Question

通常是'？'运算符以下列形式使用：

A ? B : C

然而，在B = A的情况下，我看到了以下缩写

A ? : C

这令人惊讶地奏效。是否更好地保留第二个参数（样式方式），或者某些编译器是否有机会无法处理它？</ p>

Answer 1

语言C（据我所知）不允许这样做，但是像gcc这样的编译器的快捷方式是？：c作为extension。 a?:c表示与a?a:c相同。

Answer 2

它是gcc的扩展

Conditionals with Omitted Operands

x ? : y相当于x ? x : y

Answer 3

除非我犯了错误，否则你正在使用编译器扩展（猜测，gcc）。我很确定标准不允许你省略三元运算符的第二个操作数。

Answer 4

我填写了一下。

标准使用术语 条件运算符 。

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

条件表达式不会产生左值。另外; Wikipedia; Conditional

_{注意：I.e。：C ++有：

逻辑OR表达？表达式：赋值 -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.}

足部食物：

_{^[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

所以：不明智的使用。

Answer 5

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

因此很可能其他编译器认为这是非法的。顺便说一下，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，它运行正常。

Answer 6

最好留下第二个参数。如果B有变化，你可能不记得修改上面的陈述。此外，如果您将B留在声明之外，其他人可能很难阅读您的代码并对其进行改进。

带有空第二个参数的C条件运算符（'？'）

6 个答案: