pointers

[bbcc]

Could any one help me with the meaning of this:

TAxis *xaxis = h->GetXaxis();

Many thanks in advance

[Moschops]

It says:

Please make me a new object, called xaxis. The type of xaxis , the new object I want to make, is a pointer to a TAxis object. Please make my new object xaxis equal to what you get back from calling the function GetXaxis on the object pointed to by the pointer named h.

[zb0S216C]

In the posted code, xaxis is a pointer to a structure/variable of type TAxis. xaxis is initialized with the returning value of GetXaxis( ). h is another pointer. h's type is known to us as we can't see it's definition/declaration.

You can read upon pointers on this website^[1].

References:
^[1]http://www.cplusplus.com/doc/tutorial/pointers/

_Wazzak

[bbcc]

Thank you for your responses.
I have read the documention for pointers in cplusplus, but I still feel lost when i want to read and understand another code:(

I have a question from Moschops, isn't it like that to give an object an identity we should use new

So I mean the line above is only a declaration, not making the object xaxis, am I right?
Thank you again

[zb0S216C]

If a pointer is used to allocate a block of memory with new, the pointer would look like this:

// Allocate a block of memory which is the same size of TAxis.
TAxis *xaxis( new TAxis( h->GetXaxis( ) ) );

_Wazzak

[bbcc]

o.k. I think I should be more specific in my question,

I meant if we have already defined a class :

class TArrow{
int ArrowHeadSize;
void Draw();

}

and then I say:

TArrow a;

Does it mean that object a is being built from that class TArrow or I need to use the new methos to make an instance of my class?
Many thanks again

[zb0S216C]

You only use new if you want to dynamically allocate your instance. If you do dynamically allocate a instance of TArrow, all members of that instance are dynamically allocated as well. If the instance isn't dynamically allocated, the instance is created on the program's stack.

_Wazzak

[Moschops]

To reiterate m'learned chum Framework;

This makes an object of type TArrow, named a:
TArrow a;

A complete, fully made instance of a TArrow object, made on the piece of memory known as the stack. No need for new unless you want to make it on the heap (as a general rule, make it on the stack when you can, and on the heap when you must).

[bbcc]

Could you please tell me more about Stack and Heap part of the memory.

As far as I know, heap relates mostly to the pointers and if we want to give more permanence to an object we intrduce it in heap.

Thanks again

[bbcc]

I have another confusion as well that I really apprecite if you help me with that as well:

Here I have a class with the name Quad

Quad stack_quad(1,2,3);
Quad  *stack_ptr = &strack_quad

The thing that make me confuse is that if the pointer stack_ptr to the Quad class gets the memory address of the object stack_quad or the value in that memory address?

Thanks

[zb0S216C]

The Heap^[1] refers to the main memory. When we allocate a block of memory with new, malloc( ), or calloc( ), a specified amount of memory is taken from the heap. The heap is said to be slower than the stack.

The stack^[2] is a FILO( First In Last Out ) structure that has a default size of 1MB( 1024-Bytes ). Every program has one. Each object that is created is stored on the stack.

References:
^[1]http://en.wikipedia.org/wiki/Dynamic_memory_allocation
^[2]http://en.wikipedia.org/wiki/Stack_(data_structure)

_Wazzak

[zb0S216C]

bbcc wrote:

if the pointer stack_ptr to the Quad class gets the memory address of the object stack_quad or the value in that memory address?

When a pointer points to another object of the type, it points to the address of the object it was assigned to. When we dereference a pointer using the asterisk operator, we access the value stored in the address the pointer is pointing to.

_Wazzak

[LB]

A Quad pointer points to a Quad class, not the data members in it. You *could* make an int pointer to a Quad class if the first data member in the class was an int, but don't do that...some variables are private, and for a reason.

[bbcc]

To Framework, So you mean that:

1.Quad *stack_ptr
in this case stack_ptr is a pointer to the class Quad (points to the address of Quad)

2.Quad *stack_ptr = &strack_quad
in this case stack_ptr is a dereference operator, returns the value of strack_quad

3.Quad *stack_ptr = strack_quad
what about this case then? I think it does the same as number 2

[zb0S216C]

bbcc wrote:

in this case stack_ptr is a pointer to the class Quad (points to the address of Quad)

No. stack_ptr is an instance of Quad. stack_ptr can only point to another instance of Quad.

bbcc wrote:

in this case stack_ptr is a dereference operator, returns the value of strack_quad

No. In this case, you're creating an instance of Quad, and initializing it by making it point to the address of strack_quad. NULL is another initializing value of a pointer, which basically means the pointer is pointing to nothing. When you dereference a pointer, you reveal the value the pointer is pointing to. A structure cannot contain a value. A structure holds members. The members of a structure hold the values.

bbcc wrote:

what about this case then? I think it does the same as number 2

This will generate a compiler error. You forgot the address-of( & ) operator before strack_quad.

_Wazzak