Why Do Explicit Specializations Outside a Namespace Cause Compilation Errors in C ?

Explicit Specialization Issue in Non-Namespace Scope

When compiling a C class template with explicit specialization outside of a namespace, some compilers like g may raise an error stating "Explicit specialization in non-namespace scope." This issue arises because explicit specializations of class templates and their member functions should reside within the namespace of the template's declaration.

The relevant C 03 standard for explicit specialization (section §14.7.3/2) specifies that:

"An explicit specialization shall be declared in the namespace of which the template is a member, or, for member templates, in the namespace of which the enclosing class or enclosing class template is a member."

This means that the explicit specialization of the Verify() method should be declared within the namespace where the CConstraint class is declared.

Furthermore, according to C 03, §14.7.3/3, explicit specialization of member functions requires explicit specialization of the enclosing class. To resolve this, consider forwarding the Verify() method to an explicit, potentially specialized free function within a nested namespace:

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);
    }
};

By encapsulating the explicit specialization in a nested namespace, compilers will recognize it as belonging to the template's namespace, resolving the error.

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