Thread: Structures? *brain melts*

Originally posted by Bubba
Well anyways now that the interested party is confused thanks to our ranting and raving....next question MrDoomMaster and we will see if we can't confuse you more.



Just kidding man...really..we don't mind helping. We've all been there scratching our heads before....and still do.


edit: Man this post is growing exponentially. Can't keep up.


Looks like you got it man....but the 3D analogy lost me...so let's just forget you made it and move on.

LOL thanks man, I'm getting there! But yeah, I have an obsession with creating local variables to save memory... I like to free space whenever I'm not using a variable, I believe this is good programming technique. So, having a local structure in a module when you're only going to be USING it in that module would be healthy to ME, but I'm just a noob, so if someone has an argument to this please argue, because I'm in a "learning" mode

Bubba most certainly you do know your code, as all of you do. I was a bit lost when you were getting into the complex terminology, I'm not THAT knowledgeable yet!

Originally posted by Bubba
What? When did we start talking about headers? Now I think I'm confused.

He asked the question very discreetly. It was a comment within his code.

So, having a local structure in a module when you're only going to be USING it in that module would be healthy to ME, but I'm just a noob, so if someone has an argument to this please argue, because I'm in a "learning" mode

I'll take you up on that offer.

First, by module, do you mean the entire source file, or just the function int main()? If you mean the latter, don't say module, say function.

You can't have local structures in a function, that's not possible. It wouldn't make a difference to memory usage anyway, it's just the way c++ works.

Anyone think the Bloodshed Dev-C++ compiler is any good? Is there a better? Opinions welcome!

I have an obsession with creating local variables to save memory

It wont make a difference. The variable takes up the exact same amount of space wherever it is.

Originally posted by bennyandthejets
I'll take you up on that offer.

First, by module, do you mean the entire source file, or just the function int main()? If you mean the latter, don't say module, say function.

You can't have local structures in a function, that's not possible. It wouldn't make a difference to memory usage anyway, it's just the way c++ works.

well yes, I do mean functions... different blocks of code (for organizing data, to make the code more readable)

and by "local" I mean, once I'm done with using initialized variables from a structure, I want to free up all memory that was used by that structure and all of its variables. Seems only right if I wouldn't use it again. I have learned that Local variables only last till the end of the function or block of code. Prove me wrong again if you will, I still feel like I'm missing some things lol

Here goes...

Local variables, for functions that is, have a very limited lifetime and are not in memory at all. They are created on the stack and then removed from the stack when the function returns.

Code:

void FooBar(void)
{
  int x=50;
  int y=100;
  int z=150;
}

Here is the simplified pseudocode for this function:

Create stack space for x
Create stack space for y
Create stack space for z
Set x to 50
Set y to 100
Set z to 150
Remove z from stack
Remove y from stack
Remove x from stack
Restore stack pointer to where it was prior to entering function
Return to caller


I omitted some things for clarity's sake



x,y, and z only exist within this function. After that they have gone out of scope.

Originally posted by Bubba
Here goes...

Local variables, for functions that is, have a very limited lifetime and are not in memory at all. They are created on the stack and then removed from the stack when the function returns.

Code:

void FooBar(void)
{
  int x=50;
  int y=100;
  int z=150;
}

x,y, and z only exist within this function. After that they have gone out of scope.

what is "stack"?

Originally posted by Bubba
New is basically malloc(sizeof(object)).

It is similar in the manner that it allocates an object, yes. But the standard is not anything like this:

Code:

void *operator new(size_t size) {
 void *mem;
 mem = malloc(size);
 if(!mem) {
  bad_alloc ba;
  throw ba;
 }
 return mem;
}

The C++ standard of new allocates memory differently than malloc, so yes, they are similar at heart in that they allocate memory, but not similar in the way they do it

If you declare a variable within a function, and it is not static, then yes, it will be freed when the function exits. Note that by static I mean the following:

Code:

void func()
{
       static int a;
       int b;
       
       a++;
       b++;
}

The variable a is created once, and is only destroyed at the end of the program. That is what 'static' does. b however, is created and destroyed every time the function is called.

55 replies already? And you are getting any information out of all this nonsense Now you probably got bubba started with assembly stuff

Originally posted by MrDoomMaster
what is "stack"?

The stack is (hmm how to explain ) basically a list that can have items dynamically added / removed from it. The local variables are placed on the stack until the function returns, where the locals are removed from the stack.

EDIT: Like benny said, as long as they are not static variables.

Originally posted by bennyandthejets
If you declare a variable within a function, and it is not static, then yes, it will be freed when the function exits. Note that by static I mean the following:

Code:

void func()
{
       static int a;
       int b;
       
       a++;
       b++;
}

The variable a is created once, and is only destroyed at the end of the program. That is what 'static' does. b however, is created and destroyed every time the function is called.

OH so by default, all variables are set to be "destroyed" after a function returns? Hmm... so you have to specify in the code which variables you want to remain after the function has ended? But of course, declaring a variable in the mainline would make it global also, right?