Re: Globals - C / C++ / MFC Discussion Boards

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Re: Heap, stack?

Alvaro Mendez11-Feb-02 12:10

Alvaro Mendez

11-Feb-02 12:10

Let's see if I can shed some more light on the subject.

There are basically two locations where memory can be allocated in C++: on the stack or on the heap.

The stack is used by the compiler mostly to allocate local variables whose size is well known at compile time. Every time you define a local variable (inside a function block), the compiler reserves a block of bytes for the size taken up by the variable. This is a very quick operation because all the compiler does is assign the address to the variable and move its "stack pointer" down by the size of the variable. This is called pushing the stack. When the variable goes out of scope the compiler does the opposite -- it pops the stack -- and the memory is reclaimed. Thus, memory allocated on the stack is automatically reclaimed by the compiler, and it's all very quick because it's all done in one location -- the end of the stack, if you will. The stack is also used whenever a function is called to hold its return address and its parameters (if any).

The heap is quite different. It's typically used to allocate variables that you plan to keep longer than duration of a function block, or variables whose size is not known until run-time (such as dynamic arrays). In C++, memory is taken from the heap via the new operator (or via the global C function "malloc"). When this happens, the heap manager looks for a block of memory large enough to hold the contents of your variable and returns the beginning address of that block. This block of memory stays allocated until your program calls the delete operator on that memory (or "free" if malloc was used) or until the program ends. By the way, allocating memory on the heap is much slower than allocating from the stack, so the stack should be used if the variable will not be needed outside the block where it was allocated and its size is known at compile-time. In addition, since the stack's memory is automatically reclaimed, it requires less housekeeping on the part of the programmer (to prevent memory leaks). However, the stack should ONLY be used when the variable will not be referenced outside the block where it was allocated, otherwise you'll be left with an invalid memory address which when used will cause the program to crash.

I hope to have cleared things up.

Regards,
Alvaro

Behind a beautiful woman there's usually a guy who just couldn't wait to get rid of her.

Globals

alex.barylski9-Feb-02 22:01

alex.barylski

9-Feb-02 22:01

How can I do the following, but correctly? Frown | :(

LPSTR fileName = "data.dat";

class MyClass{
  MyClass(){ CFile myFile(fileName, BLAH); }
  ~MyClass(){}
};

#define AfxSetFileName(file) (fileName = file)

I don't think I'm missing anything. Anyways i'm getting a linker error fileName already defined. Somebody wanna enlighten me and explain why...?

It's all in the one header file, if this makes a difference.

Also how do I prevent name collisions with third party libs using namespace on the global variable...? Confused | :confused:

"An expert is someone who has made all the mistakes in his or her field" - Niels Bohr