objective-c - 是缩短的问号冒号吗？ : a Objective-C syntax?

Question

Xcode 没有给出我的(thought-to-be)打字错误的错误:

 NSString *theme = [[NSUserDefaults standardUserDefaults] objectForKey:@"theme"];
 NSLog(@"Theme: %@", theme ?: @"Default");

事实证明:

 NSLog(@"Theme: %@", theme ?: @"Default");

工作原理与:

 NSLog(@"Theme: %@", theme ? theme : @"Default");

上面的缩短语法是否适用于 gcc only ？或者它是 Objective-C 的一部分？

Answer 1

它是 C 中条件表达式的 GNU 扩展:

来自 here :

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

a = x ? : y;