条件检查：if（x == 0）vs.if（！x） [英] Condition checking: if(x==0) vs. if(!x)

问题描述

if（x == 0）与 if（！x）有什么区别？还是它们总是等效的？对于 x 的不同C ++内置类型：

What's the differences of if(x==0) vs. if(!x)? Or are they always equivalent? And for different C++ build-in types of x:

• bool


• int


• char


• 指针


• iostream


• ...

• bool
• int
• char
• pointer
• iostream
• ...

推荐答案

假定从类型到支持 if（x）或 if（！x）的内容进行了转换，那么只要 operator int（）与 opterator bool（），结果将相同。

Assuming there is a conversion from a type to something that supports if (x) or if (!x), then as long as there isn't a DIFFERENT conversion for operator int() than opterator bool(), the result will be the same.

如果（x == 0）将使用最佳转换，其中包括 bool 或 void * 转换器。只要有任何可以将类型转换为标准类型的转换器。

if (x == 0) will use the "best" conversion, which includes a bool or void * converter. As long as there is any converter that can convert the type to some "standard type".

if（！x）会完全一样，它将使用任何转换为​​标准类型的转换器。

if(!x) will do exactly the same, it will use any converter that converts to a standard type.

这两个当然都假定转换器功能不是C + +11不默认转换。

Both of these of course assume the converter function isn't a C++11 "don't default convert".

当然，如果您有这样的课程：

Of course, if you have a class like this:

class
{
   int x;
  public:
   bool operator bool() { return x != 0; }
   int operator int() { return x == 0; } 
}; 

然后 if（x == 0）将执行 if（（x == 0）== 0）和 if（！x）将执行 if（！（x！= 0），这是不一样的。但是现在我们真的很想麻烦，这是非常糟糕的设计代码。

then if (x == 0) will do if ( (x == 0) == 0) and if (!x) will will do if (! (x != 0), which isn't the same. But now we're really TRYING to make trouble, and this is VERY BADLY designed code.

当然，上面的示例可能会导致任何 operator int（）错误 false 表示 x == 0 ， true 值。

Of course, the above example can be made to go wrong with any operator int() that doesn't result in false for x == 0 and true for all other values.

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