C ++类转换为布尔 [英] c++ convert class to boolean

问题描述

与所有的基本类型C ++的，可以简单地询问：

With all of the fundamental types of C++, one can simply query:

if(varname)

和的类型转换为用于评估一个布尔值。有什么办法来复制一个用户定义的类此功能？我的一个类是由一个整数标识，虽然它有一些其他的成员，我希望能够检查整数设置为NULL，以这样的方式。

and the type is converted to a boolean for evaluation. Is there any way to replicate this functionality in a user-defined class? One of my classes is identified by an integer, although it has a number of other members, and I'd like to be able to check if the integer is set to NULL in such a manner.

感谢。

推荐答案

您可以定义一个用户定义的转换操作符。这必须是一个成员函数，例如：

You can define a user-defined conversion operator. This must be a member function, e.g.:

class MyClass {
  operator int() const
  { return your_number; }
  // other fields
};

您也可以实现运营商布尔。不过，我的强烈建议反对这样做，因为你的类将成为算术前pressions可迅速导致一个烂摊子使用。输入输出流定义，例如，转换为无效* 。您可以在您可以测试布尔，但没有语言的定义隐式转换同样的方式测试无效* 从无效* 。另一种方法是定义运营商！与所需的语义。

You can also implement operator bool. However, I would STRONGLY suggest against doing it because your class will become usable in arithmetic expressions which can quickly lead to a mess. IOStreams define, for example, conversion to void*. You can test void* in the same way you can test a bool, but there are no language-defined implicit conversions from void*. Another alternative is to define operator! with the desired semantics.

在短：定义转换操作符STO整数类型（包括布尔）是一个非常糟糕的主意。

In short: defining conversion operator sto integer types (including booleans) is a REALLY bad idea.

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