Pointers and Memory Allocation 3

[Watts]

Here is a dozy of a situation, say I declare a pointer to a structure like this:

The structure contains x and y integer values.

Struct* ptr;
ptr->x = 10;
ptr->y = 35;

Is this valid and why don't I need to use the malloc() function to reserve space for this structure and return a memory address? Is the compilier doing some behind the scenes work for me? Where does the data for this structure end up (stack/heap)?

 

[jtm111]

I believe it is not valid.

Since this is a C++ forum, I will advise you to use the new and delete operators not the malloc and free functions, as the latter are discouraged in C++.

You must allocate the pointer:

ptr = new Struct;
ptr->x = 10;
ptr->y = 35;

then destroy it when finished:

delete ptr;

I assume x and y are integers, not pointers. If they were pointers, you would be responsible for allocating them as well after the ptr object was initialized;

I believe the memory is allocated to the heap.

 

[IonelBurtan]

First of all if you do this you will get a C4700 compiler warning like:
warning C4700: local variable 'ptr' used without having been initialized

Then at run-time you will have an error of type:
the memory could not be "written". - pretty obvious why

If do not get the warning, check your ProjectSettings for warnings.

Hope this helps, s-)

Blessed is he who in the name of justice and good will, shepards the week through the valley of darkness...

http://www20.brinkster.com/ionelb

 

[Zyrenthian]

NOT VALID AT ALL. You definitely need to allocate the memory. When you declare the pointer you get some random memory address. With either a malloc or a new you get a specific memory address that has memory allocated. Without doing this, you will write to the initial memory address and possibly overwrite some important information.

Matt