So, in all of the repl consoles, you need to press enter to input something. However, there are ways around this. This one is for c++.

First, we're going to need a few things, specifically iostream, stdio, and stdlib

#include <iostream>
#include <stdio.h>
#include <stdlib.h>

Next, we're going to make a function that'll return whatever key is pressed. I'll call it keypress for readability.

#include <iostream>
#include <stdio.h>
#include <stdlib.h>

int keypress() {

}

Now here's the confusing part. We're going to change how the console works. I myself don't fully understand it, but the basic premise is that we change how we input at the beginning of the function, and reset it at the end.

#include <iostream>
#include <stdio.h>
#include <stdlib.h>

int keypress() {
  system ("/bin/stty raw");

  //code will go here

  system ("/bin/stty cooked");
}

Now, we'll create a variable (c) to store the ASCII value of the key pressed, and give it a value by taking input with getc from stdin, then return the value.

#include <iostream>
#include <stdio.h>
#include <stdlib.h>

int keypress() {
  system ("/bin/stty raw");
  int c;
  c = getc(stdin);
  system ("/bin/stty cooked");
  return c;
}

Finally, we'll make sure the key that is pressed isn't visible to the user by disabling and reenabling echo.

#include <iostream>
#include <stdio.h>
#include <stdlib.h>

int keypress() {
  system ("/bin/stty raw");
  int c;
  system ("/bin/stty -echo")
  c = getc(stdin);
  system ("/bin/stty echo")
  system ("/bin/stty cooked");
  return c;
}

And that's it!

Below is a repl that demonstrates this. Try it out by pressing a key, and the console will display the ASCII value of the key.