Re: inheritance, list of objects, polymorphism

"James Kanze" <james.kanze@gmail.com> ha scritto nel messaggio
news:d3cde4c3-dd7d-473b-b71d-b82fcef30e26@o28g2000yqh.googlegroups.com...


yes i'm not so smart, but how to use all this?
why declare all distructors "virtual" ?
why use all that "const"?

#include <iostream>
#include <list>
#include <string>

using namespace std;

// Interface...
class Expression{
public:
 Expression(){}
 Expression( Expression& );
 Expression& operator=( Expression& );

 virtual ~Expression() {}
 virtual double value() = 0;
        // other functions...?
};

class OneOperandExpression : public Expression
{public:
 Expression* m_op;

 OneOperandExpression( Expression* op ):m_op( op ){}
 virtual ~OneOperandExpression() { delete m_op; }
};

class TwoOperandExpression : public Expression{
public:
 Expression* m_lhs;
 Expression* m_rhs;

 TwoOperandExpression( Expression* lhs, Expression* rhs )
  :m_lhs( lhs ),m_rhs( rhs ){}

 virtual ~TwoOperandExpression() { delete m_lhs; delete m_rhs; }
};

class ConstantExpression : public Expression{
public:
 double m_value;
 ConstantExpression( double value ):m_value( value ){}
 virtual double value(){return m_value;}
};

class NegExpression : public OneOperandExpression{
public:
 NegExpression( Expression* op ):OneOperandExpression(op){}
 virtual double value(){return - m_op->value();}
};

class AddExpression : public TwoOperandExpression{
public:
 AddExpression( Expression* lhs, Expression* rhs )
     :TwoOperandExpression( lhs, rhs ){}

 virtual double value(){return m_lhs->value() + m_rhs->value();}
};

class MulExpression : public TwoOperandExpression{
public:
 MulExpression(Expression* lhs, Expression* rhs)
     :TwoOperandExpression( lhs, rhs ){}

 virtual double value(){return m_lhs->value() * m_rhs->value();}
};

int main(void)
{// Expression x(3), y(4), z(5);
 MulExpression xy(3,4), yz(5,9);
 AddExpression p;

 cout << xy.value() << "\n";
 p.add( xy );
 cout << "pvalue=" <<p.value() << "\n";
 p.add( yz );
 p.add( z );
 cout << p.value() << "\n";
 p.add(xy);
 return 0;
}