Specialization template class with templated function - c++

Question

I have some helper template with templated function. I'm using it for other policy-based templated class to cast or not to cast some value to specific type:
// use this when needed additional value cast
template <typename T>
struct AdditionalValueStaticCaster
{
template <typename U>
static T cast(U u)
{
return static_cast<T>(u);
}
};
// use this specialization when no needed to cast value
template <>
struct AdditionalValueStaticCaster<void>
{
template <typename U>
static U cast(U u)
{
return u;
}
};
Now I want to split these classes to declaration and implementation (.h and .hh files)
So I write declaration like this:
template <typename T>
struct AdditionalValueStaticCaster
{
template <typename U>
static T cast(U u);
};
template <>
struct AdditionalValueStaticCaster<void>
{
template <typename U>
static U cast(U u);
};
And now I want to write implementation. This is ok:
template <typename T>
template <typename U>
T AdditionalValueStaticCaster<T>::cast(U u)
{
return static_cast<T>(u);
}
But this causes error error: too many template-parameter-lists
template<>
template <typename U>
U AdditionalValueStaticCaster<void>::cast(U u)
{
return u;
}
If I delete line template<> all compiles ok, but I still have a question: is it right solution or I missed something?

Answer 1

If I delete line template<> all compiles ok, but I still has question: is it right solution or I missed something?
That's correct, you just need to delete that line. AdditionalValueStaticCaster<void> is a concrete type and you're just providing the definition of a member function template on that type. Hence:
template <typename U>
U AdditionalValueStaticCaster<void>::cast(U u)
{
return u;
}
No different than the member function template on any other class type:
template <typename U>
U SomeClass::cast(U u)
{
return u;
}

Answer 2

I don't think that you can define a template with in a template.
put all the template parameters in the first template line. like this.
template <typename T, typename U>
struct AdditionalValueStaticCaster
{
static T cast(U u)
{
return static_cast<T>(u);
}
};
specialized to an actual object like this:
AdditionalValueStaticCaster< My_T_Type, My_U_Type> MyNewCaster;
and used like this
My_T_Type My_T_Type_var;
My_U_Type My_U_Type_var;
My_T_Type_var = MyNewCaster.cast(My_U_Type_var);
Sorry I don't have a compiler to check this on, but syntax should be close.