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请能者解答：读C++大师Andrew Koenig>时产生的一点疑问 |
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作者：未知 来源：月光软件站 加入时间：2005-5-13 月光软件站 |
第八章“一个面向对象的范例”
范例是关于 +，-，*，/等数学表达式的处理。
程序如下：（C++Builder6.0 build 通过，运行不对。书内程序用明显少数字符错误，已改。）
#include
class Expr;
class Expr_node
{
friend ostream& operator << ( ostream&, const Expr& );
friend class Expr;
int use;
protected:
Expr_node(): use(1) {}
virtual ~Expr_node() { }
virtual void print( ostream& ) const = 0;
};
//-----------------------------------------------------
class Expr
{
friend ostream& operator << ( ostream&, const Expr& );
Expr_node * p;
public:
Expr( int );
Expr( const string&, Expr );
Expr( const string&, Expr, Expr );
~Expr() { delete p; }
public:
Expr( const Expr& t ) { p = t.p; ++p->use; }
Expr& operator = ( const Expr& t );
};
//----------------------------------------------------
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 << ")"; }
};
ostream& operator << ( ostream& o, const Expr& t )
{
t.p->print( o );
return o;
};
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;
};
//--------------------------------------------------
void main()
{
// Expr t = Expr( "*", Expr( "-", 5 ), Expr( "+", 3, 4 ));
// Expr t0 = Expr( 123 ); cout << t0 << endl;
Expr t00( 5 ); cout << t00 << endl; // 此例可以。
Expr t01( "-", t00 ); //不行
// t00 = Expr( "-", t00 );
// cout<<"sfsfsffs"<
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