Templated class hierarchy does not warn about duplicate member name

March 19, 2024

I have been refactoring, moving members from children classes to a common parent.

I’d like to use compiler warnings/errors to protect against the same variable appearing in child and parent (if I forgot to remove from a child).

The below produces a warning with Clang 16/17:

struct P
{
    int a;
};


struct C : public P
{
    int a;
};

int main()
{
    return 0;
}

warning: non-static data member 'a' of 'C' shadows member inherited from type 'P' [-Wshadow-field]

However, in my situation I had a templated hierarchy:

template<class E>
struct P
{
    int a;
};

template<class E>
struct C : public P<E>
{
    int a;
};

int main()
{
    C<double> c;
    return 0;
}

and this does not produce a warning. Why is this?

>Solution :

This is a consequence of dependent names and template specialization. When you do

template<class E>
struct C : public P<E>
{
    int a;
};

all you can really do is check that it is syntactically correct. We don’t know what E is yet so we can’t know what P<E> will turn into. There could be a specialization introduced after C that specializes P for double that has no members so there would be no member hiding.