Thread: Questions on Memory

Keep in mind that I am relatively new to programming in general, so these questions may be a little foolish.

1) How much memory does a normal variable use?
2) What is the difference memory-wise between a normal variable and a pointer?
3) How much memory does a pointer use?
4) What is the difference between a normal variable or pointer and dynamically allocated memory?
5) Why do you have to free dynamically allocated memory, but not regular variables or pointers?

Like I said before, I am new to programming. I have read tutorials on C++, but I don't quite follow certain parts, especially those parts dealing with memory, hence the questions. Thanks in advance for any help!

For your first three questions start with this link and for the rest see this.

Jim

Thanks for the links. So, from what I understand:

A variable always takes a certain amount of memory based on what type it is (int, char, float, short int, etc.). If you try to give a variable a value outside of its range, it will return an error.

A pointer always uses the same amount of memory, regardless of the type, because it does not actually contain a value, simply the adress of a value.

Dynamically allocated memory allows you to declare a pointer or array of a size which is not previously determined.

A couple more questions:

1) How does the memory of a normal variable or pointer get freed?
2) Because the identifier of an array is equivalent to the adress of its first element, is an array in essence a pointer? How much memory would it then use?

Okay, a few more questions. Hopefully the last!

Say for example, I create the following class:

Code:

class Arithmetic
{
public:
	int AddTo(int Num);
	int AddToTen();
};

and then do something like this:

Code:

#define ten     10

int Arithmetic::addToTen()
{
	Arithmetic number;
	Arithmetic sum;
	cout<<"Please Input A Number: ";
	cin>>number;
	sum = number.AddTo(ten);
	return sum;
}

1) how much memory does number or sum take up (because I didn't give it a type)?
2) how much memory does ten take up (because, again, I defined it, but didn't give it a type)
3) how much memory does a function use? (I'm assuming it's varied?)

Originally Posted by Mr.777

1. No Size.

How can it not take up any memory? If I give it a value, it has to record that value somewhere in memory, right?

Originally Posted by Mr.777

How may you even assign it any value???
You have no variables in your class, nor any constructor....
So?

I apologize. I am still learning the language, as you can see, and my knowledge of classes is limited. If I understand correctly, creating an object of a class (number and sum in my previous example) allows you to assign different values to each variable based on which class object you are currently referencing. Which means you can't assign a value to an object of the class itself, right? So object1.x != object2.x, even though you have only declared x once, correct?

I guess my question then is, if you have only declared x once yet it holds different values, it has to occupy two different locations in memory (one for object1.x and one for object2.x), correct?

Yeah, I'd already seen that. Thanks for all the help!

just to be pedantic, I think I should clear a few things up.

* if I recall correctly, the C++ standard does not mandate that pointers be implemented as memory addresses, but every implementation I've seen implements them as such.

* an instance of a class need not take up any memory if it has no data members and no virtual functions. I suspect that most implementations implement it as 1 byte - GCC does, but I haven't had an opportunity to test others.

* regarding your question "how much memory does ten take up", it takes up no memory at all, because it is a preprocessor macro. the preprocessor replaces references to 'ten' with the literal value 10, which then takes up memory space - as much space as an int takes, although with optimizations turned on, it's likely that the compiler will put that value in a register or put it in the assembly code as a literal, where it will take no memory at all. you wouldn't be to refer to it by a pointer. for example:

Code:

#define ten 10
int* ptrToTen = &ten;

is not legal code, because 'ten' is not a variable.