AKMA’s Random Thoughts

A variable leads a simple life, full of activity but quite short (measured in nanoseconds, usually). It all begins when the program finds a variable declaration, and a variable is born into the world of the executing program. There are two possible places where the variable might live, but we will venture into that a little later.

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.

The most basic duality that exists with variables is how the programmer sees them in a totally different way than the computer does. When you're typing away in Project Builder, your variables are normal words smashed together, like software titles from the 80s. You deal with them on this level, moving them around and passing them back and forth.

To address this issue, we turn to the second place to put variables, which is called the Heap. If you think of the Stack as a high-rise apartment building somewhere, variables as tenets and each level building atop the one before it, then the Heap is the suburban sprawl, every citizen finding a space for herself, each lot a different size and locations that can't be readily predictable. For all the simplicity offered by the Stack, the Heap seems positively chaotic, but the reality is that each just obeys its own rules.

Earlier I mentioned that variables can live in two different places. We're going to examine these two places one at a time, and we're going to start on the more familiar ground, which is called the Stack. Understanding the stack helps us understand the way programs run, and also helps us understand scope a little better.

Note first that favoriteNumbers type changed. Instead of our familiar int, we're now using int*. The asterisk here is an operator, which is often called the "star operator". You will remember that we also use an asterisk as a sign for multiplication. The positioning of the asterisk changes its meaning. This operator effectively means "this is a pointer". Here it says that favoriteNumber will be not an int but a pointer to an int. And instead of simply going on to say what we're putting in that int, we have to take an extra step and create the space, which is what does. This function takes an argument that specifies how much space you need and then returns a pointer to that space. We've passed it the result of another function, , which we pass int, a type. In reality, is a macro, but for now we don't have to care: all we need to know is that it tells us the size of whatever we gave it, in this case an int. So when is done, it gives us an address in the heap where we can put an integer. It is important to remember that the data is stored in the heap, while the address of that data is stored in a pointer on the stack.