初始化为NULL的指针数组的成员会占用内存吗？

Question

I use SDL_Surface pointers, and I'm wondering if this:

SDL_Surface* Images[8][7];

where some of the individual surface pointers are initialized to NULL in its class constructor and then never used, will take up more memory than if those were kept separate. As you can imagine, using an enumeration would make accessing these surface pointers easier and require no if or switch statements. For example, Images[0][3] to Images[0][7] may all be NULL, NULL, etc. Thanks in advance for replies!

Edit: "kept separate" meaning have a pointer variable name and a smaller array to avoid having NULL as a value for some of the images of which there are less than 7.

Addendum: Thanks everyone for the lightning replies, it's clear to me now; I'll go forward with my 2-dimensional array and there will be other arrays as well for other image groups.

Answer 1

Do members of an array of pointers initialized to NULL take up memory?

A pointer is a type which occupies enough memory to store an address location.
So Yes they will occupy/consume memory once they are declared just like any other data type which occupies memory irrespective of whether its variable is initialized or not.

Answer 2

The array is not dynamically allocated, so each element has memory allocated at compile time. But the null-initialized pointers do not point to additional allocated memory.

I hope that's what you're asking.

Answer 3

初始化为NULL的指针使用内存，但只使用指针所需的字节数。

Answer 4

Whether you declare your pointers in an array, or individually, one by one, the same amount of memory will be consumed. Of course, if you can declare fewer pointers by declaring them individually, then less space will be consumed.

So the following two lines both reserve space for 3 pointers:

SDL_Surface* surfaces[3];
SDL_Surface *surface1, *surface2, *surface3;

Given the small number of pointers involved in your code sample, you should opt for the approach that results in the most readable code. Only if your array was huge and had only a small proportion of pointers non-null would it be worth getting concerned about the overhead of the unused pointers in the array.

Answer 5

A null pointer takes up the same space as any other pointer to data. What you will save on will be the memory space allocated for data objects.

As to whether you should use an enum or something else - my main concern would be that C++ arrays of arrays aren't ideal. If you have an entire row or column of the array is empty, I certainly wouldn't bother defining it. Just keep it readable and maintainable.

Answer 6

A pointer is merely another data type, like int , double , etc. that happens to have the purpose of storing memory addresses. Typically, a pointer will take 32 or 64 bits depending on your architecture, but could take more/less on other hardware.

Just like an int array takes space even if the values are 0's, so too does a pointer array take space even if its values are 0. (NULL is typically just represented as a 0.)

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For example, assuming your architecture/compiler uses a 32-bit int:

int array[30];

The above code will reserve 30 x 4 bytes = 120 bytes. It doesn't matter what values you store.

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Let's assume now that you're on a 64-bit machine:

int *ptr_array[30];

The above code will reserve 30 x 8 bytes = 240 bytes, regardless of what pointer values you store.

You can imagine this as writing 30 individual pointers:

int *ptr1, *ptr2, *ptr3, ..., *ptr30;

Note that we're only talking about the size that the pointers consume in memory. Typically, you would have memory reserved elsewhere that the pointers point to (when non-null). That memory would of course vary depending on how much you have allocated. In that sense, having the pointers be null implies that there's nothing for them to point to, and can translate to reduced memory consumption.