <numeric>
#include <string>
#include <vector>
#include <thread>

using namespace std;

mutex m;

struct X
{
    void foo(int i, const string& str)
    {
        
        lock_guard<mutex> lk(m);
        cout << "foo cur thread id :" << this_thread::get_id() << endl;
        cout << str << ' ' << i << '\n';
    }

    void bar(const string& str)
    {
        lock_guard<mutex> lk(m);
        cout << "bar cur thread id :" << this_thread::get_id() << endl;
        cout << str << '\n';
    }

    int operator()(int i)
    {
        lock_guard<mutex> lk(m);
        cout << "operator cur thread id :" << this_thread::get_id() << endl;
        cout << i << '\n';
        return i + 10;
    }
};


int main()
{
    X x;

    cout << "main thread id :" << this_thread::get_id() << endl;
    
    auto a1 = async(&X::foo, &x, 42, "Hello");
    
    auto a2 = async(launch::deferred, &X::bar, x, "world!");
    
    auto a3 = async(launch::async, X(), 43);
    a2.wait();                    
    cout << a3.get() << '\n'; 
} 

输出为

可以看到a1,a3和主线程不同
a2与主线程id一致