Organizing classes 3/3

Modularized class

In previous post, I show the design of composite class.
It looks almost nice but it' a bother to write/use accessor functions to each component.

The third idea here I introduce is something I call 'modularized class'.

class Object
{
public:
    virtual ~Object() {} // make Object polymorphic.
};

// component classes deriving Object 'virtual'ly.
class Input : public virtual Object
{
    ...
};
class AIParam : public virtual Object
{
    ...
};
class Position : public virtual Object
{
    ...
};
class Health : public virtual Object
{
    int health;
public:
    void apply_damage(int damage) {

        // when this object inherits TimeStatus, calcurate damage by the status
        TimeStatus* s = dynamic_cast<TimeStatus*>(this);
        if( s ) {
            // do some culcuration.
        }
        health -= damage;
    };
};
class TimeStatus : public virtual Object
{
    ...
};

template <typename... Modules>
class ModularObject : public Modules...
{
};

// Player, Enemy, Obstacle class inherits each component class.
// template class ModularObject is used for utility.
typedef ModularObject<Input, Position, TimeStatus, Health> Player;
typedef ModularObject<AIParam, Position, TimeStatus, Health> Enemy;
typedef ModularObject<Position,Health> Obstacle;

// use like this. (using template)
template <typename T>
void attack(int damage, T* obj)
{
    // when obj does not inherit Health, this gets compile error.
    obj->apply_damage(damage);
}

// or use dynamic_cast
void attack(int damage, Object* obj)
{
    // when obj does not inherit Health, nothing happens.
    Health* h = dynamic_cast<Health*>(obj);
    if( h ) {
        h->apply_damage(damage);
    }
}

This approach uses multiple inheritance. and virtual inheritance which solves 'diamond problem'.

It's easy to add/remove features, and can access components directory.

I think this is the best solution for organizing class for game object.

Organizing classes 2/3

Using composition

In the previous post, we tried to implement some features of game object by class inheritance, but it was not so good for maintenance.

Now, we will try to solve this problem by 'composite class'

// base class has interface to get all components, returning nullptr by default.
class Object
{
public:
    virtual Input* input() { return nullptr; }
    virtual AIParam* ai_param() { return nullptr; }
    virtual Position position() { return nullptr; }
    virtual Health* health() { return nullptr; }
    virtual TimeStatus* time_status() { return nullptr; }
};

// player class has actual component and returns its pointer.
class Player : public Object
{
    Input input;
    Position position;
    Health health;
    TimeStatus time_status;    
public:
    virtual Input* input() { return &input; }
    virtual Position position() { return &position; }
    virtual Health* health() { return &health; }
    virtual TimeStatus* time_status() { return &time_status; }
};

// enemy class is same as Player except first field.
class Enemy : public Object
{
    AIParam ai_param;
    Position position;
    Health health;
    TimeStatus time_status;    
public:
    virtual AIParam* ai_param() { return &ai_param; }
    virtual Position position() { return &position; }
    virtual Health* health() {  return &health; }
    virtual Health* time_status() { return &time_status; }
};

// obstacle class has Position and Health.
class Obstacle : public Object
{
    Position position;
    Health health;    
public:
    virtual Position position() { return &position; }
    virtual Health* health() {  return &health; }
};

// Health component, implements apply_damage()
class Health
{
    Object* owner;
    int health;
public:
    void apply_damage(int damage) {
        if( owner->time_status() ) {
            // calc damage by status;
        }
        health -= damage;
    }
};

In this design, A derived classes is composed by feature classes(i.e., component) and has accessor function to each component.
Each component has pointer to Object in order to access other component.

for example, attack() function will look like this.

void attack(int damage, Object* obj)
{
    if( obj->health() ) {
        obj->health()->apply_damage(damage);
    }
}

Comparing previous design(using inheritance), it's easier to add/remove features.
But accessing components by getter function is not looks pretty.

Now, I propose an other idea in next post.

Organizing classes 1/3

In most of game programs, It is the common problem that how to organize classes.

We have a class Player, Enemy, Obstacle, Bullet, ...and these objects' features are partially same, and partially different.And if it is a online game, it updates repeatedly.

Using inheritance

Let's dive into code.

// the Base class of all. has position data.
class Object
{
    int pos_x;
    int pos_y;
};

// Player class has input data, inherits Object.
class Player : public Object
{
    float input_dir;
};

// Enemy class has some kind of AI parameter, also inherites Object.
class Enemy : public Object
{
    int ai_param;
};

// Obstacle class has no data here, and inherits Object.
class Obstacle : public Object
{
};

Here we declare Object, Player, Enemy, Obstacle class.
In the sense of 'Inheritance', this organization seems meaningful.

