How do I check if a Key is pressed on C++

check if a key is pressed, if yes, then do stuff

Consider 'select()', if this (reportedly Posix) function is available on your os.

'select()' uses 3 sets of bits, which you create using functions provided (see man select, FD_SET, etc). You probably only need create the input bits (for now)

from man page:

'select()' "allow a program to monitor multiple file descriptors, waiting until one or more of the file descriptors become "ready" for some class of I/O operation (e.g., input possible). A file descriptor is considered ready if it is possible to perform a corresponding I/O operation (e.g., read(2) without blocking...)"

When select is invoked:

a) the function looks at each fd identified in the sets, and if that fd state indicates you can do something (perhaps read, perhaps write), select will return and let you go do that ... 'all you got to do' is scan the bits, find the set bit, and take action on the fd associated with that bit.

The 1st set (passed into select) contains active input fd's (typically devices). Probably 1 bit in this set is all you will need. And with only 1 fd (i.e. an input from keyboard), 1 bit, this is all quite simple. With this return from select, you can 'do-stuff' (perhaps, after you have fetched the char).

b) the function also has a timeout, with which you identify how much time to await a change of the fd state. If the fd state does not change, the timeout will cause 'select()' to return with a 0. (i.e. no keyboard input) Your code can do something at this time, too, perhaps an output.

fyi - fd's are typically 0,1,2... Remembe that C uses 0 as STDIN, 1 and STDOUT.

Simple test set up: I open a terminal (separate from my console), and type the tty command in that terminal to find its id. The response is typically something like "/dev/pts/0", or 3, or 17...

Then I get an fd to use in 'select()' by using open:

// flag options are: O_RDONLY, O_WRONLY, or O_RDWR
int inFD = open( "/dev/pts/5", O_RDONLY );

It is useful to cout this value.

Here is a snippet to consider (from man select):

  fd_set rfds;
  struct timeval tv;
  int retval;

  /* Watch stdin (fd 0) to see when it has input. */
  FD_ZERO(&rfds);
  FD_SET(0, &rfds);

  /* Wait up to five seconds. */
  tv.tv_sec = 5;
  tv.tv_usec = 0;

  retval = select(1, &rfds, NULL, NULL, &tv);
  /* Don't rely on the value of tv now! */

  if (retval == -1)
      perror("select()");
  else if (retval)
      printf("Data is available now.\n");  // i.e. doStuff()
      /* FD_ISSET(0, &rfds) will be true. */
  else
      printf("No data within five seconds.\n"); // i.e. key not pressed

As mentioned by others there's no cross platform way to do this, but on Windows you can do it like this:

The Code below checks if the key 'A' is down.

if(GetKeyState('A') & 0x8000/*Check if high-order bit is set (1 << 15)*/)
{
    // Do stuff
}

In case of shift or similar you will need to pass one of these: https://msdn.microsoft.com/de-de/library/windows/desktop/dd375731(v=vs.85).aspx

if(GetKeyState(VK_SHIFT) & 0x8000)
{
    // Shift down
}

The low-order bit indicates if key is toggled.

SHORT keyState = GetKeyState(VK_CAPITAL/*(caps lock)*/);
bool isToggled = keyState & 1;
bool isDown = keyState & 0x8000;

Oh and also don't forget to

#include <Windows.h>

Disclaimer: This answer was provided before the question was edited to limit its scope to a specific OS

There is no portable function that allows to check if a key is hit and continue if not. This is always system dependent.

Solution for linux and other posix compliant systems:

Here, for Morgan Mattews's code provide kbhit() functionality in a way compatible with any POSIX compliant system. He uses the trick of desactivating buffering at termios level.

Solution for windows:

For windows, Microsoft offers _kbhit()

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