C ++：如何在几秒钟后执行一个函数？

Question

Firstly, I'm using VS2008 (doesn't support C++11). I can't upgrade and need to use native libraries only because it needs to be compiled on another persons' compiler which I don't have control over.

I would like to run the code automatically after 5 seconds without having to poll how many seconds have elapsed.

This is my incomplete code

#include <windows.h>
#include <iostream>
void runMeAfterFiveSeconds(){
     cout<<"I'm activated!"<<endl;
}
void main(){
     while(1){
          cout<<"hello there!"<<endl;
          Sleep(2000);
     }
}

Example output

hello there!
hello there! //after 2 seconds
hello there! //after 4 seconds
I'm activated! //after 5 seconds
hello there! //after 6 seconds
hello there! //after 8 seconds
hello there! //after 10 seconds
I'm activated! //after 10 seconds
...

Answer 1

This example shows how to do it using a very simple scheduling algorithm. No spawning of additional threads is required.

#include <stdio.h>
#include <windows.h>

int main(int argc, char ** argv)
{
   DWORD now                      = timeGetTime();
   DWORD nextPrintHelloThereTime  = now;
   DWORD nextPrintImActivatedTime = now+5000;

   while(1)
   {
      now = timeGetTime();
      DWORD nextEventTime = (nextPrintHelloThereTime < nextPrintImActivatedTime) ? nextPrintHelloThereTime : nextPrintImActivatedTime;

      DWORD millisecondsToSleep = nextEventTime-now;
      Sleep(millisecondsToSleep);

      now = timeGetTime();
      if (now >= nextPrintHelloThereTime)
      {
         printf("hello there!\n");
         nextPrintHelloThereTime += 2000;
      }
      if (now >= nextPrintImActivatedTime)
      {
         printf("I'm activated!\n");
         nextPrintImActivatedTime += 5000;
      }
   }
}

Answer 2

It really depends on what code you want to execute and how you want it to be executed. The very simple way of doing so would be creating a separate thread and Sleep() in it. So, since you cannot upgrade from Visual Studio 2008 (which, if I remember correctly, does not support C++11), you have to use either native Windows threads or some library implementation like Boost.Thread. To look up how to use Windows threads, see MSDN documentation on _beginthreadex() function . A short tutorial about Boost.Thread can bee seen here .

Quick examples of both, taken directly from the links I provided:

1) Windows threads:

// crt_begthrdex.cpp
// compile with: /MT
#include <windows.h>
#include <stdio.h>
#include <process.h>

unsigned Counter; 
unsigned __stdcall SecondThreadFunc( void* pArguments )
{
    printf( "In second thread...\n" );

    while ( Counter < 1000000 )
        Counter++;

    _endthreadex( 0 );
    return 0;
} 

int main()
{ 
    HANDLE hThread;
    unsigned threadID;

    printf( "Creating second thread...\n" );

    // Create the second thread.
    hThread = (HANDLE)_beginthreadex( NULL, 0, &SecondThreadFunc, NULL, 0, &threadID );

    // Wait until second thread terminates. If you comment out the line
    // below, Counter will not be correct because the thread has not
    // terminated, and Counter most likely has not been incremented to
    // 1000000 yet.
    WaitForSingleObject( hThread, INFINITE );
    printf( "Counter should be 1000000; it is-> %d\n", Counter );
    // Destroy the thread object.
    CloseHandle( hThread );
}

2) Boost.Thread:

struct callable
{
    void operator()();
};

boost::thread copies_are_safe()
{
    callable x;
    return boost::thread(x);
} // x is destroyed, but the newly-created thread has a copy, so this is OK

In the second example, you could as well have used a plain function pointer as boost::thread constructor argument. Moreover, you could use a pointer to function with multiple arguments - a luxury Windows API's threads do not provide.

Answer 3

You're probably just going to need to create a thread like so:

#include <windows.h>
#include <iostream>
#include <thread>
void runMeAfterFiveSeconds(){
     while(true){
          sleep(5000);
          cout<<"I'm activated!"<<endl;   
     }     
}
void main(){
     std::thread th(runMeAfterFiveSeconds);
     while(1){
          cout<<"hello there!"<<endl;
          Sleep(2000);
     }
}

Answer 4

You're going to have to either make a thread (Coding Orange's answer, probably the better way), or just write it all out.

void runMeAfterFiveSeconds(){
     cout << "I'm activated!" <<endl;        
}

void main(){
     while(1){
          cout << "hello there!" << endl;
          Sleep(2000);
          cout << "hello there!" << endl;
          Sleep(3000);
          runMeAfterFiveSeconds();
          Sleep(1000);
          cout << "hello there!" << endl;
          Sleep(2000);
          cout << "hello there!" << endl;
          Sleep(2000);
          cout << "hello there!" << endl;
          runMeAfterFiveSeconds();
     }
}