Any Tips on learning C++??

**Elysia** · 07-01-2010

You are aware that those lines do absolutely nothing?

**cuo741** · 07-01-2010

Originally Posted by Elysia

You are aware that those lines do absolutely nothing?

Well i created member variables so that the user can assign values to them.

**Elysia** · 07-01-2010

Yes, but those specific lines of code do exactly nothing. They serve no purpose whatsoever.
You created those variables in your class definition. And you assign them later on in the code.

**cuo741** · 07-01-2010

Originally Posted by Elysia

Yes, but those specific lines of code do exactly nothing. They serve no purpose whatsoever.
You created those variables in your class definition. And you assign them later on in the code.

Oh i think i know what you mean, are you saying that i can just take those lines out and the program would still work fine since those member variables are already defined in the class definition?

**Elysia** · 07-01-2010

Yes, that's right.

**cuo741** · 07-01-2010

Ahhhh thanks a lot :], hey i was wondering if you could explain public and private members and functions to me please?im reading my book right now but, i dont really get it.

**Elysia** · 07-01-2010

Let's take the car analogy again.
Your car has lots of components. An engine, an electrical system and who knows what else.
But the engineers of the car don't want you to go poking around in its internals, right? You hit the gas pedal and the car accelerates. You don't go around poking in the injectors or such.
Therefore, it would be convenient to allow you to use the gas pedal, but not access the injectors and whatnot.
We call the pedals the the interface. The functions that you use to use the functionality of the object. These functions are public.
The other stuff, like the injectors and things that shouldn't be accessed by you are private.

So to translate that into C++, basically you create an object from a class. Then you use its members. But we only want the users of the class to access certain members. So the members they can access are public, and those that they cannot are private.

Code:

class MyClass
{
public:
    void foo() {}
private:
    void bar() {}
};

MyClass m;
m.foo(); // OK: foo is public.
m.bar(); // Compile error: bar is private.

**cuo741** · 07-01-2010

Originally Posted by Elysia

Let's take the car analogy again.
Your car has lots of components. An engine, an electrical system and who knows what else.
But the engineers of the car don't want you to go poking around in its internals, right? You hit the gas pedal and the car accelerates. You don't go around poking in the injectors or such.
Therefore, it would be convenient to allow you to use the gas pedal, but not access the injectors and whatnot.
We call the pedals the the interface. The functions that you use to use the functionality of the object. These functions are public.
The other stuff, like the injectors and things that shouldn't be accessed by you are private.

So to translate that into C++, basically you create an object from a class. Then you use its members. But we only want the users of the class to access certain members. So the members they can access are public, and those that they cannot are private.

Code:

class MyClass
{
public:
    void foo() {}
private:
    void bar() {}
};

MyClass m;
m.foo(); // OK: foo is public.
m.bar(); // Compile error: bar is private.

So i dont get why object "m" cant access m.bar(), isnt it an instance of class MyClass?

**Elysia** · 07-01-2010

"m" isn't trying to access bar(); you, the user, is trying to access it. And you are not "m". Therefore it yields a compile error.
OTOH, this works:

Code:

class MyClass
{
private:
    void bar() {}
public:
    void foo() { bar(); }
};

MyClass m;
m.foo(); // OK: foo is public.

It works because the call to bar is inside foo which is inside the class. So basically, if MyClass is the car, you've basically told it to accelerate and it calls appropriate functions inside itself to make this happen.
But you can't tell it to pump more gasoline directly (as the user), because that function is private. OTOH, the car can do it since it's its internals.

**cuo741** · 07-01-2010

so what im wondering is who CAN access private members then?

**cuo741** · 07-01-2010

Oh wait so im reading in my book, private functions can only be called upon by other functions in the class?

**laserlight** · 07-01-2010

Originally Posted by cuo741

Oh wait so im reading in my book, private functions can only be called upon by other functions in the class?

And friend functions, and member functions of friend classes.

**cuo741** · 07-01-2010

Originally Posted by laserlight

And friend functions, and member functions of friend classes.

What do you mean friend functions??

**Elysia** · 07-01-2010

A class can say "this function is my friend". They are called friend functions. Similarly to classes. I'm sure your book will discuss them in due time.