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> More Advanced C++, Polymorphic / Abstract / Virtual example
qwijibow
post May 6 2005, 01:21 PM
Post #1


Way Out Of Control - You need a life :)
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This example assumes you already know basic c++ functions and classes.
it is by no means a complete c++ tutorial, but should give you a good idea of the power of c++.

In this tutorial, i will be using virtualisation, abstraction, and polymorphism to draw a simple picture.
the code is not complete, because i dont want to tie it into any specific operating system.
i will use onlt standard template library finctions, and graphics requires the use of OS specific API's/

We are going to write a program, which holds an array or different shapes.
and (if complete) would draw thenm ot the screen.. (for this example, the will just output somthing like "I am a circle, radius 12, x=20, y=50"

It doesnt sound impressive, but we will be using a single array to hold different classes without using a container. In other words, its like having an array that holds some integers, some booleans, and some strings. it is normally not allowed.

first, before the code, some simple definitions.

=============================
Virtual functions:
=============================
A virtual function is a function which apears to exist to some parts of the code, but aprears not to exist to other parts of the code.

a Pure virtual function is the same, but has no function body. if a class has at least one pure virtual function, then it is considered abstract. meaning it cannot be used directly, but can be used as a template for other functions.

Take a look at the following example program

CODE

#include<iostream>
using namespace std;

class base_class {
public:
void function1() {
 cout << "function1 of base_class" << endl;
}

virtual void function2() {
 cout << "function2 of base_class" << endl;
}
};

class sub_class : public base_class {
public:
void function2() {
 cout << "function2 of sub_class" << endl;
}
};

class sub_class2 : public base_class {};

int main() {

base_class BASE;
sub_class SUB_A;
sub_class2 SUB_B;

BASE.function1(); // outputs "function1 of base_class"
BASE.function2(); // outputs "function2 of base_class"

SUB_A.function1(); // outputs "function1 of base_class"
SUB_A.function2(); // outputs "function2 of sub_class"

SUB_B.function1(); // outputs "function1 of base_class"
SUB_B.function2(); // outputs "function2 of base_class"

return 0;
}


virtual function2 in the base class apears to exist, except to the class sub_class, because this class defines its own function2, it over-rides the virtual function.

we could have made function2 a pure virtual function be declairing it like so....
CODE

virtual void function2() = 0;


a pure virtual function exists ONLY to be over-ridden.
you cannot declair an instance of a class containing pure virtual functions, and so if this had been the case, the line

CODE
base_class BASE;
would have caused a compile error.
a class containing pure virtual functions is called abstract.

=============================
Polymorphic objects. (the clever pointer)
=============================

In C++, the pointer rules have been relaxed.
a pointer of type XYZ can point to any class derived from XYZ

the following code example is perfectly legal in c++

CODE

class commom{};

class tree : public common{};
class bird : public common{};
class car : public common{};
class pan_galactic_gargle_blaster : public common{};

int main() {

common* array[5];

array[0] = new common;
array[1] = new tree;
array[2] = new bird;
array[3] = new car;
array[4] = new pan_galactic_gargle_blaster;

return 0;
}


=====================================================
MAIN example, combining the 2 examples above.
======================================================
Armed with the ability to have an array of different types, and the ability to over ride functions,
we can do some very clever things.

we are going to write a simpe drawing program.
it will be an array of different shapes, the base class will have a draw() function
which draws different shapes, depending on how it is used. (polymorphic, to change shape! lol )

// include the usual stuff.
CODE

#include<iostream>
#include<vector>

using std::vector;
using std::cout;
using std::cin;


now we need our common base class
CODE

class shape {

protected:
int x_position;
int y_position;
int width;
int height;

public:
virtual void draw() = 0;

void setX(int x) {
 x_position = x;
}

void setY(int y) {
 y_position = y;
}

void setW(int w) {
 width = w;
}

void setH(int h) {
 height = h;
}
};


This is ab abstract class because it contains a pure virtual function draw.
the size variables, and manipulation functions are common to all shapes, so they go in the base class.

