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I am currently struggeling with partial template specialization of a template template (template ) class. I know we can realize the following problem with inheritance but the goal is to use only templates and template specialization.

Let's consider this piece of code:

template< typename T >
struct V4;
template< >
struct V4< float > 
 using point_type = std::array< float, 4 >;
;
template< typename T >
struct V12;
template< >
struct V12< int > 
 using point_type = std::array< int, 12 >;
;

template< typename T, template< typename = T > class IM >
struct Rectangle;
template< template< typename > class IM >
struct Rectangle< float, IM > 
 float get( size_t i ) /*...*/ 
;

template< typename T, template< typename = T > class IM >
struct Polygon;
template< template< typename > class IM >
struct Polygon< int, IM > 
 float get( size_t i ) /*...*/ 
;

The classes V2 and V12 can be considered as some kind of a wrapper classes for builtin-types or aggregates. The classes Rectangle and Polygon uses V2 or V12 and are partial specialized.
So far so good. We can use this like the following:

Rectangle< float, V4 > a;
std::cout << a.get(1) << "n";

Now what we want to achieve is a class which encapsulates some calculations on the introduces structs for instance. It should be called like this:

std::cout << CalculateSomething< Rectangle< float, V4 > >::doIt( ) << "n";

CalculateSomething should be partial specializable regarding to wether it works with an Rectangle or an Polygon. So a possible specialization should look like this:

template< typename T >
struct CalculateCenter< Rectangle< T, V2 >, T > 
 static T doIt( ) 
 /*...*/
 
;
template< typename T >
struct CalculateCenter< Polygon< T, V12 >, T > 
 static T doIt( ) 
 /*...*/
 
;

The question now is: How to declare the template class CalculateCenter?

Thanks in advance

The answer seems rather straightforward:

template < class T >
struct CalculateCenter;

template< typename T >
struct CalculateCenter< Rectangle< T, V2 > > 
 static T doIt( ) 
 /*...*/
 
;

template< typename T >
struct CalculateCenter< Polygon< T, V12 > > 
 static T doIt( ) 
 /*...*/
 
;

[Live example]