conversion operator as standalone function

Why does C++ require that user-defined conversion operator can only be non-static member? Why is it not allowed to use standalone functions as for other unary operators开发者_StackOverflow? Something like this:

operator bool (const std::string& s) { return !s.empty(); }

The one reason I can think of is to prevent implicit conversions being applied to the thing being cast. In your example, if you said:

 bool( "foo" );

then "foo" would be implicitly converted to a string, which would then have the explicit bool conversion you provided applied to it.

This is not possible if the bool operator is a member function, as implicit conversions are not applied to *this. This greatly reduces the possibilities for ambiguity - ambiguities normally being seen as a "bad thing".

By keeping the conversion operator within the class you give the author of the class control of how it could be converted (It prevents users from creating implicit conversions). As an implementer I would consider this an advantage, as implicit conversions does have its issues

There is a difference being able to pass one object as another, and having it to go through a conversion function. The former communicates that the object is of a given type, while the latter shows new readers that there is a difference between the two types and that a conversion is necessary.

Implicit user-defined conversions are frowned upon anyway. Don't use them. Just pretend that they aren't there. Let alone thinking about newer ways to introduce them.

Anyway, I guess they aren't there because the way they are they can do enough unexpected things. Including a new header which introduces such a conversion for a class defined somewhere else might lead to even more confusing errors.

There's a group of operators that have to be overloaded as non-static member functions: assignment, subscripting, function call, class member access, conversion functions.

I guess the standard's committee or Stroustrup simply felt it might be just too confusing if it was allowed to inject these very special behaviors to classes from outside.

I suppose the best way to get the answer would be to e-mail the author.