#include <iostream>#include <string>using namespace std;class Expr_node{    //friend ostream& operator<<(ostream&, const Expr_node&);    friend class Expr;private:    int use;public:    Expr_node(): use(1) { }    virtual void print(ostream&) const = 0;    virtual ~Expr_node() { }};/*ostream& operator<<(ostream& o, const Expr_node& e){    e.print(o);    return o;}*/class Expr{    friend ostream& operator<<(ostream&, const Expr&);private:    Expr_node *p;public:    Expr(int);    Expr(const string&, Expr);    Expr(const string&, Expr, Expr);    Expr(const Expr&);    Expr& operator=(const Expr&);    ~Expr() { if(--p->use==0)delete p; }};class Int_node: public Expr_node{    friend class Expr;    int n;    Int_node(int k): n(k) { }    void print(ostream &o) const { o << n; }};class Unary_node: public Expr_node{    friend class Expr;    string op;    Expr opnd;    Unary_node(const string& a,Expr b): op(a), opnd(b) { }    void print(ostream& o) const { o << "(" << op << opnd << ")";}};class Binary_node: public Expr_node{    friend class Expr;    string op;    Expr left;    Expr right;    Binary_node(const string& a,Expr b,Expr c):        op(a),left(b),right(c) { }    void print(ostream& o) const { o << "(" << left << op << right << ")";}};Expr::Expr(int n) { p = new Int_node(n); }Expr::Expr(const string& op, Expr t) { p = new Unary_node(op, t); }Expr::Expr(const string& op, Expr left, Expr right){    p = new Binary_node(op, left, right);}Expr::Expr(const Expr& t){    p = t.p;    ++p->use;}Expr& Expr::operator=(const Expr& rhs){    rhs.p->use++;    if(--p->use == 0){        delete p;    }    p = rhs.p;    return *this;}ostream& operator<<(ostream& o,const Expr& t){    (t.p)->print(o);    return o;}int main(){    Expr t = Expr("*", Expr("-",5), Expr("+", 3, 4));    cout << t << endl;    return 0;}