Which operator to overload in order to use my class in an if-statement? [duplicate]

问题

For example, I have defined a class

class MyClass
{
   ....
};

which operator do I have to overload in order to do the if comparison with a MyClass object?

eg:

MyClass cc;
if ( cc ) // compile error
{
   //do some stuff
}

I tried with

 bool operator == ( int value ) ; //guess the if () will call this

 bool operator != ( int value ) ;

but both give me a compile error!

回答1:

You should provide a bool conversion operator:

struct MyClass
{
  explicit operator bool() const { return true; }   
};

Here, an explicit operator is used to prevent unwanted implicit conversions to other types, specifically numeric types. Note that this is only possible since C++11.

回答2:

operator bool() is the one you want. It takes care of type conversion from your class to type bool.

回答3:

You have to provide a conversion operator for bool or for something convertible to bool. If you have C++11, the best way is this:

class MyClass
{
public:
  explicit operator bool () const {
    ...
  }
};

If you don't have C++11 (or at least its support for explicit conversion operators), things get a bit more tricky (because an implicit conversion can kick you real painfully when you least expect it). See the safe bool idiom for more information on this.

回答4:

You could override operator bool(), but according to your example, you might also consider creating simple method that will return bool. Then its usage could for example look like this:

MyClass cc;
if (cc.isValid())
{
   // do some stuff
}

which would be more straightforward and also easier to read in this case. Custom operators are great for many things, but don't force it. Sometimes it's better to just keep it simple :)

来源：https://stackoverflow.com/questions/15616779/which-operator-to-overload-in-order-to-use-my-class-in-an-if-statement

标签

c++

operator-keyword

overwrite