表达式树程序

在c++沉思录举了一个表达式树的例子，将例如(-5)*(3+4)的式子表示成树的形式。可以通过如下形式：

Expr t = Expr("*", Expr("-", 5), Expr("+", 3, 4));
cout<<t<<" = "<<t.eval()<<endl;

创建树并将树打印出来。

这个程序主要用于理解动态绑定和句柄，程序实现代码如下：

#include <iostream>
#include <string>
/*
所有节点类型的父类 
*/
class Expr_node{
//    friend  ostream& operator<<(ostream&, const Expr&);
    friend class Expr;
    int use;
protected:
    Expr_node():use(1){}
    virtual int eval()const = 0;
public:
    virtual void print(ostream &)const = 0;//书中这个方法是protected修饰的，但是放在那里会报错 
    virtual ~Expr_node(){}
};

/**
句柄类 
*/
class Expr{
    //重载操作符 
    friend ostream& operator<<(ostream&, const Expr&);
    Expr_node * p;
public:
    Expr(int);
    Expr(const string &, Expr);
    Expr(const string &, Expr, Expr);
    Expr(const Expr&t){ p = t.p; ++p->use; };
    Expr & operator=(const Expr &);
    ~Expr(){if(--p->use==0)delete p;}
    int eval()const {return p->eval();}
    //ostream& operator<<(ostream&, const Expr&)const;
};
class Int_node: public Expr_node{
    friend class Expr;
    int n;
    Int_node(int k):n(k){}
    void print(ostream &o)const{o<<n;}
    int eval()const{return 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 << ")";}
    int eval()const;
};
int
Unary_node::eval()const
{
    if(op == "-")
        return -opnd.eval();
    throw "error,bad op " + op + "in unarnode";
}

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){}
    int eval()const;
    void print(ostream& o)const
    {
        o << "(" << left << op << right<< ")";
    }

};
int
Binary_node::eval()const
{
    int op1 = left.eval();
    int op2 = right.eval();
    if(op == "-") return op1 - op2;
    if(op == "+") return op1 + op2;
    if(op == "*") return op1 * op2;
    if(op =="/" && op2 != 0) return op1 / op2;
    throw "error, bad op " + op + "in  binarynode" ;
}

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::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<<" = "<<t.eval()<<endl;
}