Question

I am using a global array (I know they are evil). This is not so much a problem, but I don’t know why it works one way and not the other, as they should be the same. (that and otherwise I need to deallocate memory after in the currently working way).

Anyway, I have this Snake class. I want to make, this being the global array:

Snake snakes[8];

I have several (or will anyway) several classes and function that interact with this. The problem in question occurs with a setter function in the Snake class. It compiles fine, but segfaults. It turned out, this was a null pointer (0x0). Not sure why. The stack trace in this case came from a function call from another file, which includes the snake header and has this:

extern Snake * snakes;

An array’s name is a pointer, so I figured this should work. Oddly enough, it does not. I have no idea why.

But, when I change the declaration from what it was to:

Snake * snakes;

And later allocate it like so:

snakes = new Snake [8];

It works! But, then I need to deallocate something else I didn’t want to, when the number of snakes is constant. (8).

Also, the function call in question accesses the 0th element, where there were 8 elements each time.

Any idea what causes this?

On a side note, are global variables still evil when in a named namespace? (not anonymous). I ask because I am trying to get into the OO habit and logically structure things, which I am certain will pay off compared to my last endeavors, the encapsulation, organization, etc, all make sense. (and maybe in a few years, compile time). Still, having to include a scoped variable in a function call for setting seems… bad. Is that proper or is there a better way? (then global, namespace or otherwise)

EDIT: The error is a segfault, at the line of the member function.

Snake member function:

void setValue(Some value here...) {this-> value = input)

The call from the file that externed the global array:

snakes[0].setValue(some value here...)

Answer 1

An array’s name is not a pointer. In most contexts it decays into a pointer, but when it doesn’t, it doesn’t. So if you have:

Snake snakes[8];

in one source file and

extern Snake *snakes; // don't do this

in another, you’ll get all kinds of mysterious problems, including crashes. For an array, the extern declaration must declare an array:

extern Snake snakes[];

Incidentally, I know this because I had exactly the same problem many years ago, and that’s when I figured out how to do this.