C++, function pointer to the template function pointer

Question

I am having a pointer to the common static method

class MyClass
{
  private:
    static double ( *pfunction ) ( const Object *, const Object *);
    ...
};

pointing to the static method

 class SomeClass
 {
  public:
    static double getA ( const Object *o1, const Object *o2);
    ...
 };

Initialization:

double ( *MyClass::pfunction ) ( const Object *o1, const Object *o2 )  = &SomeClass::getA;

I would like to convert this pointer to the static template function pointer:

template <class T>
static T ( *pfunction ) ( const Object <T> *, const Object <T> *); //Compile error

where:

 class SomeClass
 {
  public:
    template <class T>
    static double getA ( const Object <T> *o1, const Object <T> *o2);
    ...
 };

But there is the following compile error:

error: template declaration of : T (* pfunction )(const Object <T> *o1, const Object <T> *o2)

Thanks for your help...

Answer 1

In the second case, getA is not a function anymore but a function template, and you can't have a pointer to function template.

What you can do is have pfunction point to a particular getA instance (ie: for T = int) :

class MyClass {     static double (*pfunction)(const Object<int> *, const Object<int> *); };  double (*MyClass::pfunction)(const Object<int> *o1, const Object<int> *o2)  = &SomeClass::getA<int>; 

But I don't think there is a way to get pfunction to point on any possible instance of getA.

Answer 2

template is a template :) it's not a concrete type and cannot be used as a member. e.g. you cannot define following class:

class A
{
    template <class T> std::vector<T> member;
}

because template <class T> std::vector<T> member; is something that potentially can be specialized to many different types. you can do something like this:

template <class T>
struct A
{
 static T (*pfunction)();
};

struct B
{
 template <class T>
 static T getT();
};

int (*A<int>::pfunction)() = &B::getT<int>;

here A<int> is a specialized template and so has specialized member