What does this ? operator in C++ do?

Question

Consider this function

template<class T> inline bool cx(T &a, T b) {return a < b ? a = b, 1 : 0;}

Can anyone explain what exactly this is doing? It seems different from the typical condition ? true : false format.

Then I am not understanding which piece is what, because what I see is `condition ? condition, true: false` — AJJ, Jul 24 '15 at 16:50
Then you should ask what `a = b, 1 ` means. Or how the full thing is parsed, taking into account operator precedence. But it is a standard conditional expression. The code does seem to be written to confuse though. — juanchopanza, Jul 24 '15 at 16:51
This question cries out for two dupes: What is the ternary ?: operator, and What is the comma operator. — πάντα ῥεῖ, Jul 24 '15 at 16:54
@NathanOliver Nope, OP understands the "typical" `?` usage, but is confused by the `a = b, 1` part. — Paul Roub, Jul 24 '15 at 16:55
@PaulRoub Then shouldn't this what is the `,` for? http://stackoverflow.com/questions/54142/how-does-the-comma-operator-work — NathanOliver, Jul 24 '15 at 16:56

score 9 · Accepted Answer · edited May 23 '17 at 12:24

9

We could make it more clear like so:

return a < b ? (a = b, 1) : 0;

The parenthesized bit means "assign b to a, then use 1 as our value".

Comma-separated lists of values in C and C++ generally mean "evaluate all of these, but use the last one as the expression's value".

edited May 23 '17 at 12:24

Community

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answered Jul 24 '15 at 16:52

Paul Roub

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I wasn't aware you could combine statements with commas like this in C++ (I always assumed things needed to be separated by semicolons). – AJJ Jul 24 '15 at 16:57
@ArukaJ Those are mainly expressions. The statement is `return ;`. – juanchopanza Jul 24 '15 at 16:59
@ArukaJ You can't combine _statements_ with commas, only _expressions_. – celticminstrel Jul 24 '15 at 17:35

score 4 · Answer 2 · answered Jul 24 '15 at 16:53

This combination is a little tricky, because it combines a comma operator with the conditional expression. It parses as follows:

a < b is the condition,
a = b, 1 is the "when true" expression
0 is the "when false" expression

The result of the comma operator is its last component, i.e. 1. The goal of employing the comma operator in the first place is to cause the side effect of assigning b to a.

QuestionC · Answer 3 · 2015-07-24T16:57:17.713

3

The , operator just evaluates all the expressions, left to right, and evaluates to the value of the rightmost expression.

Your code is the same as...

if (a < b)
{
  a = b;
  return 1;
}
else
{
  return 0;
}

edited Jul 24 '15 at 16:57

answered Jul 24 '15 at 16:53

QuestionC

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... and won't compile. – Paul Roub Jul 24 '15 at 16:56
1

@juanchopanza This code is equivalent to `return a < b ? a = b, 1 : 0;` – QuestionC Jul 24 '15 at 16:58
@QuestionC Fair enough. – juanchopanza Jul 24 '15 at 16:59

score 3 · Answer 4 · answered Jul 24 '15 at 16:55

You can execute several expression using ,

In this case if a < b, then assign b to a and return 1. According C++ grammar:

conditional-expression:  
    logical-or-expression
|   logical-or-expression ? expression : assignment-expression

where

expression:  
    assignment-expression
|   expression , assignment-expression

assignment-expression:   
    conditional-expression
|   logical-or-expression assignment-operator initializer-clause
|   throw-expression

score 1 · Answer 5 · answered Jul 24 '15 at 16:53

1

Read it as:

if ( a < b )
{
    a = b;
    return ( 1 );
}
else
{
    return ( 0 );
}

answered Jul 24 '15 at 16:53

unxnut

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score 0 · Answer 6 · answered Jul 24 '15 at 16:53

a < b ? a = b, 1 : 0 is parsed as (a < b) ? (a = b, 1) : 0, a normal conditional operator. When a < b is true, a = b, 1 is evaluated by assigning b to a and then "returning" 1. The net effect of cx(a,b) is thus to assign the larger value to a and to return 1 if a changed, 0 otherwise.

What does this ? operator in C++ do?

6 Answers6