Polymorphism runtime errors

[hornet07]

I've been reading up on polymorphism and tried a sample code. It builds with no errors, but on run it opens a new window that displays this :Unhandled exception at 0x00AF7250 in polymorphism.exe: 0xC0000005: Access violation reading location 0xCCCCCCCC. Funny thing is, even though it says this, it still displays a part of the output correctly. I'll post the code first and then the output.

// header.h


#ifndef HEADER_H
#define HEADER_H

#include <string>
using std::string;

class Shape
{
public:
	virtual string type();
	virtual float area();
	virtual float perimeter();
};

class Circle : public Shape
{
public:
	Circle(float rad);

	string type();
	float area();
	float perimeter();

protected:
	float mRadius;
};

class Rectangle : public Shape
{
public:
	Rectangle(float w, float l);

	string type();
	float area();
	float perimeter();

protected:
	float mWidth;
	float mLength;
};


#endif // HEADER_H




// implementation.cpp

#include "header.h"


string Shape::type()
{
	return "undefined";
}

float Shape::area()
{
	return 0.0f;
}

float Shape::perimeter()
{
	return 0.0f;
}

Circle::Circle(float rad)
	: mRadius(rad)
{}

string Circle::type()
{
	return "Circle";
}

const float PI = 3.14f;

float Circle::area()
{
	return PI * mRadius * mRadius;
}

float Circle::perimeter()
{
	return PI * 2 * mRadius;
}


Rectangle::Rectangle(float w, float l)
	: mWidth(w), mLength(l)
{}

string Rectangle::type()
{
	return "Rectangle";
}

float Rectangle::area()
{
	return mWidth * mLength;
}

float Rectangle::perimeter()
{
	return 2 * mWidth + 2 * mLength;
}




// main.cpp

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


int main()
{
	Shape* shapes[10];

	shapes[0] = new Circle(1.0f);
	shapes[1] = new Circle(2.0f);
	shapes[2] = new Circle(3.0f);
	shapes[3] = new Circle(4.0f);
	shapes[4] = new Circle(5.0f);

	shapes[0] = new Rectangle(1.0f, 2.0f);
	shapes[1] = new Rectangle(2.0f, 4.0f);
	shapes[2] = new Rectangle(3.0f, 1.0f);
	shapes[3] = new Rectangle(4.0f, 6.0f);
	shapes[4] = new Rectangle(5.0f, 2.0f);

	for (int i = 0; i < 10; ++i)
	{
		string type = shapes[i]->type();
		float area = shapes[i]->area();
		float peri = shapes[i]->perimeter();
		cout << "Shape[" << i << "]'s ";
		cout << "Type = " << type << ", ";
		cout << "Area = " << area << ", ";
		cout << "Perimeter = " << peri << endl;
	}

	// Delete memory
	for (int i = 0; i < 10; ++i)
	{
		delete shapes[i];
	}


	cin.ignore();
	cin.get();
	return 0;

} // end main 

Output is this:
Shape[0]'s Type = Rectangle, Area = 2, Perimeter = 6
Shape[1]'s Type = Rectangle, Area = 8, Perimeter = 12
Shape[2]'s Type = Rectangle, Area = 3, Perimeter = 8
Shape[3]'s Type = Rectangle, Area = 24, Perimeter = 20
Shape[4]'s Type = Rectangle, Area = 10, Perimeter = 14

So it correctly outputs the Rectangle stats but not the Circle ones, which should be first.

Something of note: Originally, the base class(Shape) constructor and methods implementations were in the header file and I got a ton of linker errors on build(strange). After I moved them with the other implementations in the implementation.cpp there were no build errors. Might be related.

PS. Somewhat off-topic, the buttons for code and output don't work anymore. I don't know how to manually format text into output.

[Peter87]

You are storing the rectangles at the same array indices as the cirlces so the shapes array ends up with 0-4 pointing to the rectangles and 6-9 uninitialized.

Also, you should make the Shape destructor virtual otherwise deleting subclass objects through a Shape pointer will not work correctly.

[hornet07]

Can you provide a heavy metal plate with a bulls-eye on it? I want to smash my head in it until I learn that copy paste needs a follow-up....