template specialization for template parameters

Question

I have two templates

template<typename Policy>
class X : public Policy
{ }

template<typename T>
class SimplePolicy 
{ }

SimplePolicy can be used as the template parameter to X . Is it possible to specialize X for all occurrences of SimplePolicy -ies (ie, for any T )? Or what would be the best strategy to proceed, if that's not possible?

Thanks!

Answer 1

Yes, you can do it like this:

#include <iostream>

using namespace std;

template<typename T>
class SimplePolicy 
{ };

template<typename Policy>
class X : public Policy
{   
    public: void Do() { cout << "Standard" << endl; }
};  

template<typename Inner>
class X<SimplePolicy<Inner> >
{   
    public: void Do() { cout << "Special" << endl; }
};  

class A{};

int main()
{   
    X<A> xa; 
    X<SimplePolicy<A>> xs; 

    xa.Do();
    xs.Do();
} 

Answer 2

Yes it is possible using partial template specialization

#define STATIC_ASSERT(exp) \
    typedef char static_assert_ ## __LINE__ [(exp) ? 1 : -1]

template<typename Policy>
class X : public Policy
{
public:
    enum { value = 0 };
};

template<typename T>
class SimplePolicy 
{ };

class CleverPolicy 
{ };

template <typename T>
class X< SimplePolicy<T> > : public SimplePolicy<T>
{ 
public:
    enum { value = 1 }; 
};

int _tmain(int argc, _TCHAR* argv[])
{
    STATIC_ASSERT(X<SimplePolicy<int> >::value == 1);
    STATIC_ASSERT(X<SimplePolicy<float> >::value == 1);
    STATIC_ASSERT(X<CleverPolicy>::value == 0);
    return 0;
}