[c++] What does '?' do in C++?

int qempty()
{
    return (f == r ? 1 : 0);
}

In the above snippet, what does "?" mean? What can we replace it with?

It's the conditional operator.

a ? b : c

It's a shortcut for IF/THEN/ELSE.

means: if a is true, return b, else return c. In this case, if f==r, return 1, else return 0.

The question mark is the conditional operator. The code means that if f==r then 1 is returned, otherwise, return 0. The code could be rewritten as

int qempty()
{
  if(f==r)
    return 1;
  else
    return 0;
}

which is probably not the cleanest way to do it, but hopefully helps your understanding.

This is a ternary operator, it's basically an inline if statement

x ? y : z

works like

if(x) y else z

except, instead of statements you have expressions; so you can use it in the middle of a more complex statement.

It's useful for writing succinct code, but can be overused to create hard to maintain code.

You can just rewrite it as:

int qempty(){ return(f==r);}

Which does the same thing as said in the other answers.

Just a note, if you ever see this:

a = x ? : y;

It's a GNU extension to the standard (see https://gcc.gnu.org/onlinedocs/gcc/Conditionals.html#Conditionals).

It is the same as

a = x ? x : y;

It is called the conditional operator.

You can replace it with:

int qempty(){ 
    if (f == r) return 1;
    else return 0;
}