Ion Gaztañaga wrote:
I mean:
template<class T, member_hook T::* P> class value_traits;
compiles fine, but I just want to have one template parameter
template<member_hook T::* P> class value_traits;
This is a compilation error. Can C++ deduce both the parent class and the pointer to member? I don't see any reason that should forbid this, but I can find the right syntax.
There is none. There's no such thing as implicit template parameters. Each parameter must be explicitly specified. P is a non-type template parameter; it takes a constant value, not a type. The constant value is a pointer-to-member. (Typically an offset inside a class, but of course that's implementation-dependent.) A pointer-to-member needs a class it points into. In your case, which class is it? You only have T there, so the compiler looks for a type called T. Not finding one, it throws an error. If there was an implicit template parameter there, that would make the code extremely fragile: as long as no type T is defined at the point of the template declaration, the template has two parameters - probably the compiler would, not finding a type T, assume it's an implicit template parameter. If at any point in time a type T is made visible to the template declaration, the meaning suddenly changes. This is of course unacceptable. And it works the other way, too. Consider: struct FooBarZoot { ... }; template <int FoobarZoot::* P> class Oops { ... }; How many template parameters does Oops have? That's right, two, because FoobarZoot is different from FooBarZoot and thus an implicit template parameter. The error message would be very helpful, too. As you can see, it's imperative that every template parameter is specified explicitly. Sebastian Redl