41

通常是“？” 运算符以下列形式使用：

A ? B : C

但是，在 B = AI 的情况下，会看到以下缩写

A ? : C

这出人意料地有效。将第二个参数留在（风格方面）更好，还是某些编译器无法处理这个问题？

4

6 回答 6

39

语言 C 不允许这样做（据我所知），但 gcc 等编译器具有快捷方式 a?:c 作为扩展名。 a?:c意思相同a?a:c。

于 2012-04-13T14:55:59.820 回答
17

它是 gcc 的扩展

带有省略操作数的条件

x ? : y相当于x ? x : y

于 2012-04-13T14:58:23.970 回答
3

除非我大错特错，否则您使用的是编译器扩展（猜测是 gcc）。我很确定标准不允许您省略三元运算符的第二个操作数。

于 2012-04-13T14:56:28.090 回答
3

我稍微填一下。

该标准使用术语条件运算符。

语法
  条件表达式：
          逻辑或表达式
          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.

于 2012-04-13T21:08:38.547 回答
1

我在网上做了一些研究，根据维基百科，这种行为得到了 C 的 GNU 扩展的支持。http://en.wikipedia.org/wiki/%3F:#C

所以其他编译器很可能认为这是非法的。顺便说一句，这个运算符被称为三元条件，因此您可以浏览它。

编辑：

我检查了 gcc 和 apple llvm，它工作正常。

于 2012-04-13T15:11:16.613 回答
0

最好保留第二个参数。如果 B 发生变化，您可能不记得修改上面的语句。此外，如果您将 B 排除在语句之外，其他人可能难以阅读您的代码并对其进行改进。

于 2012-04-13T15:04:49.817 回答