void GetFlag(C<T> a);
看楼主的意思是要把这个函数用模板了。
而楼主错误地使用在了整个类上。

class C
{
public:
    template <typename T>
    void GetFlag(C<T> a);
    void get(T b);
};

#include <iostream>
using namespace std;
class A
{
public :
   int Flag() const{return 1;}
};
class B:public A
{
public :
   int Flag() const{return 2;}
};

class C
{
public:
    template <typename T>
    int GetFlag(T a)
    {
         return a.Flag();
    }
};

int main()
{
  A a;
  B b;
  C c;
  cout << c.GetFlag(a) << endl;
  cout << c.GetFlag(b) << endl;
  return 0;
}

#include <iostream> 

using namespace std; 

class A
{
public:
protected:
private:
};
class B
{
public:
protected:
private:
};

template<class T>
class C
{
public:
    void Get(C<T> a)
    {
    
    }
    void get(T b)
    {

    }
protected:
private:
};

void main()
{
    C<A> a;
    C<B> b;
    C<A> c;
    //a.Get(b);//error
    a.Get(c);
    B bb;
    //a.get(bb);//error
    A aa;
    a.get(aa);
}