#include <iostream>
 
template <typename T> struct extra {};
 
template <> struct extra<int> {
public:
    int get() const;
};
 
template <typename T>
class base : public extra<T> {
    friend class extra<T>;
public:
   void set(T data) { data_=data; }
private:
    T data_;
};
 
int extra<int>::get() const{ return static_cast<const base<int>*>(this)->data_;}
 
int main()
{
    base<int> b;
 
    b.set(42);
    std::cout << b.get() << std::endl;
}
 

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