Next, let's think to add data 'poisoned' which is bool value and represents the object get poisoned.
Well, what kind of object can get poisoned??
In this case, Player and Enemy. Obstacle can not get poisoned.

// Player class has input data, inherits Object.
class Player : public Object
{
    float input_dir;
    bool poisoned;
};

// Enemy class has some kind of AI parameter, also inherits Object.
class Enemy : public Object
{
    int ai_param;
    bool poisoned;
};

// Obstacle class has no data here, and inherits Object.
class Obstacle : public Object
{
};

Here we met some trouble... 'bool poisoned' is duplicated. This means poison-related implementation will duplicate too.
And in the future, when it comes to add 'cursed' or 'paralyzed', we will get same problem.

One idea to solve this problem is to create new base class.

//Living Object may got poisoned, paralyzed, ... and some other status.
class LivingObject : public Object {
{
    bool poisoned;
    bool paralyzed;
    ...
};

class Player : public LivingObject
{
    float input_dir;
};

class Enemy : public LivingObject
{
    int ai_param;
};

class Obstacle : public Object
{
};

Creating new class and make existing classes inherit the new class...easy! ...really?

Additionally, what if we need to declare data 'health'?
Ok, LivingObject has health.....No. It's not good. because we need 'health' for Obstacles in order to get destroyed by others.

Well, how can we re-organize the class tree !?
It's painful to re-organize the class tree. And if it's already released product, it will take big risk.

=> to be continued to next post.

What is OOP(Object Oriented Programmming)?? -- the real meaning in C++ - 2/2

In previous post, We learn meanings of Class, Methods in C++,which is the set of data and how to process that.

And see the simple video game's Player class in C++ like

class Player
{
private:
    int pos_x;
    int pos_y;    

public:
    void apply_input(float direction);
    void move_to(int x, int y);
};

Ok next, we will make class Enemy.

class Enemy
{
private:
    int hate;
    int pos_x;
    int pos_y;    

public:
    void apply_hate(int hate);
    void move_to(int x, int y);
};

Class Enemy does not require 'apply_input()' method because it will not move by joystick input.
And Enemy has hate data(declared as int to make it simple), which will used for enemy's behaviour.

mmm... It seems to be OK, but the Player and Enemy has same data and methods.
It's waste of codes.

Let's think about 'pos_x' and 'pos_y'.
These are attributes of all object in the game.
So we declare Object class which have position data and related method.

class Object
{
private:
    int pos_x;
    int pos_y;
public:
    void move_to(int x, int y);
};

and the class Player and Enemy 'Inherits' the class Object.

class Player : public Object
{
public:
    void apply_input(float direction);
};

class Enemy : public Object
{
private:
    int hate;
public:
    void apply_hate(int hate);
};

Inheritance in C++ means addition of data and methods.
In this example, Object has position of object.
And in addition, Player has method 'apply_input', Enemy has data 'hate' and a method 'apply_hate()'.

Inheritance is usable when some code is common in some classes.

What is OOP(Object Oriented Programmming)?? -- the real meaning in C++ - 1/2

Object Oriented Programming(OOP) -- when you start to learning programming, this may the first paradigm you will meet. And most explanations of OOP I saw in books or websites, does not explain the real meaning of OOP.

You may see this sort of explanation.

Animals ----- Birds
         +- Fishes
         +- Mammals

'Animals','Fishes','Birds','Mammals' are Class. 'Animals' is a parent Class of other ones. and Birds has a method of 'fly', Fishes has a method of 'swim', and so on.

With this kind of explanation, You can understand the meanings of words 'Class' or 'Method' in abstract concept. But you can't learn the real meanings of C++.

In C++, the 'Class' means the set of data and how to process that.
'How to process that?' is the Class Method.

Let's see some example.
In video game, a player is a typical object.
the Player class is declared like this.

class Player
{
private:
    int pos_x;
    int pos_y;    

public:
    void apply_input(float direction);
    void move_to(int x, int y);

};

You see the data 'pos_x','pos_y' and methods 'apply_input()','move_to()'.
As you can easily understand, 'pos_x' and 'pos_y' means position of the player.
The method 'apply_input()' applies input data of joystick, 'move_to()' moves the player to specified position.

Simple, isn't it !?

For comparision, let's see the C language implementation of this Player.

struct Player
{
    int pos_x;
    int pos_y;
};

void apply_input(struct Player*, float direction);
void move_to(struct Player*, int x, int y);

In C, struct can not have methods. so we must declare functions outside of the struct, receiving pointer to the struct.

so, class in C++ is the set of data and how to process it.

Would you like to learn more?
Visit the next post.