这是三元条件：正确的（目标）C的语法？ [英] Is this ternary conditional ?: correct (Objective) C syntax?

问题描述

我不认为这是可能的，但显然在Objective C中，它是允许的：

I didn't think this was possible but apparently in Objective C it is allowed:

int a = b ?: c;

所以你看他们在这里做什么，他们离开了三元前pression的第二部分，这样，如果b为非零，B作为第二部分。

So you see what they're doing here, they're leaving out the second part of the ternary expression, such that if b is nonzero, b is used as the second part.

这是聪明的，但据我所知，这是对K＆安培; R C，大概ANSI C

It's clever but as far as I know this is against K&R C, and probably ANSI C.

如果没有，我已经错过了一个非常聪明的语法招了多年......唉！

If not, I've been missing out of a terribly clever syntax trick for years...alas!

更新：
这是gcc。

Update: It is gcc.

推荐答案

http://en.wikipedia.org /维基/％3F％3A

一个GNU扩展C允许省略第二个操作数，并使用隐式的第一个操作数作为第二个也：

A GNU extension to C allows omitting the second operand, and using implicitly the first operand as the second also:

a = x ? : y;

  
  

这位前pression相当于

The expression is equivalent to

a = x ? x : y;

  
  

除非如果x是一个前pression，它仅计算一次。所不同的是显著如果评估前pression有副作用。

except that if x is an expression, it is evaluated only once. The difference is significant if evaluating the expression has side effects.

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