1.最近几天在看c++ 沉思录。里面举的例子确实让我茅塞顿开，醍醐灌顶哈。

2.我把代码贴上去吧。根据原书的代码敲的。

这一章的内容要解决的是如何表示一个表达式树。

作者将所有类型的表达式都隔离开了，就用一个类来提供所有的接口，管理内存。让用户避免记住麻烦的用法和内存管理。

相当巧妙

//Expr_node.h
#include 
using namespace std;class Expr;
class Expr_node
{friend ostream& operator<<(ostream& o, const Expr& t);friend ostream& operator<<(ostream& o, const Expr_node& e){e.print(o);return o;}friend class Expr;int use;
protected:Expr_node() : use(1) { }virtual void print(ostream& ) const = 0;virtual int eval() const = 0;virtual ~Expr_node() {}
};class Expr
{friend ostream& operator<<(ostream&, const Expr&);friend class Expr_node;Expr_node* p;
public:Expr(int); // create a int_node;Expr(const string &, Expr);Expr(const string &, Expr, Expr);Expr(const string& , Expr, 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();}
};class Int_node: public Expr_node 
{friend class Expr;int n;Int_node(int k):n(k) {}void print(ostream& o) const { o << n;}virtual int eval() const { return n;}
};
//unary node
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;
};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 << "( " <

//Expr_node.cpp
#include "Expr_node.h"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 string& op, Expr left, Expr middle, Expr right)
{p = new Ternary_node(op, left, middle, 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 Unary_node::eval() const 
{if (op == "-")return -opnd.eval();throw "error, bad op " + op + " int UnaryNode";
}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";
}void Ternary_node::print(ostream& o) const
{o<<"( "<

//main.cpp
#include #include "Expr_node.h"using namespace std;
int main(int argc, char const *argv[])
{Expr t = Expr("*", Expr("-", 5), Expr("+", 3,4));cout<