`operator type&` confusion

Question

I have user-defined type:

class String::CharProxy
{
public:
    const char* operator&() const;
    char* operator&();
    operator char() const;
    operator char&();
};

The problem is when I'm trying to perform some explicit casts, the wrong operator is called:

CharProxy p(...);
static_cast<char>(p); // operator char&() call instead of operator char() const

I want operator char() const to be called when casting to char and operator char&() - when casting to char& only.

Could anyone explain how this mechanism works? Am I mistaken anywhere?

Answer 1

In your static_cast, both operator char() const and operator char&() are candidate functions as per [over.match.conv] because both char and char& are convertible to char via a standard conversion sequence.

Between the two functions, the compiler then decides by standard overload resolution rules. In these, operator char&() is found to be a better fit because its implicit this parameter has the same cv-qualifiers as the object you're passing into it (that is: there is no const qualifier on p). You can explicitly call the other operator with

static_cast<char>(static_cast<CharProxy const &>(p));

But in more realistic terms: 1. user-defined conversion functions are a hornets' nest that are best avoided 99% of the time, and 2. if your operator char() const has so different semantics from your operator char&() that you cannot use the latter where you can use the former, that seems like a very dangerous thing you're building there. Certainly it's not POLA-compliant.