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c++templatessyntaxsemantics

Meaning of template syntax with scope qualifier

I saw this recently:

template <class U> struct ST
{
...
};
template <class U, class V>
struct ST<U V::*>
{
...
};

I assume that the second template is a specialization of the first.

But what is the semantics of U V::* ???

Solution

  • That means "pointer-to-member of class V where the type of the member is U ". For instance,

    struct X
{
    int x = 0;
};

// ...

int X::*p = &X::x;     // <== Declares p as pointer-to-member

ST<decltype(&X::x)> s; // <== Will instantiate your template specialization,
                       //     with U = int and V = X

ST<int X::*> t;        // <== Will instantiate your template specialization,
                       //     with U = int and V = X