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EMAIL: rom@wanadoo.fr
IP: 217.128.51.62
URL: http://www.mobikool.net
DATE: 01/08/2004 08:18:33 AM

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But some variables are immortal. These variables are declared outside of blocks, outside of functions. Since they don't have a block to exist in they are called global variables (as opposed to local variables), because they exist in all blocks, everywhere, and they never go out of scope. Although powerful, these kinds of variables are generally frowned upon because they encourage bad program design.

We can see an example of this in our code we've written so far. In each function's block, we declare variables that hold our data. When each function ends, the variables within are disposed of, and the space they were using is given back to the computer to use. The variables live in the blocks of conditionals and loops we write, but they don't cascade into functions we call, because those aren't sub-blocks, but different sections of code entirely. Every variable we've written has a well-defined lifetime of one function.

When the machine compiles your code, however, it does a little bit of translation. At run time, the computer sees nothing but 1s and 0s, which is all the computer ever sees: a continuous string of binary numbers that it can interpret in various ways.

Note the new asterisks whenever we reference favoriteNumber, except for that new line right before the return.

Being able to understand that basic idea opens up a vast amount of power that can be used and abused, and we're going to look at a few of the better ways to deal with it in this article.

Since the Heap has no definite rules as to where it will create space for you, there must be some way of figuring out where your new space is. And the answer is, simply enough, addressing. When you create new space in the heap to hold your data, you get back an address that tells you where your new space is, so your bits can move in. This address is called a Pointer, and it's really just a hexadecimal number that points to a location in the heap. Since it's really just a number, it can be stored quite nicely into a variable.

Each Stack Frame represents a function. The bottom frame is always the main function, and the frames above it are the other functions that main calls. At any given time, the stack can show you the path your code has taken to get to where it is. The top frame represents the function the code is currently executing, and the frame below it is the function that called the current function, and the frame below that represents the function that called the function that called the current function, and so on all the way down to main, which is the starting point of any C program.

This will allow us to use a few functions we didn't have access to before. These lines are still a mystery for now, but we'll explain them soon. Now we'll start working within the main function, where favoriteNumber is declared and used. The first thing we need to do is change how we declare the variable. Instead of

Let's take a moment to reexamine that. What we've done here is create two variables. The first variable is in the Heap, and we're storing data in it. That's the obvious one. But the second variable is a pointer to the first one, and it exists on the Stack. This variable is the one that's really called favoriteNumber, and it's the one we're working with. It is important to remember that there are now two parts to our simple variable, one of which exists in each world. This kind of division is common is C, but omnipresent in Cocoa. When you start making objects, Cocoa makes them all in the Heap because the Stack isn't big enough to hold them. In Cocoa, you deal with objects through pointers everywhere and are actually forbidden from dealing with them directly.

That gives us a pretty good starting point to understand a lot more about variables, and that's what we'll be examining next lesson. Those new variable types I promised last lesson will finally make an appearance, and we'll examine a few concepts that we'll use to organize our data into more meaningful structures, a sort of precursor to the objects that Cocoa works with. And we'll delve a little bit more into the fun things we can do by looking at those ever-present bits in a few new ways.

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