#include <iostream>
 
// interface
 
template<class Imp>
class Interface{
  public:
    inline void print(void){
      std::cout<<"value: ";
      return asImp().print();
    }
  private:
    typedef Imp Implementation;
    inline Implementation& asImp(void){return static_cast<Implementation&>(*this);}
};
 
// add
class Add:public Interface<Add>{
  public:
    inline void print(void){std::cout<<value<<std::endl;++value;}
  private:
    int value;
};
 
// main
int main(void){
  Interface<Add> foo;
  foo.print();
  foo.print();
}

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