Problem casting derived class pointer to base class pointer

[littleneutrino]

Dear All,

I have basically the following situation:

int main(int argc char* argv[])
{

// vectors that will contain pointers to BaseBox objects
vector<cBaseBox*> BaseVector;

// pointer to cDerivedBox object
cDerivedBox pDerBox = new cDerivedBox;

// initialization
pDerbOX->name = "Big Box";
pDerBox->weight = 10;
pDerBox->height = 10;
pDerBox->color = "white";
pDerBox->thickness = 0.5;

// pointer to a BaseBox object
cBaseBox* pBasBox;

// casting of derived class pointer to base class pointer
pBasBox = dynamic_cast<cBaseBox*>(pDerBox);

// include pointer to base object in vector
BaseVector.push_back(pBasBox);

return 0;

}

class cBaseBox
{
private:
string name;
double weight;
double height;
}

class cDerivedBox : public cBaseBox
{
private: 
string color;
double thickness;
}

I am getting the following runtime error:

Unhandled exception at 0x777e15de in Boxes.exe: 0xC0000005: Access violation reading location 0xfeeefee2.

I have pinpointed the problem to the line

BaseVector.push_back(pBasBox);

So in essence there seems to be a problem when casting my derived class pointer to a base class pointer. While debugging I looked at the base pointer after casting, and my guess is that it would have been stripped of those fields that correspond to the derived class; however that is not happening.

Could you please tell me what am I doing wrong?

Cheers,
Little

[coder777]

pDerBox is not a pointer.
on line 8: cDerivedBox *pDerBox = new cDerivedBox; // Note that *

[littleneutrino]

My bad: transcription error. The code should look like

int main(int argc char* argv[])
{

// vectors that will contain pointers to BaseBox objects
vector<cBaseBox*> BaseVector;

// pointer to cDerivedBox object
cDerivedBox* pDerBox = new cDerivedBox;

// initialization
pDerbOX->name = "Big Box";
pDerBox->weight = 10;
pDerBox->height = 10;
pDerBox->color = "white";
pDerBox->thickness = 0.5;

// pointer to a BaseBox object
cBaseBox* pBasBox;

// casting of derived class pointer to base class pointer
pBasBox = dynamic_cast<cBaseBox*>(pDerBox);

// include pointer to base object in vector
BaseVector.push_back(pBasBox);

return 0;

}

class cBaseBox
{
private:
string name;
double weight;
double height;
}

class cDerivedBox : public cBaseBox
{
private: 
string color;
double thickness;
}

[Peter87]

Please post real code.

[vlad from moscow]

The code you showed shall not be compiled because the derived class has no access to private members of the base class.
For example this statement

pDerbOX->name = "Big Box";

is invalid because name is a private data member of the base class.

[littleneutrino]

Here is a minimum working example that reproduces the problem, straight from VS 2005:

/* 
	Main: PLAYING WITH BOXES
*/

#include "stdafx.h"
#include <iostream>
using namespace std;

#include <vector>
#include <string>

#include "Warehouse.h"

int _tmain(int argc, _TCHAR* argv[])
{
	int nRetCode = 0;
	bool ret;
	
	string Name = "test01";

	CWarehouse Warehouse(Name);
	ret = Warehouse.Load();
	if (ret == false){
		fprintf(stderr, "Loading error..\n");
		return 1;
	}
	
	return nRetCode;
}

//
// Warehouse.h
//

#pragma once
#include <string>
#include <vector>
using namespace std;

#include "BaseBox.h"
#include "DerivedBox.h"

class CWarehouse
{
public:

	CWarehouse(std::string);
	~CWarehouse(void);

	// data box name
	string BoxName;

	// Boxes vectors
	vector<CBaseBox*> BaseBoxes;
	vector<CDerivedBox*> DerivedBoxes;
	
	// number of boxes
	int NumBoxes;

	bool Load();

};

//
// Warehouse.cpp
//

#include <iostream>
using namespace std;
#include <string>

#include "Warehouse.h"
#include "BaseBox.h"
#include "DerivedBox.h"

// constructor
CWarehouse::CWarehouse(std::string name)
: NumBoxes(0)
{
    BoxName = name;

	BaseBoxes.clear();	
	DerivedBoxes.clear();
	
