C conditional operator ('?') with empty se

问题:

This question already has an answer here:

• ?: ternary conditional operator behaviour when leaving one expression empty 2 answers

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?

回答1:

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.

回答2:

Its a gcc's extension

Conditionals with Omitted Operands

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

回答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.

回答4:

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.

回答5:

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.

回答6:

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.