Nested class template specialization

Question

A class:

template<typename C, typename T> class A {     template <typename U>     class Nested{};      Nested<T> n; }; 

And I want to specialize Nested. Here what I tried:

template<typename C, typename T> class A {     template <typename U>     class Nested{};      template <>     class Nested<int>{}; // by my logic this should work by I have a compilation error "explicit specialization in non-namespace scope 'class A<C, T>'"      Nested<T> n; }; 

My next attempt:

template<typename C, typename T> class A {     template <typename U>     class Nested{};      Nested<T> n; };  template<> A<>::Nested<int>{}; // What is the correct syntax to do it here? Now I have an error "wrong number of template arguments (0, should be 2)" 

Here on stackoverflow I found a solution:

template<typename C, typename T> class A {     template <typename U, bool Dummy = true>     class Nested{}; // why need of this Dummy??      template <bool Dummy>     class Nested<int, Dummy>{}; // why need to provide an argument??      Nested<T> n; }; 

It perfectly works, but I can't understand how. Why to provide a dummy template argument? Why can't I use raw specialization template<> class Nested<int, true>{} or template<> class Nested<int>{}?

Answer 1

It's forbidden to create explicit specialization in class-scope:

An explicit specialization shall be declared in a namespace enclosing the specialized template.

But it's not forbidden to create partial specialization:

A class template partial specialization may be declared or redeclared in any namespace scope in which its definition may be defined (14.5.1 and 14.5.2).

this

template <bool Dummy> class Nested<int, Dummy>{}; // why need to provide an argument?? 

is partial specialization and it's allowed to create such specialization in class-scope. You also cannot fully specialize nested class, in not-specialized outer class. You can do this:

template<> template<> class A<int, double>::Nested<int> { }; 

but you cannot do

template<typename C, typename T> template<> class A<C, T>::Nested<int> { };