C++ Expression Templates

Listing 1: Source code for object-oriented technique

class AbstractExpr {
public:
   virtual double eval() const = 0;
};

class TerminalExpr : public AbstractExpr {
};

class NonTerminalExpr : public AbstractExpr {
};

class Literal : public TerminalExpr {
public:
   Literal(double v) : _val(v) {}
   double eval() const { return _val; }
private:
   const double _val;
};

class Variable : public TerminalExpr  {
public:
   Variable(double& v) : _val(v) {}
   double eval() const { return _val; }
private:
   double& _val;
};

class BinaryExpr : public NonTerminalExpr {
protected:
   BinaryExpr(const AbstractExpr* e1, const AbstractExpr* e2) 
     : _expr1(e1),_expr2(e2) {}
   virtual ~BinaryExpr () 
   { delete const_cast<AbstractExpr*>(_expr1); 
     delete const_cast<AbstractExpr*>(_expr2); 
   }
   const AbstractExpr* _expr1;
   const AbstractExpr* _expr2;
}; 

class Sum : public BinaryExpr {
public:
   Sum(const AbstractExpr* e1, const AbstractExpr* e2) 
     : BinExpr(e1,e2) {}
   double eval() const 
   { return _expr1->eval() + _expr2->eval(); }
};

...
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