Object Oriented Programming Advanced Usage – Arma Reforger

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This guideline requires the understanding of Object Oriented Programming Basics.

Casting

Casting is the act of "presenting" a value as another type. For example, if a class hierarchy is Animal > Dog > Cocker, a dog is an animal, a cocker is a dog (that is an animal), but a dog is not especially a cocker.

class Animal {}
class Dog : Animal {}
class Cocker : Dog {}
class Labrador : Dog {}

Upcasting

Upcasting means seeing the class as one of its parents:

void Method()
{
	Cocker cocker = new Cocker();
	Animal animal = cocker;			// OK, as a cocker is an animal
	string sentence = cocker;		// error: cocker does -not- inherit from string
};

Downcasting

Downcasting means seeing a parent class as a specific child - this must be done by manually casting:

void Method(Dog dog)
{
	Cocker cocker1 = dog;				// error: "Dog" is too generic to be casted as Cocker - it could be e.g a Labrador
	Cocker cocker2 = Cocker.Cast(dog);	// OK: manual casting tells the code "the developer knows what he is doing"
										// if 'dog' is not castable as a Cocker, null is returned - the code does not crash

	Cocker cocker3;
	cocker3.Cast(dog);					// alternative method
}

Manual Casting

void Method()
{
	float value1 = 4.9;
	int value2 = value1;						// value2 = 4 as integer casting -truncates- the value, not rounds it
	string value3 = "result = " + (bool)value2;	// "result = true", as a non-zero integer is true when casted to bool
}

Template

A template is a class that allows a generic management for multiple types. Its methods cannot assume anything about the type.

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The generic type is by convention declared by the T letter.

// template class Item with generic type T
class Item<Class T>
{
	protected T m_data;

	void Item(T data)
	{
		m_data = data;
	}

	void SetData(T data)
	{
		m_data = data;
	}

	T GetData()
	{
		return m_data;
	}

	void PrintData()
	{
		Print(m_data);
	}
};

void Method()
{
	Item<string> stringItem = new Item<string>("Hello!");	// template class Item declared with type "string". In Item<string> class, all Ts are substituted with 'string'
	Item<int> intItem = new Item<int>(72);					// template class Item declared with type "int". In Item<int> class, all Ts are substituted with 'int'

	string_item.PrintData();	// prints "m_data = 'Hello!'"
	int_item.PrintData();		// prints "m_data = 72"
}