c++多线程（十二） - std::packaged_task

你的名字 2022-04-23 14:48 82阅读 0赞

packaged\_task 是类模板，它的模板参数是各种可调用对象。packaged\_task把可调用对象包装起来，作为线程入口函数。

#include<iostream>
    #include<future>
    using namespace std;
    int myThread(int num)
    {
    	cout << "Thread start id = " << this_thread::get_id() << endl;
    	chrono::milliseconds time(5000); //休息5秒
    	this_thread::sleep_for(time);
    	cout << "Thread end id = " << this_thread::get_id() << endl;
    	return num * 2;
    }
    
    int main()
    {
    	cout << "Main start id = " << this_thread::get_id() << endl;
    	packaged_task<int(int)> package(myThread);  //把函数myThread通过packaged_task包装起来。
    	thread thread(ref(package),5);             
    	thread.join();                              //等待线程执行完毕
    	future<int> result = package.get_future();  //future对象里保存线程的执行结果
    	cout << "result = " << result.get() << endl;
    	return 0;
    }

执行结果：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpeWF6aGVuMjAxMQ_size_16_color_FFFFFF_t_70][]

packaged\_task可是使用lambda表达式作为参数，而且packaged\_task对象，也是一个可调用对象。

#include<iostream>
    #include<future>
    using namespace std;
    int main()
    {
    	cout << "Main start id = " << this_thread::get_id() << endl;
    	packaged_task<int(int)> package([](int num) {
    		cout << "Thread start id = " << this_thread::get_id() << endl;
    		chrono::milliseconds time(5000); //休息5秒
    		this_thread::sleep_for(time);
    		cout << "Thread end id = " << this_thread::get_id() << endl;
    		return num * 2;
    	});
    	  
    	package(5);                                 //直接调用，并未创建新线程
    	future<int> result = package.get_future();  //future对象里保存线程的执行结果
    	cout << "result = " << result.get() << endl;
    	return 0;
    }

执行结果：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpeWF6aGVuMjAxMQ_size_16_color_FFFFFF_t_70 1][]

注意，直接调用packaged\_task对象时，并未创建新的线程，从运行结果可以看出，线程id始终为22296。

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpeWF6aGVuMjAxMQ_size_16_color_FFFFFF_t_70]: /images/20220307/ef2ffa2a36df4afc855b480562fc81c7.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpeWF6aGVuMjAxMQ_size_16_color_FFFFFF_t_70 1]: /images/20220307/76996f9b9cd5411284f0d32876c59c61.png