什么时候Objective-C的隐式转换为BOOL？

问题描述

我看了，现在我偏执到 BOOL 键入 ID 的隐式铸造变量。

假设我有一个较低位的危险的指针归零，因此，它强制转换为 BOOL 将产生 NO ，即使它指向一个有效的对象。我们称之为危险的指针富。

如何做简单的if语句工作？
请问if语句投富到 BOOL 时，计算该条件？还是将它转换为一个纯粹的布尔？

 如果（富）
{
  [myArray的ADDOBJECT：美孚]。
}
 

它的工作原理相同的方式在三元前pressions？

  //这是否休息的时候foo是危险？
self.titleLabel.hidden = foo的？是：否;
 

还是会如果我这样做三元只有突破：

  //我pretty肯定现在打破
self.titleLabel.hidden =（（BOOL）富）？是：否;
 

我觉得我失去了用C基本的东西有关逻辑运算，以及它们如何与 BOOL 。请帮赐教。

解决方案

Never, since this phrase is semantically incorrect. A cast is, by definition, explicit. What you're asking about is called an "implicit type conversion" (coercion - thanks, Josh!).

Also, strictly speaking, the conditionals in Objective-C are not special: they're inherited from C. So expressions, when needed to evaluated as booleans, aren't treated as BOOL, but as int.

// Does this break when foo is 'dangerous'?`
self.titleLabel.hidden = foo ? YES : NO;

No, it doesn't. (expr) when used as a "boolean" expression, is equivalent to (expr != 0). No risks here.

// I'm pretty sure it breaks now
self.titleLabel.hidden = ((BOOL)foo) ? YES : NO;

Now this can break, since BOOL is just typedeffed to signed char, which is 8 bit on iOS. Thus, if foo, which is a (32-bit) pointer, gets truncated to 8 bits, and the lower 8 bits of the pointer were all zero, but the pointer itself wasn't nil, this will incorrectly report false. You don't want to do this superfluous, ugly and dangerous cast. If you want to be more explicit, write

var = (obj != nil) ? valueOne : valueTwo;

instead.

这篇关于什么时候Objective-C的隐式转换为BOOL？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！