Can handle class be used to simulate "Pointer" in C language?

If I define a "Ptr" class as following:

classdef Ptr < handle
    properties
        data
    end
end

then, objects of Ptr will behave like pointers in C language.

Is this true, guys?

[Shunchao Wu]

Why is the "Bytes" 0 in Variable Editor?

The "Bytes" of a field of a struct, or a property of a object, is 0.

Verified on 2008a and 1010b.

Answer 1

[Daniel Shub]

Objects of the handle class are not identical to pointers. When you pass an object to a function (whether it is a value class or a handle class), MATLAB does not immediately make a copy of the object. It is almost like it passes a pointer (but it is not a pointer). MATLAB has a complicated COW and tries not to copy the object unless it needs to. With a high level language of MATLAB you have to give up some of the control on memory management and hope MATLAB behaves in a reasonable manner.

[Shunchao Wu]

Thanks, Daniel

I agree that MATLAB has its own way to manage memory.

I find another interesting thing that graphics functions, e.g., figure(), return a double number, named graphic handle, but not a figure object.

And I can not figure out where the figure object stored. Is it sotred in currenct workspace? A persistent variable in some functions? Or in a memory segment whose adress only MATLAB knows?

[per isakson]

Undocumented Matlab at, http://undocumentedmatlab.com/ , might interest you.

[Shunchao Wu]

Great website!

Thanks, isakson.

Answer 2

[Titus Edelhofer]

Hi,

"then objects of Ptr will behave like pointers". If the behaviour you mean is that once you change a Ptr, all other variables that "point" to the same object are changed as well, then yes. Example:

x = Ptr;
y = x;  % both x and y "point" to the same object
x.data = 2; % now y.data is 2 as well

But I would say that "reference" like the & in C++ comes nearer to what handle classes are ...

Titus

[Shunchao Wu]

Thanks, Titus

I think Pointer is like handle classes more.

You can see it from following code:

>> n1=dlnode(1);

>> n2=n1;

>> n1=dlnode(2);

>> n1

n1 =

Doubly-linked list node with data:

2

>> n2

n2 =

Doubly-linked list node with data:

1

[Friedrich]

Titus is right here. Its a reference:

The handle class is the superclass for all classes that follow handle semantics. A handle is a reference to an object. If you copy an object's handle, MATLAB copies only the handle and both the original and copy refer to the same object data. If a function modifies a handle object passed as an input argument, the modification affects the original input object.

[Shunchao Wu]

Oh, I thought Titus reffered to the C++'s reference. My misunderstanding.

I agree that it's a MATLAB's reference, for C++'s reference cannot change its target.

[Titus Edelhofer]

hmm, why not? When you pass an object by reference to a function, the function may change the underlying/referenced object. Of course only, as long as you don't pass a const reference ... Or do I remember entirely wrong, since my C++ programming is about 10 years ago by now ;-).

[Shunchao Wu]

en, Titus is right.

The only difference between handle class and C++'s reference is, C++'s reference can not change to another value since initialization.