Detecting when a key is released

Question

I want to check when a key is released, but I can't do so without having an infinite cycle, and this puts the rest of the code on pause. How can I detect if a key is released while running the rest of my program without an infinite cycle? This is the code I found and that I have been using:

#include "stdafx.h"
#include <windows.h>
#include <iostream>
#include <fstream>
using namespace std;

int main()
{
    int counter=0;
    ofstream myfile;
    short prev_escape = 0, curr_escape = 0;
    myfile.open("c:\\example.txt");
    while(true)
    {
        if(GetAsyncKeyState(VK_ESCAPE))
            curr_escape = 1;
        else
            curr_escape = 0;
        if(prev_escape != curr_escape)
        {
            counter++;
            if(curr_escape)
            {
                myfile <<"Escape pressed : " << counter << endl;
                cout<<"Escape pressed !" << endl;
            }
            else
            {
                myfile <<"Escape released : " << counter << endl;
                cout<<"Escape released !" << endl;
            }
            prev_escape = curr_escape;
        }        
    }
    myfile.close();
    return 0;
}

"while running the rest of my program"? First of all, decide when specifically you want to check for the keypress. — Kos, Sep 22 '14 at 12:32
possible duplicate of [Keyboard Input & the Win32 message loop](http://stackoverflow.com/questions/2441457/keyboard-input-the-win32-message-loop) — ivan_pozdeev, Sep 22 '14 at 12:33
JustJeff, Thanks, I will investigate. Kos, I need it to be checked at all times. — Puértolas Luis, Sep 22 '14 at 12:45
You are supposed to have a window which gets messages like [WM_KEYUP](http://msdn.microsoft.com/en-us/library/windows/desktop/ms646281(v=vs.85).aspx). — nwp, Sep 22 '14 at 12:53
Do you need a global detection (no matter what the active app is), or should it work only when your application is the active one? — Paul, Sep 22 '14 at 13:26
Paul, it is an Opengl game, where I need to run the whole game, while constantly checking if a key is pressed or released — Puértolas Luis, Sep 22 '14 at 16:25
You normally test the keyboard from inside your main game loop — David Heffernan, Sep 22 '14 at 16:51
Yes thank you, I solved the problem, I just don't know how to close this post as solved, i'm new. — Puértolas Luis, Sep 22 '14 at 17:28

Ytivarg · Answer 1 · 2019-02-14T01:51:16.293

if (GetAsyncKeyState(VK_SHIFT) < 0 && shift == false)
    {
        shift = true;
    }
    if (GetAsyncKeyState(VK_SHIFT) == 0 && shift == true)
    {
        shift = false;
    }

This is a more refined version that I used in my CLI game which I adopted from Davids answer just now (thank you David, I was racking my brain trying to figure this out myself). The top executes when shift is pressed, the bottom is executed when shift is released.

EDIT: The bool "shift" has to be initialized to false before hand for this to work.

score 3 · Accepted Answer · edited May 23 '17 at 12:12

3

First of all, the way you test the return value of GetAsyncKeyState() is incorrect. Test for the return value being negative to detect whether or not the key is down. So your if should read:

if (GetAsyncKeyState(VK_ESCAPE) < 0)

If you wish for that code to execute without blocking your main thread, then you'd need to put the busy loop into a separate thread. That's probably still a poor idea because you are running a busy loop.

In a GUI process you would have a window message loop that would be able to receive WM_KEYDOWN messages. But you have a console application. In that case you might be best using PeekConsoleInput which you would call periodically to check whether or not there is an escape key press waiting in the input buffer. An example of how to do that can be found here: Intercept ESC without removing other key presses from buffer.

edited May 23 '17 at 12:12

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answered Sep 22 '14 at 14:05

David Heffernan

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Thanks, I don't know how to explain it, but that solved the problem! – Puértolas Luis Sep 22 '14 at 16:50
it didn't worked for me, it is also works if i will keep holding this key. – Haseeb Mir Mar 22 '18 at 17:04
@HaSeeBMiR What doesn't work? I don't think you read the answer carefully enough. – David Heffernan Mar 22 '18 at 17:38
@DavidHeffernan you have to check if keystate is zero like this.`if (GetAsyncKeyState(VK_ESCAPE) == 0)` – Haseeb Mir Mar 23 '18 at 22:27
@HaSee What on earth are you talking about? – David Heffernan Mar 23 '18 at 22:33
if key is released it state is 0. – Haseeb Mir Mar 23 '18 at 22:46
@HaSee That's not what I'm talking about in this answer. It's also wrong. Key is down if and only if GetAsyncKeyState returns negative value. – David Heffernan Mar 23 '18 at 22:58
i printed the state of key in infinite loop without pressing any key it printed out value 0. and when pressed it printed out **32768**. – Haseeb Mir Mar 24 '18 at 01:44
@HaSee GetAsyncKeyState returns a signed 16 bit value and 32768 is outside the range of possible signed 16 bit values. Clearly you are doing something wrong. I can't diagnose it with no code, in comments to a 4 year old question. If you need help, ask a question. – David Heffernan Mar 24 '18 at 06:50
I was printing with %hu format because some bug with %h format , actually it was in range . I solved it anyways. – Haseeb Mir Mar 25 '18 at 08:45

Detecting when a key is released

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