How to perform an action when a key is released after being pressed(С++)?

Question

I am making a program which should press another key when a key is released after being pressed. Such as, if you release an a key then the program should press d, if release d then press a. The program should emulate a user keystroke, and so that it does this even when another program is on top.

How can I do this?

Here is my code so far:

Header.h

#include <iostream>
#include <windows.h>
#include <conio.h>
//#include<stdlib.h>

bool isOn = false;
//bool a = false;
//bool d = false;

Source.cpp

#include "Header.h"

void OnOff();

void Action();

int main() 
{
    printf("Press: Home to start\n");
    while (true)
    {
        OnOff();
        if (isOn)
            Action();
        //std::cout << a << "," << d << std::endl;
    }
}

void Action() 
{
    // code should be here
}

void OnOff()
{
    if (_kbhit()) {
        switch (_getch())
        {
        case 71:
            if (!isOn)
            {
                printf("ON\n");
                isOn = true;
                break;
            }
            else if (isOn)
            {
                printf("OFF\n");
                isOn = false;
                break;
            }

        }
    }
}

Answer 1

NOTE: This answer was written before the following sentence was added to the question:

The program should emulate a user keystroke, and so that it does this even when another program is on top.

Therefore, this answer now only partially answers the question.

The function _getch will only inform you when the user presses a key, but not when the user releases a key. If you want to be informed of both "key down" and "key up" events, then I suggest that you use the function ReadConsoleInput instead.

Here is an example:

#define WIN32_LEAN_AND_MEAN

#include <Windows.h>
#include <iostream>
#include <cctype>

void HandleKeyEvent( const KEY_EVENT_RECORD &key_event );
void HandleMouseEvent( const MOUSE_EVENT_RECORD &mouse_event );

int main( void )
{
    HANDLE hStdin;
    INPUT_RECORD input[128];
    DWORD num_input;

    hStdin = GetStdHandle( STD_INPUT_HANDLE );

    while ( ReadConsoleInput( hStdin, input, sizeof input / sizeof *input, &num_input ) )
    {
        for ( DWORD i = 0; i < num_input; i++ )
        {
            if ( input[i].EventType == KEY_EVENT )
            {
                HandleKeyEvent( input[i].Event.KeyEvent );
            }
        }
    }

    std::cerr << "Error calling ReadConsoleInput!\n";
}

void HandleKeyEvent( const KEY_EVENT_RECORD &key_event )
{
    std::cout << "Key ";
    if ( std::isprint( static_cast<unsigned char>(key_event.uChar.AsciiChar) ) )
        std::cout << (char)std::toupper( static_cast<unsigned char>(key_event.uChar.AsciiChar) );
    else
        std::cout << "<unprintable>";
    if ( key_event.bKeyDown)
    {
        std::cout << " down!\n";
    }
    else
    {
        std::cout << " up!\n";
    }
}

This program has the following output when I press and release the W, A, S, D keys.

Key W down!
Key W up!
Key A down!
Key A up!
Key S down!
Key S up!
Key D down!
Key D up!