Question

I ask this question following the issue I raised here.

The point is quite simple. Suppose you have two classes of this kind:

template < class Derived >
class Base {
...
operator const Derived&() const {
    return static_cast< const Derived& >(*this);
  }
...
};

class Specialization : public Base<Specialization> {
...
};

Then suppose you have a type conversion like this one:

template < class T >
functionCall( const Base<T>& param) {
  const T & val(param);
  ...
}

The question is: what should be the standard conforming behavior of this conversion?

Should it be the same as const T & val(static_cast<const T &> (param) ) or should it recursively iterate until stack overflow? Notice that I obtain the first behavior compiling with GNU g++ and the second compiling with Intel icpc.

I already tried to peek at the standard (section 5.9 on static_cast and section 12.3 on conversions) but due to my lack of experience I was not able to figure out the answer.

My many thanks in advance to anybody taking the time to help me out with this.

Answer 1

Looking at [expr.static.cast] in n3337 (first working draft after the Standard):

2/ An lvalue of type “cv1 B,” where B is a class type, can be cast to type “reference to cv2 D,” where D is a class derived (Clause 10) from B, if a valid standard conversion from “pointer to D” to “pointer to B” exists […]

4/ Otherwise, an expression e can be explicitly converted to a type T using a static_cast of the form static_cast<T>(e) if the declaration T t(e); is well-formed, for some invented temporary variable t [..]

Therefore, I would interpret that gcc’s behavior is the correct one, ie the expression:

static_cast<Derived const&>(*this)

should not invoke recursively operator Derived const& () const.

I deduce this from the presence of the Otherwise keyword which implies an ordering of the rules. The rule 2/ should be tried before the rule 4/.