此回是针对上回所设计方案的扩展，下面将会演示
（1）如何增加计算表达式的值的功能
（2）如何增加新的节点类型（已经有表示单个数、一元操作符、二元操作符的节点类型Int_node\Unary_node\Binary_node，将演示如何增加三元操作符(Ternary_node)）
完整代码如下：
#include
#include
using namespace std;
class Expr_node
{
friend class Expr; //友元类可以被继承
int use; //引用计数
public:
virtual void print(ostream&) const = 0;
public:
Expr_node():use(1) {}
virtual ~Expr_node() {}
virtual int eva l() const = 0;
};
class Expr //句柄类
{
friend ostream& operator<<(ostream &o, const Expr &e);
private:
Expr_node *p; //指向基类的指针
public:
Expr(int n);
Expr(const string &op, Expr t);
Expr(const string &op, Expr left, Expr right);
Expr(const string &op, Expr left, Expr middle, Expr right);
Expr(const Expr &t);
Expr& operator=(const Expr&);
~Expr()
{
if(--p->use == 0)
delete p;
}
int eva l() const
{
return p->eva l();
}
};
class Int_node: public Expr_node
{
private:
int n;
public:
Int_node(int k):n(k) {}
void print(ostream &o) const
{
o << n;
}
int eva l() const { return n; }
};
class Unary_node: public Expr_node
{
private:
//friend class Expr;
string op;
Expr opnd;
public:
Unary_node(const string &a, Expr b):op(a), opnd(b) {}
void print(ostream &o) const
{
o << "(" << op << opnd << ")";
}
int eva l() const
{
if(op == "-")
return -opnd.eva l();
throw "error!错误的一元操作符";
}
};
class Binary_node: public Expr_node
{
private:
//friend class Expr;
string op;
Expr left;
Expr right;
public:
Binary_node(const string &a, Expr b, Expr c):op(a), left(b), right(c) {}
void print(ostream &o) const
{
o << "(" << left << op << right << ")";
}
int eva l() const
{
int op1 = left.eva l();
int op2 = right.eva l();
if(op == "-") return op1 - op2;
if(op == "+") return op1 + op2;
if(op == "*") return op1 * op2;
if(op == "/") return op1 / op2;
throw "error! 错误的二元操作符！";
}
};
class Ternary_node: public Expr_node
{
private:
string op;
Expr left;
Expr middle;
Expr right;
public:
Ternary_node(const string& a, Expr b, Expr c, Expr d):op(a), left(b), middle(c), right(d) {}
void print(ostream &o) const
{
o << "(" << left << " " << middle << ":" << right << ")";
}
int eva l() const
{
if(left.eva l())
return middle.eva l();
else
return right.eva l();
}
};
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(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 &e)
{
e.p->print(o);
return o;
}
void main()
{
Expr t = Expr("*",
Expr("-", Expr(5)),
Expr("+", Expr(3), Expr(4)));
cout << t << "=" << t.eva l() << endl;
Expr t2 = Expr("-", Expr(5), Expr(" :", Expr(0), Expr(1), Expr(2)));
cout << t2 << "=" << t2.eva l() << endl;
}
运行结果如下：
((-5)*(3+4))=-3
(5-(0 1