C++ cin keypress event

Question

I believe this is a very simple question, but I can't find a simple answer to it. I have an infinite loop, e.g. while(1), for(;;), and I need to break from the loop on a keypress. What is the easiest way to do this?

P.S.: I can't use getch, cin.ignore, or cin.get because it stops the loop.

Answer 1

Well, what you want is asynchronous input. All of the methods provided by cin wait for enter. You will have to use system-specific functions for that, or use a library that will do it for you.

What you need to do is to not only process your logic in a while loop, but also listen from message pipe from your OS. If you want some more information about that one, please drop a comment.

EDIT: There's one other way, but I wouldn't recommend it as it can be non-portable I believe. The following code compiles and runs under VS2012RC.

#include<iostream>
#include<conio.h>

using namespace std;

int main()
{
   cout << "Enter a character";
   getch();
}

Answer 2

There is no "keyboard" in C++. You only have an opaque input data stream which your terminal popu­lates occasionally with its own keyboard input. This is almost always a buffered, editable line-wise input, so you have no way of knowing when any given key was pressed.

You need a platform-specific method to communicate directly with your terminal on a lower level. One such library, fairly wide-spread and portable, is ncurses (a compatible Windows variant exists). Portable graphical application frameworks such as SDL and Allegro also provide raw keyboard handling.

Answer 3

Below is a Windows console code that uses kbhit() and has an infinite loop. But if keyboard is hit, it breaks the loop and exits. If you have <conio.h> , try this :

#include <iostream> 
#include <conio.h>

using namespace std;


int main()
{


   while (1)
   { 
     if (kbhit()) break;

   }

  return 0;
}

Answer 4

This checks if "left arrow" is being pressed or not:

GetKeyState(VK_LEFT)

Also this does not wait for anything. Just checks some flags.

Some other keys defined in winuser.h:

#define VK_NUMPAD0        0x60
#define VK_NUMPAD1        0x61
#define VK_NUMPAD2        0x62
#define VK_NUMPAD3        0x63
#define VK_NUMPAD4        0x64
#define VK_NUMPAD5        0x65
#define VK_NUMPAD6        0x66
#define VK_NUMPAD7        0x67
#define VK_NUMPAD8        0x68
#define VK_NUMPAD9        0x69

#define VK_CLEAR          0x0C
#define VK_RETURN         0x0D

#define VK_SHIFT          0x10
#define VK_CONTROL        0x11
#define VK_MENU           0x12
#define VK_PAUSE          0x13
#define VK_CAPITAL        0x14

#define VK_KANA           0x15
#define VK_HANGEUL        0x15  /* old name - should be here for compatibility */
#define VK_HANGUL         0x15
#define VK_JUNJA          0x17
#define VK_FINAL          0x18
#define VK_HANJA          0x19
#define VK_KANJI          0x19

#define VK_ESCAPE         0x1B

#define VK_CONVERT        0x1C
#define VK_NONCONVERT     0x1D
#define VK_ACCEPT         0x1E
#define VK_MODECHANGE     0x1F

#define VK_SPACE          0x20
#define VK_PRIOR          0x21
#define VK_NEXT           0x22
#define VK_END            0x23
#define VK_HOME           0x24
#define VK_LEFT           0x25
#define VK_UP             0x26
#define VK_RIGHT          0x27
#define VK_DOWN           0x28
#define VK_SELECT         0x29
#define VK_PRINT          0x2A
#define VK_EXECUTE        0x2B
#define VK_SNAPSHOT       0x2C
#define VK_INSERT         0x2D
#define VK_DELETE         0x2E
#define VK_HELP           0x2F

winuser.h must be included in windows.h