Thread: template and polymorphism

Hi,
I wonder if I can use template with polymorphism. The problem is that it seems impossible to define a pointer to an uninstantiated template. For instance, here is a toy example,

Code:

#include <iostream>
#include <cstring>
using namespace std;
template<class T>
class A {
 public:
 T a;
 virtual void f(){ cout<< "A"<< endl;}
};

template<class T>
class B: public A<T> {
 public:
 void f(){ cout<< "B"<< endl;}
};

template<class T>
class C: public A<T> {
 public:
 void f(){ cout<< "C"<< endl;}
};

int main(int argc, char **argv){
A * ptr; // error: missing template arguments before ‘*’ token
if (strcmp(const_cast<const char*>(argv[1]), "B") == 0){
 if  (strcmp(const_cast<const char*>(argv[2]), "int") == 0) ptr = new B<int>;
 else  if(strcmp(const_cast<const char*>(argv[2]), "float") == 0) ptr = new B<float>;
} else if(strcmp(const_cast<const char*>(argv[1]), "C") == 0){
 if  (strcmp(const_cast<const char*>(argv[2]), "int") == 0) ptr = new C<int>;
 else  if (strcmp(const_cast<const char*>(argv[2]), "float") == 0) ptr = new C<float>;
}
ptr->f();
return 0;
}

Is there any possible way to use template and polymorphism?
THanks and regards!

Even if you inherit, the T's need to be the same right? So could you do something like:

Code:

A<int>* ptr;

(obviously I didn't try).
But I don't know of a way to be able to have one thing that could be either an A<int> or an A<float> or whatever, if that's what you're asking. (Someone will probably come along with the way where possible. I don't expect it to be easy.)

If use

Code:

A<int>* ptr;

g++ will complain at later code line:

error: cannot convert ‘B<float>*’ to ‘A<int>*’ in assignment

Similarly for

Code:

A<float>* ptr;

Really appreciate if someone can come up with a solution.

I want to see if it is possible to achieve something like defining a single "pointer" pointing to a "base class" without type argument specified i.e. uninstantiated template base class, so code can be shared not only within polymorphism of each instantiation but also across various type argument. Just like the way we use virtual function.

A new question is that if the type argument can be determined dynamically? something like this

Code:

A <argv[1]> a;

I know it is wrong the type keyword is not string. How to do it if possible?

Thanks!