Outer class operator for a template class inside a template class

Question

I am trying to write an external class template operator for a template class inside a template class.

I hope the following snippet will explain what I mean.

enum MyEnum {};

template <MyEnum a>
class ClassWithTemplateClass {
 public:
    template <bool B> 
    class TemplateClass {
        // ...
    };
};

When I wrote the statement as follows:

template <MyEnum enumVal, bool B>
auto operator<<(ClassWithTemplateClass<enumVal>::TemplateClass<B> &a, int b) {
    // ...
    return a;
}

Compiler Error:

error: operator declaration <<as not functional

Can you tell me how I should write this statement?

Answer 1

ClassWithTemplateClass<enumVal>:: , , , . enumVal , ::, .

<< (1) TemplateClass:

enum MyEnum { X, Y, Z };

template <MyEnum E>
struct ClassWithTemplateClass
{
    template <bool B> 
    struct TemplateClass
    {
        friend auto& operator<<(TemplateClass& a, int b)
        {
            return a;
        }
    };
};

TemplateClass ClassWithTemplateClass<?>::TemplateClass<?>

DEMO

(2) ClassWithTemplateClass:

enum MyEnum { X, Y, Z };

template <MyEnum E>
struct ClassWithTemplateClass
{
    template <bool B> 
    struct TemplateClass
    {
    };

    template <bool B>
    friend auto& operator<<(TemplateClass<B>& a, int b)
    {
        return a;
    }
};

DEMO 2

, (3) ( , ), B:

enum MyEnum { X, Y, Z };

template <MyEnum E>
struct ClassWithTemplateClass
{
    template <bool B> 
    struct TemplateClass
    {        
    };
};

template <bool B>
auto& operator<<(ClassWithTemplateClass<X>::TemplateClass<B>& a, int b)
{
    return a;
}

template <bool B>
auto& operator<<(ClassWithTemplateClass<Y>::TemplateClass<B>& a, int b)
{
    return a;
}

template <bool B>
auto& operator<<(ClassWithTemplateClass<Z>::TemplateClass<B>& a, int b)
{
    return a;
}

DEMO 3

(4) TemplateClass SFINAE, :

enum MyEnum { X, Y, Z };

template <MyEnum E>
struct ClassWithTemplateClass
{
    template <bool B> 
    struct TemplateClass
    {
        static constexpr MyEnum ClassWithTemplateClass_E = E;
        static constexpr bool TemplateClass_B = B;
    };
};

template <typename T
        , MyEnum E = T::ClassWithTemplateClass_E
        , bool B = T::TemplateClass_B>
auto& operator<<(T& a, int b)
{
    return a;
}

DEMO 4

(5) operator<< , , , :

enum MyEnum { X, Y, Z };

template <MyEnum E>
struct ClassWithTemplateClass;

template <MyEnum E, bool B>
auto& print(typename ClassWithTemplateClass<E>::template TemplateClass<B>& a, int b);

template <MyEnum E>
struct ClassWithTemplateClass
{
    template <bool B> 
    struct TemplateClass
    {
        friend auto& operator<<(TemplateClass& a, int b)
        {
            return print<E, B>(a, b);
        }
    };
};

template <MyEnum E, bool B>
auto& print(typename ClassWithTemplateClass<E>::template TemplateClass<B>& a, int b)
{
    return a;
}

DEMO 5