#include <iostream>

class Expr_node {
    friend std::ostream& operator << (std::ostream& lhs, const Expr_node& rhs) ;

  protected:
    virtual void print(std::ostream&) const = 0 ;
    virtual ~Expr_node() {}
};

std::ostream& operator << (std::ostream& lhs, const Expr_node& rhs) {
    rhs.print( lhs ) ;
    return lhs ;
}

class Int_node: public Expr_node {
    // friend class Expr;
  public :
    int n ;

    Int_node(int k): n(k) {}
    void print(std::ostream& o) const {
        o << n;
    }
};

int main() {
    Int_node n(1) ;
    operator<<( std::cout, n );
    return 0;
}