Capture characters from standard input without waiting for enter to be pressed

Question

That's not possible in a portable manner in pure C++, because it depends too much on the terminal used that may be connected with stdin (they are usually line buffered). You can, however use a library for that:

1. conio available with Windows compilers. Use the _getch() function to give you a character without waiting for the Enter key. I'm not a frequent Windows developer, but I've seen my classmates just include <conio.h> and use it. See conio.h at Wikipedia. It lists getch(), which is declared deprecated in Visual C++.

2. curses available for Linux. Compatible curses implementations are available for Windows too. It has also a getch() function. (try man getch to view its manpage). See Curses at Wikipedia.

I would recommend you to use curses if you aim for cross platform compatibility. That said, I'm sure there are functions that you can use to switch off line buffering (I believe that's called "raw mode", as opposed to "cooked mode" - look into man stty). Curses would handle that for you in a portable manner, if I'm not mistaken.

On Linux (and other unix-like systems) this can be done in following way:

#include <unistd.h>
#include <termios.h>

char getch() {
        char buf = 0;
        struct termios old = {0};
        if (tcgetattr(0, &old) < 0)
                perror("tcsetattr()");
        old.c_lflag &= ~ICANON;
        old.c_lflag &= ~ECHO;
        old.c_cc[VMIN] = 1;
        old.c_cc[VTIME] = 0;
        if (tcsetattr(0, TCSANOW, &old) < 0)
                perror("tcsetattr ICANON");
        if (read(0, &buf, 1) < 0)
                perror ("read()");
        old.c_lflag |= ICANON;
        old.c_lflag |= ECHO;
        if (tcsetattr(0, TCSADRAIN, &old) < 0)
                perror ("tcsetattr ~ICANON");
        return (buf);
}

Basically you have to turn off canonical mode (and echo mode to suppress echoing).

I found this on another forum while looking to solve the same problem. I've modified it a bit from what I found. It works great. I'm running OS X, so if you're running Microsoft, you'll need to find the correct system() command to switch to raw and cooked modes.

#include <iostream> 
#include <stdio.h>  
using namespace std;  

int main() { 
  // Output prompt 
  cout << "Press any key to continue..." << endl; 

  // Set terminal to raw mode 
  system("stty raw"); 

  // Wait for single character 
  char input = getchar(); 

  // Echo input:
  cout << "--" << input << "--";

  // Reset terminal to normal "cooked" mode 
  system("stty cooked"); 

  // And we're out of here 
  return 0; 
}

CONIO.H

the functions you need are:

int getch();
Prototype
    int _getch(void); 
Description
    _getch obtains a character  from stdin. Input is unbuffered, and this
    routine  will  return as  soon as  a character is  available  without 
    waiting for a carriage return. The character is not echoed to stdout.
    _getch bypasses the normal buffering done by getchar and getc. ungetc 
    cannot be used with _getch. 
Synonym
    Function: getch 


int kbhit();
Description
    Checks if a keyboard key has been pressed but not yet read. 
Return Value
    Returns a non-zero value if a key was pressed. Otherwise, returns 0.

libconio http://sourceforge.net/projects/libconio

or

Linux c++ implementation of conio.h http://sourceforge.net/projects/linux-conioh

If you are on windows, you can use PeekConsoleInput to detect if there's any input,

HANDLE handle = GetStdHandle(STD_INPUT_HANDLE);
DWORD events;
INPUT_RECORD buffer;
PeekConsoleInput( handle, &buffer, 1, &events );

then use ReadConsoleInput to "consume" the input character ..

PeekConsoleInput(handle, &buffer, 1, &events);
if(events > 0)
{
    ReadConsoleInput(handle, &buffer, 1, &events);  
    return buffer.Event.KeyEvent.wVirtualKeyCode;
}
else return 0

to be honest this is from some old code I have, so you have to fiddle a bit with it.

The cool thing though is that it reads input without prompting for anything, so the characters are not displayed at all.