What does the question mark character ('?') mean in C++?

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

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

8 Answers

This is commonly referred to as the conditional operator, and when used like this:

condition ? result_if_true : result_if_false 

... if the condition evaluates to true, the expression evaluates to result_if_true, otherwise it evaluates to result_if_false.

It is syntactic sugar, and in this case, it can be replaced with

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

Note: Some people refer to ?: it as "the ternary operator", because it is the only ternary operator (i.e. operator that takes three arguments) in the language they are using.

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.

Just a note, if you ever see this:

a = x ? : y; 

It's a GNU extension to the standard (see ).

It is the same as

a = x ? x : y; 

You can just rewrite it as:

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

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

It is called the conditional operator.

You can replace it with:

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

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.

It read as:

If f == r then return 1 else return 0