Why Does Explicit Specialization of a Member Function in a C Template Class Cause a 'Non-Namespace Scope' Error?

Explicit Specialization in Non-Namespace Scope

Query:

In a C template class CConstraint, why does the compiler generate the error "Explicit specialization in non-namespace scope" when a member function is explicitly specialized?

Response:

The issue arises because the explicit specialization in CConstraint is not declared within a namespace. According to the C 03 standard, section 14.7.3/2, explicit specializations must reside within the template's namespace or, for member templates, within the namespace of the enclosing class or class template.

Furthermore, C 03 section 14.7.3/3 restricts the explicit specialization of member functions unless the containing class itself is explicitly specialized.

Solution:

To resolve this issue, one approach is to forward the Verify() member function to a specialized free function defined within a separate namespace, as shown below:

namespace detail {
    template <typename TL> void Verify(int, int[]) {}
    template <>            void Verify<int>(int, int[]) {}
}

template<typename T> class CConstraint {
    // ...
    template <typename TL> void Verify(int position, int constraints[]) {
        detail::Verify<TL>(position, constraints);
    }
};

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