}

// destructor
CWarehouse::~CWarehouse(void)
{
	for(vector<CBaseBox*>::iterator it = BaseBoxes.begin(); it != BaseBoxes.end(); ++it)
		delete (*it);

	for(vector<CDerivedBox*>::iterator it = DerivedBoxes.begin(); it != DerivedBoxes.end(); ++it)
		delete (*it);
	
	BaseBoxes.clear();
	DerivedBoxes.clear();
}

bool CWarehouse::Load()
{
	//////////////////////////////////////
	/////////// load box data ///////////
	//////////////////////////////////////

	
	// pDerBox: pointer to derived box
	CDerivedBox* pDerBox = new CDerivedBox;

	// reads box name
	pDerBox->Name = "New Box";	

	// inserts pointer to derived class in list of derived boxes
	DerivedBoxes.push_back(pDerBox);

	// creates pointer to base class
	CBaseBox* pBasBox = new CBaseBox;

	// casts derived class pointer to base class pointer
	pBasBox = dynamic_cast<CBaseBox*>(pDerBox);

	// inserts pointer to base class in list of base boxes
	BaseBoxes.push_back(pBasBox);  // THIS LINE IS CAUSING THE PROBLEM
	
	// increases number of load devices
	NumBoxes++;

	return true;
};

//
// BaseBox.h
//

#pragma once
#include <string>

class CBaseBox
{
public:
	CBaseBox(void);
	~CBaseBox(void);

	// box name 
	std::string Name;
	
    // lid information
	int NumLids;
	
	double Capacity;
	double Weight;			
};

//
// BaseBox.cpp
//

#include "BaseBox.h"

CBaseBox::CBaseBox(void)
{
	Capacity = 0;
	Weight = 0.0;

    NumLids = 0;
}

CBaseBox::~CBaseBox(void){}

//
// DerivedBox.h
//

#pragma once
#include "BaseBox.h"
#include <string>

class CDerivedBox : public CBaseBox
{
public:
	CDerivedBox(void);
	~CDerivedBox(void);

	std::string color;
	double thickness;
};

//
// DerivedBox.cpp
//

#include "DerivedBox.h"

CDerivedBox::CDerivedBox(void): CBaseBox()
{
	thickness = 0;
}

CDerivedBox::~CDerivedBox(void){}

[Cubbi]

There are several memory management errors in this program:

1) A destructor of a base class must be either public and virtual or protected and non-virtual:
replace
~CBaseBox(void);
with
virtual ~CBaseBox(void);

2) Do not delete objects twice
At minimum, replace
pBasBox = dynamic_cast<CBaseBox*>(pDerBox); // this pointer points at the same object as pDerBox
with
pBasBox = dynamic_cast<CBaseBox*>(new CDerivedBox); // this is a different Derived Box
(your CWarehouse attempts to call delete on every pointer it holds, and you gave it both pointers)

3) Don't leak memory:
replace
CBaseBox* pBasBox = new CBaseBox;
with
CBaseBox* pBasBox;

(P.S dynamic_cast is not necessary in this case, you can just assign directly:
pBasBox = new CDerivedBox;
)

[vlad from moscow]

The code you showed shall not crash in the statement you marked

BaseBoxes.push_back(pBasBox); // THIS LINE IS CAUSING THE PROBLEM

in spite of the memory leak produced by the statements

CBaseBox* pBasBox = new CBaseBox;
pBasBox = dynamic_cast<CBaseBox*>(pDerBox);


It is the destructor that leads to the crash

CWarehouse::~CWarehouse(void)
{
	for(vector<CBaseBox*>::iterator it = BaseBoxes.begin(); it != BaseBoxes.end(); ++it)
		delete (*it);

	for(vector<CDerivedBox*>::iterator it = DerivedBoxes.begin(); it != DerivedBoxes.end(); ++it)
		delete (*it);
	
	BaseBoxes.clear();
	DerivedBoxes.clear();
}

You are trying to delete the same pointer twice. The first time in vector<CBaseBox *> and the second time in vector<CDerivedBox *>.
As Cubbi said already there is no need to use dynamic_cast. And you shall declare the destructor as virtual.

[littleneutrino]

Thank you very much guys, you nailed the problem!

Cheers,
Little