C++Talk.NET

Joaquín M López Muñoz · Guest

The following piece of code is legal C++:

struct A
{
int x;
};

struct B:A{};

int main()
{
int B::* p=&B: Mad

;
}

which is reasonable enough, as A: Mad

is part of B public interface via
inheritance. Perhaps a little metaphorically, we can regard this
aliasing of a pointer to member of base as a pointer to
member of derived like a sort of contravariance rule.
But in the context of template non-type arguments, the rule does not
apply:

struct A
{
int x;
};

struct B:A{};

template<int B::*Ptr>
struct foo{};

int main()
{
// error: argument of type "int A::*" is incompatible with
// template parameter of type "int B::*"
foo<&B: Mad

> f;
}

I'm sure there's some place in the standard implicitly or explicitly
banning the conversion in this particular context (though I couldn't
find it). My questions are:

* Is there any sensible reason for this banning that escapes me?
Why here (template args) and not anywhere else?
* If not, would it be a good idea to file a DR?

Thanks in advance,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

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