now lets add some shapes/
CODE

class triangle : public shape {

public:
void draw() {

 cout << "i am a triangle" << endl;
 cout << " height:" << height << endl;
 cout << " width:" << width << endl;
 cout << " X:" << x_position << endl;
 cout << " Y:" << y_position << endl;
}
};

class circle : public shape {

public:
void draw() {

 cout << "i am a circle" << endl;
 cout << " radius:" << height << endl;
 cout << " X:" << x_position << endl;
 cout << " Y:" << y_position << endl;
}
};

class rectangle : public shape {

public:
void draw() {

 cout << "i am a rectangle" << endl;
 cout << " height:" << height << endl;
 cout << " width:" << width << endl;
 cout << " X:" << x_position << endl;
 cout << " Y:" << y_position << endl;
}
};

class dot : public shape {

public:
void draw() {

 cout << "i am a dot" << endl;
 cout << " X:" << x_position << endl;
 cout << " Y:" << y_position << endl;
}
};


each class needs to override the pure virtual function in the shape class.
if a class failed to override the draw() function, then that class would in turn become abstract.

finally, let se out super cool porgram in action.

CODE

int main() {

vector<shape*> shapes;

shapes.push_back( new triangle() );
shapes.push_back( new circle() );
shapes.push_back( new rectangle() );
shapes.push_back( new dot() );
shapes.push_back( new triangle() );
shapes.push_back( new circle() );
shapes.push_back( new rectangle() );
shapes.push_back( new dot() );

// set all the shapes member variables (idealy, use random)
for(int n=0; n< shapes.size(); n++) {
 shapes[n]->setX(1);
 shapes[n]->setY(2);
 shapes[n]->setH(3);
 shapes[n]->setW(4);
}

// draw the picture !
for(int n=0; n< shapes.size(); n++) {
 shapes[n]->draw();
}

      // clean up, prevent memory leaks.
for(int n=0; n< shapes.size(); n++) {
 delete shapes[n];
}

return 0;
}


the vector MUST be a vector of pointers to shapes, and not a vector of shapes.
the the intelligent pointer that knows what functions to call.

and just for fun, the output of the compiled program.
QUOTE
bash-2.05b$ g++ TEST.cpp -o TEST
bash-2.05b$ ./TEST
i am a triangle
        height:3
        width:4
        X:1
        Y:2
i am a circle
        radius:3
        X:1
        Y:2
i am a rectangle
        height:3
        width:4
        X:1
        Y:2
i am a dot
        X:1
        Y:2
i am a triangle
        height:3
        width:4
        X:1
        Y:2
i am a circle
        radius:3
        X:1
        Y:2
i am a rectangle
        height:3
        width:4
        X:1
        Y:2
i am a dot
        X:1
        Y:2


Even advanced C++ isnt too complicated when you understand whats happening.

Any questions / comments / surgestions ?
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CarolinaBlues
post Nov 8 2005, 05:13 AM
Post #2


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Can you make it easier to understand, You explained it kind of confusingly

Notice from moonwitch:
don't quote an unneeded long quote.


This post has been edited by moonwitch: Nov 11 2005, 10:54 AM
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warbird
post Nov 8 2005, 05:58 PM
Post #3


Little MechBirdie
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Wow, great tutorial. I couldn't find any good tutorials yet, so I stopped trying to find good C++ resources on the net and focust on books and things. I must say I realy like this one. Could you also write us a tutorial about game programming in C++? Then I realy would be delighted tongue.gif.

-=jeroen=-
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qwijibow
post Feb 27 2006, 01:10 AM
Post #4


Way Out Of Control - You need a life :)
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From: Nottingham England
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QUOTE
Could you also write us a tutorial about game programming in C++


I can do the next best thing...
Post a link to a great game programming tutorial.

(well, not really a game programming tutorial, its an OpenGL tutorial, that uses games as its example programs. but still very good)

http://nehe.gamedev.net/
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