C conditional operator ('?') with empty second parameter

Question

Typically the '?' operator is used in the following form:

A ? B : C

However in cases where B = A I have seen the following abbreviation

A ? : C

This surprisingly works. Is it better to leave the second parameter in (style wise), or is their a chance certain compilers won't be able to handle this?

Looking like [Groovy](http://en.wikipedia.org/wiki/Groovy_%28programming_language%29)-like syntax. — Lion, Apr 13 '12 at 15:24

score 41 · Accepted Answer · edited Apr 13 '12 at 15:08

41

It is not permitted by the language C (as far as I know), but compilers such as gcc have the shortcut a?:c as an extension. a?:c means the same as a?a:c.

edited Apr 13 '12 at 15:08

undur_gongor

15,657
5
63
75

answered Apr 13 '12 at 14:55

Juri Robl

5,614
2
29
46

22

It means the same... with the caveat that `a` does not contain side-effects. `a?:c` only executes `a` once, whereas `a?a:c` would execute the side-effects of `a` twice. – mbauman Apr 13 '12 at 21:05
It works for Clang too. – 4LegsDrivenCat Dec 01 '21 at 20:05

score 17 · Answer 2 · answered Apr 13 '12 at 14:58

17

Its a gcc's extension

Conditionals with Omitted Operands

x ? : y is equivalent to x ? x : y

answered Apr 13 '12 at 14:58

Prasoon Saurav

91,295
49
239
345

2

The page you linked to contradicts itself. On the one hand it says "This example is perfectly equivalent to `x ? x : y`", which means `x` is evaluated twice, but on the other hand the last paragraph states that `x` would be evaluated only once, which would make it perfectly equivalent to `x || y`, not `x ? x : y` – Celada Apr 13 '12 at 15:03
1

@Celada : I think it means to say that `x ? : y` is **roughly** equivalent to `x ? x : y` except `x` gets evaluated only once in the former case. – Prasoon Saurav Apr 13 '12 at 15:05
3

@Celada : `x || y` evaluates to 0 or 1 which is not the case with this operator. – Prasoon Saurav Apr 13 '12 at 15:07
1

I stand corrected. I thought `||` returned value value of its first argument if the first argument was something that is considered true (nonzero). – Celada Apr 13 '12 at 15:12
It's one of ISO C99 features. It works fine for me on `GCC` 4.4.1 and `TCC`(that's an ANSI C compiler) version 0.9.25 on Windows plataform. – Jack Apr 13 '12 at 16:20
It works for Clang too, not only GCC. – 4LegsDrivenCat Dec 01 '21 at 20:05

score 3 · Answer 3 · answered Apr 13 '12 at 14:56

3

Unless I'm badly mistake, you're using a compiler extension (at a guess, gcc). I'm pretty sure the standard does not allow you to omit the second operand to the ternary operator.

answered Apr 13 '12 at 14:56

Jerry Coffin

476,176
80
629
1,111

Morpfh · Answer 4 · 2012-04-13T21:30:33.340

I fill in a bit.

The standard uses the term conditional operator.

_{Syntax
  conditional-expression:
          logical-OR-expression
          logical-OR-expression ? expression : conditional-expression}

A conditional expression does not yield an lvalue. Also; Wikipedia; Conditional

_{Note: I.e.: C++ has:

logical-OR-expression ? expression : assignment-expression}

_{Constraints:
* The first operand shall have scalar type^[1].
* One of the following shall hold for the second and third operands:
   — both operands have arithmetic type^[2];
   — both operands have the same structure[3] or union type^[4];
   — both operands have void type^[5];
   — both operands are pointers to qualified or unqualified[6] versions of compatible
     types[7];
   — one operand is a pointer and the other is a null pointer constant^[8]; or
   — one operand is a pointer to an object or incomplete type^[9] and the other 
     is a pointer to a qualified or unqualified version of void.}

Foot food:

_{^[1] Scalar type     : Arithmetic types and pointer types.
^[2] Arithmetic type : Integer and floating types.
^[3] Structure type  : A sequentially allocated nonempty set of member objects (and, in
                     certain circumstances, an incomplete array), each of which has an
                     optionally specified name and possibly distinct type.
^[4] Union type      : An overlapping nonempty set of member objects, each of which has
                     an optionally specified name and possibly distinct type.
^[5] Void type       : An empty set of values; it is an incomplete type that cannot be
                     completed.
^[6] Qualified type  : 1998 (const and volatile), 1999 (restrict), respectively 
                     2011 (_Atomic). ^*
^[7] Compatible type : Their types are the same.
^[8] Null ptr. const.: NULL; implementation-defined null pointer constant.
^[9] Incomplete type : Types that describe objects but lack information needed to determine 
                      their sizes.}

^* Type qualifiers in C

So: Not wise to use.

LuisEspinoza · Answer 5 · 2012-04-13T19:26:01.500

1

i did a little research in the web, acording to wikipedia, this behavior is supported by a GNU extension of C. http://en.wikipedia.org/wiki/%3F:#C

So it is very probable that other compilers consider this illegal. By the way, this operator is called ternary conditional so you can browse about it.

EDIT:

I checked in gcc and apple llvm and it works fine.

edited Apr 13 '12 at 19:26

answered Apr 13 '12 at 15:11

LuisEspinoza

8,508
6
35
57

score 0 · Answer 6 · answered Apr 13 '12 at 15:04

0

It is better to leave the second parameter in. If B ever changes, you may not remember to modify the statement above. Further, other people may have a difficult time reading your code and improving upon it if you leave B out of the statement.

answered Apr 13 '12 at 15:04

Mike Foss

311
2
6

C conditional operator ('?') with empty second parameter

6 Answers6

Linked

Related