[Solved] Question on class object

[zodiac976]

I have a question about a class containing an object of
another class inside it as private.

Example:

Code: Select all

class Class1
{
     public:
     private:
          Class2 class2;
};

class Class2
{
     public:
     private:
};

I think I see what it does but would using inheritance or
friendship be a better choice?

If anyone could explain the purpose of doing something
like this I would appreciate it.

[Scoody]

If you want to make a specialized version of a class, ie. extend the baseclass, you use inheritance.
Example:
Car - base class
Sportscar - specialized version, inherits the base of car and adds new functions and improves the existing

And one class using another, if you got a Wheel-class, then the previous Car-class would contain the wheel.

Look up "has-a is-a relationship" on google, might be along those lines what you're looking for.

[zodiac976]

I understand the use of inheritance and friendship, I was
just wondering why you would declare an object of another
class inside a class as a private member.

[Falco Girgis]

Dude, that should be way more common than inheritance and friendship. Don't let whatever OO guide you're reading make you think differently.

What if the class1 is a player, and class2 is a weapon? The player has a weapon. What if class1 is a GUI, and class 2 is a button? The GUI has a button. What if class1 is a dude, and class2 is his penis? The dude has a penis.

You should have noticed the "has a" relationship. If you can ever say that class1 "has" class2, you don't use inheritance. That's how you handle the scenario.

[zodiac976]

Dude, that should be way more common than inheritance and friendship. Don't let whatever OO guide you're reading make you think differently.

What if the class1 is a player, and class2 is a weapon? The player has a weapon. What if class1 is a GUI, and class 2 is a button? The GUI has a button. What if class1 is a dude, and class2 is his penis? The dude has a penis.

You should have noticed the "has a" relationship. If you can ever say that class1 "has" class2, you don't use inheritance. That's how you handle the scenario.

So I would use something like this to sorta connect
the classes together:

class Character
{
public:
private:
Weapon weapon;
};

class Weapon
{
public:
private:
};

Man I must have confused Inheritance or something...

Inheritance is like extending a base class right?
How would I access an object of a class inside
the class? Is it the same as accessing a regular
private variable?

The book I read wasn't that good I mostly learned
all this stuff through sitting down and doing trial
and error with some googling and also asking
questions.

I went through 2 classes in college of c++ and basically
had to teach myself this language. The instructor wasn't
very good and was too slow paced in my opinion but anyways...

[thejahooli]

What if class1 is a dude, and class2 is his penis? The dude has a penis.

Not always

[avansc]

What if class1 is a dude, and class2 is his penis? The dude has a penis.

Not always

you said you would keep that a secret ;(

[Innerscope]

Inheritance is like extending a base class right?
How would I access an object of a class inside
the class? Is it the same as accessing a regular
private variable?

It's the same as accessing on object. You're just doing it within that class.
Here is another example to help illustrate inheritance:
Animal - Base Class
Mammal - Extended Animal
Reptile - Extended Animal
Human - Extended Mammal

Do you see what is happening here? A human now contains all of the traits(functions and variables) of a mammal. Because mammal contains all of the traits of animal, a human does as well. The reptile is an extension of animal by itself and does not contain the traits of mammal nor human.
Hope that helps/makes sense

[dandymcgee]

Also, back to accessing private members:

See this.

[zodiac976]

Inheritance is like extending a base class right?
How would I access an object of a class inside
the class? Is it the same as accessing a regular
private variable?

It's the same as accessing on object. You're just doing it within that class.
Here is another example to help illustrate inheritance:
Animal - Base Class
Mammal - Extended Animal
Reptile - Extended Animal
Human - Extended Mammal

Do you see what is happening here? A human now contains all of the traits(functions and variables) of a mammal. Because mammal contains all of the traits of animal, a human does as well. The reptile is an extension of animal by itself and does not contain the traits of mammal nor human.
Hope that helps/makes sense

I understand inheritance now and how to access private variables
such as: int, double, string, etc but how would I access the private
object: Weapon weapon; inside class Character?

[dandymcgee]

I understand inheritance now and how to access private variables
such as: int, double, string, etc but how would I access the private
object: Weapon weapon; inside class Character?

The exact same way you access variables. What's your question?

Inside class Character you can make calls to weapon just like you'd access character from main:

Code: Select all

void Character::EquipWeapon()
{
     weapon.equip();
}

void Character::UnequipWeapon()
{
     weapon.unequip();
}

[zodiac976]

I understand inheritance now and how to access private variables
such as: int, double, string, etc but how would I access the private
object: Weapon weapon; inside class Character?

The exact same way you access variables. What's your question?

Inside class Character you can make calls to weapon just like you'd access character from main:

Code: Select all

void Character::EquipWeapon()
{
     weapon.equip();
}

void Character::UnequipWeapon()
{
     weapon.unequip();
}

I know but when I run it in working code I get type specifier errors
and of course weapon isn't a member of character error.

Code: Select all

class Character
{
public:
private:
Weapon weapon; //<---isn't a member of character and type specifier
};

class Weapon
{
public:
private:
};

I know how to make it a member but the declaration of it must
be wrong?

[avansc]

you have to declare your classes before you define them. especially if you use classes in classes.

[zodiac976]

you have to declare your classes before you define them. especially if you use classes in classes.

I tried declaring class Weapon first also but it had no effect.
I must be declaring it wrong or something...I will post a piece
in a minute to show you what I mean.

[zodiac976]

Code: Select all

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

class Weapon
{
	public:
		void dispWeapon();
	private:
		string weapName;
	protected:
};

void Weapon::dispWeapon()
{
	weapName = "knife";
	cout << weapName << endl;
}

class Character
{
	public:
		void aFunc();
	private:
		Weapon weapon;
	protected:
};

void Character::aFunc()
{
	cout << "aFunc\n";
}

int main()
{
	Character character;

	character.aFunc();

	character.weapon.dispWeapon();   //I know this is wrong but how am I supposed to access
							               //class Weapon through class Character
	return 0;
}

error C2248: 'Character::weapon' : cannot access private member declared in class 'Character'
see declaration of 'Character::weapon'
see declaration of 'Character'