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Pointers in C++

Posted on 2004-08-16
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Last Modified: 2010-04-01
Hi

My problem is to do with pointers. What I am trying to do is pass a pointer to a function defined in a seperate file. That function is then going to create an object which the pointer will then reference. Is this at all possible? At the moment, I seem to be getting back nothing from the function, or it is not changing the pointer that I'm passing. Basically it goes like this:


object *obj;

define_pointer(obj);


in the other file:

int define_pointer(object *obj)
{
      obj = create_and_initialise_object(); // this function does return a pointer to the proper object
}

I'm probably doing something stupid, like scoping issues or something, but I don't know what it is.

For some more info: I'm doing this for a DirectDraw project , and the pointer that I'm passing is a LPDIRECTDRAWSURFACE7, and I've got a function which creates the surface and passes a pointer back.

Any help you could provide would be much appreciated.

Thanks
phoenix260182
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Question by:phoenix260182
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jkr earned 500 total points
ID: 11812006
>>I'm probably doing something stupid, like scoping issues or something, but I don't know what it is.

No, for the above, you either need a pointer to a pointer or a reference to a pointer, e.g.

int define_pointer(object **obj)
{
     *obj = create_and_initialise_object(); // this function does return a pointer to the proper object
}

or

int define_pointer(object*& obj)
{
     obj = create_and_initialise_object(); // this function does return a pointer to the proper object
}

The above will work.
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LVL 86

Expert Comment

by:jkr
ID: 11812028
BTW, in your originaly code

int define_pointer(object *obj)
{
     obj = create_and_initialise_object(); // this function does return a pointer to the proper object
}

only the 'local' value of 'obj' will change, i.e. point to the proper object. Once the function returns, the variable in the calling code will remain unchanged, since you are passing the pointer 'by value'. My examples above illustrate how to pass 'by reference', so that the contents of the data passed can be changed by the called function.
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Expert Comment

by:Axter
ID: 11812443
FYI:

Most interface are setup so that the pointer is in the return value.
Example:

object *define_pointer(void)
{
    return create_and_initialise_object();
}

object *obj = define_pointer();

This is a cleaner method, and it's more portable with older compilers.
This method also allows you to create and initialize the object at the same time.
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