Accessing data in a derived classes

[Tex08]

Again, I am trying to implement a composite pattern to handle a binary expression. Compiling with g++ (required), I receive the following error trying to access data in a derived class:

make
g++ -c main.cc
main.cc: In function `int main()':
main.cc:23: error: base operand of `->' has non-pointer type `BooleanExp'
*** Error code 1
make: Fatal error: Command failed for target `main.o'

Here is the code that I believe is relevant to the problem:
MAIN.CC

// Second attempt

#include <iostream>
#include "BooleanExp.h"
#include "Literal.h"
#include "Variable.h"
#include "Op_Negate.h"

using namespace std;


int main() {


BooleanExp * a, * b, * c;
a = new Literal(false);
b = new Variable("S", true);
c = new Op_Negate( *a );

cout << "Value of a: " << a->GetValue() << endl;
cout << "Name  of b: " << b->GetName()  << endl;
cout << "Value of b: " << b->GetValue() << endl;
cout << "Value of Negate: " << c->GetRight()->GetValue() << endl;
cout << "So far so good\n";

delete a, b, c;

}

LITERAL.H & LITERAL.CC

 // Will be Literal.h


#ifndef LITERAL_H
#define LITERAL_H

#include "BooleanExp.h"


class Literal : public BooleanExp  {
public:
	
	~Literal() {}
	Literal( bool );

	bool GetValue();

private:
	bool _value;

};
#endif

// Will be Literal.cc

#include <iostream>
#include "Literal.h"

using namespace std;

Literal::Literal(bool v) {
	cout << "In Literal constructor\n";
	_value = v;
	cout << "Liter Constructor _value =" << _value << endl;

};

bool Literal::GetValue() {
	return _value;
};

OP_NEGATE.H & OP_NEGATE.CC

// Will be Op_Negate.h


#ifndef OP_NEGATE_H
#define OP_NEGATE_H

#include<iostream>
#include "BooleanExp.h"

using namespace std;

class Op_Negate : public BooleanExp {
public:
	~Op_Negate() {}
	Op_Negate( BooleanExp );	

	virtual BooleanExp GetLeft();
	virtual BooleanExp GetRight();

private:
	BooleanExp _right;

};

#endif

// Op_Negate.cc


#include <iostream>
#include "Op_Negate.h"
#include "BooleanExp.h"

using namespace std;

Op_Negate::Op_Negate(BooleanExp x) {
	cout << "Op_Negate constructor\n";
	_right = x;
	};

BooleanExp Op_Negate::GetLeft() {};

BooleanExp Op_Negate::GetRight() {return _right;};

The problem lies in c->GetRight()->GetValue() statement, in attempt to access the derived class' data.

Thank you in advance for any suggestions.

Tex

[boxfish]

c->GetRight()->GetValue()

Op_Negate::GetR ight() does not return a pointer, so you can't use the arrow operator on it. Mabe you mean
c->GetRight().Get Value()
or maybe you want Op_Negate::GetR ight() to return the BooleanExp pointer &_right?

[Tex08]

My ultimate goal is to represent a boolean expression as a 'tree' using the Composite Pattern. I will need access to the item _right in order to implement an evaluation function via the visitor pattern.

So I need to figure out a way to access the _right item (there will be other classes that contain left and right elements for other operands). I need to access the derived classes private data to access the values stored there. Example:

c->GetRight()->GetValue()
~~~~~~~~~~ ---> trying to access the object returned by GetRight(), via the GetValue function for that class.

[boxfish]

There should be no problem accessing the Op_Negate class's private data through an accessor function like GetRight. That's not what the error is about. Your error says you can't use the arrow operator on something that's not a pointer. Either use the dot operator, or make GetRight return a pointer to _right, &_right, right?