c++ - 我如何在 C++ 中使用异步函数？

Question

我是一名初学者 C++ 程序员。据我所知，语句将逐行执行，因此如果我在 main 中调用一个函数，被调用函数的主体将首先执行，然后再执行其余代码...

如下代码所示:

int main()
{
 ABC();
 // ...
}

void ABC()
{
// ...
}

所以我当前程序的执行是同步，但我希望它是异步。

抱歉标题不好!我找不到更好的问题标题!如果你可以请编辑它。

感谢您对像我这样的初学者程序员的帮助:D

Answer 1

提供示例而不是让初学者阅读长篇文章。所提供的文章或链接可能会随时下线，从而导致人们再次提出相同的问题。

Async 通常不适合初学者，但既然你想学习它，这里有一个如何做的例子。您可以只复制代码并粘贴然后运行它。研究代码。他们的评论非常好，您可以理解。 看看哪一个最适合您。

方法一: 需要 C++ 11

应适用于 Windows、Mac 和 Linux*

 #include <iostream>
    #include <future> //Import the asynchronous Library

    using namespace std;

    //The function you want to call asynchronously
    void ABC()
    {
    cout<<"Hello From ABC Function"<<endl;
    }

    int main()
    {
        /*Setup **ABC function**, We will call it **MyABCFunction**, but you can call it anything else.
       Inside MyABCFunction, we call **sync function** and pass it the name of the **function** we want
        to call which is "ABC()". You don't include the "()", just the name "ABC".*/

      future<void> MyABCFunction(async(ABC));

      //Call ABC function
      MyABCFunction.get();


      //Show message from the Main Function
      cout << "Hello From Main Function." <<endl;
      return 0;
    }

由于您是 C++ 的新手，我会提到您不应该使用“using namespace std”，因为它会导致程序变得更大和其他命名冲突。

让我们在下面修复它:

    #include <iostream>
    #include <future> //Import the asynchronous Library

    /*No "using namespace std". Instead, each c++ library function must be begin with "std::"
    which includes Standard library for the function needed
    */

    //The function you want to call asynchronously
    void ABC()
    {
        //std:: before cout and endl
       std::cout<<"Hello From ABC Function"<<std::endl;
    }

    int main()
    {
      //std:: before future and async
      std::future<void> MyABCFunction(std::async(ABC));

      //Call ABC function
      MyABCFunction.get();

      //std:: before cout and endl
      std::cout << "Hello From Main Function." <<std::endl;
      return 0;
    }

方法二:

不需要 C++ 11

仅适用于 Windows(最简单和最短的方法)

#include <iostream> //For text on screen
#include <windows.h>  //Must include the use HANDLE class
#include <process.h>   // needed for _beginthread()

//Function prototype of ABCD function to be called in a thread.
void ABCD(void *param);

int main()
{


    int val = 0;
    HANDLE handle; //Create a handle (Only on Windows OS)

    /*Initialize the handle and begin thread. */
    /*_beginthread takes the name of the function to be called "ABCD" */
    /*_beginthread takes the stack size of the thread 0*/
    /*_beginthread takes the parameter to be passed to the "ABCD" which is 0(val) since void ABCD(void *param) takes void which means no parameter*/

    handle = (HANDLE) _beginthread( ABCD,0,&val);

 /*Do infinite loop on the main function to prove that main and ABCD function are running at the same time*/
    while(1)
    {
        std::cout<<"thread from main function"<<std::endl;
    }

    //Wait for ACBD to finish before exiting the program
    WaitForSingleObject(handle,INFINITE);
    return 0;
}

//ABCD function to be called from the thread
void ABCD(void *param)
{
    /*Do infinite loop on the main function to prove that ABCD and main function are running at the same time*/
    while(1)
    {
        std::cout<<"thread from ABCD function"<<std::endl;
    }
    _endthread(); // End thread. Won't be called since we are looping forever in while(1) loop

}

有关_beginthread的更多信息

方法三:

不需要 C++ 11

POSIX 标准 适用于 Windows、Mac 和 Linux

我们将在 ABCD 函数中从 1 到 10,000 计数，同时在 main 函数中从 1 计数到 5,000。

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <iostream>

void *ABCD(void *arg)
{

    //count from 1 to 10000 from the ABCD function thread
    for(int i=1; i<=10000;i++)
    {
        std::cout<<"thread from ABCD function "<<i<<std::endl;
    }
}

int main()
{

    pthread_t myPthread;

    //Create and call the ABCD function to start counting
    pthread_create(&myPthread, NULL, ABCD, NULL);

    //count from 1 to 5,000 from the main function thread
    for(int i=1; i<=5000;i++){
        std::cout<<"thread from main function"<<std::endl;
    }
}

这里的问题是 main 函数会先完成计数，因为它计数到 5,000，而 main 函数计数到 10,000。当 main 函数先完成时，即使 ABCD 函数没有完成计数，它也会终止整个程序。 为了解决这个问题，我们使用pthread_join函数来等待ABCD函数完成，然后我们的程序才能终止。

完整代码如下:

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <iostream>

void *ABCD(void *arg)
{

    //count from 1 to 10000 on from ABCD function thread
    for(int i=1; i<=10000; i++)
    {
        std::cout<<"thread from ABCD function "<<i<<std::endl;
    }
}

int main()
{

    pthread_t myPthread;

    //Create and call the ABCD function to start counting
    pthread_create(&myPthread, NULL, ABCD, NULL);

    //count from 1 to 5,000 on from the main function thread
    for(int i=1; i<=5000; i++)
    {
        std::cout<<"thread from main function"<<std::endl;
    }

    //Wait for ABCD function to finish before we exit
    int a =0;
    pthread_join(myPthread, (void **)&a);
}

我希望这可以帮助所有 c++ 线程初学者。我建议你靠一下semaphore一旦您了解了我提供的方法 3 的基础知识。 有关 pthread_t 的更多信息