Question

How can I specify that a method should take as parameter a pointer to a location in memory that can hold a specified number of values? For example, if I have:

- (void)doSomethingWith:(int *)values;

I’d like to make it clear that the int * passed in should point to an allocated space in memory that’s able to hold 10 such values.

Answer 1

To directly answer your question, use an array argument with a bounds, e.g.:

- (void)takeTenInts:(int[10])array

Which specifies that the method takes an array of 10 integers.

Only problem is the C family of languages do not do bounds checking, so the following is valid:

int a[10], b[5];

[self takeTenInts:a]; // ok
[self takeTenInts:b]; // oops, also ok according to the compiler

So while you are specifying the size, as you wish to do, that specification is not being enforced.

If you wish to enforce the size you can use a struct:

typedef struct
{
   int items[10];
} TenInts;

- (void)takeTenInts(TenInts)wrappedArray

Now this doesn’t actually enforce the size at all[*], but its as close a you can get with the C family (to which the word “enforcement” is anathema).

If you just wish to know the size, either pass it as an additional argument or use NSArray.

[*] It is not uncommon to see structures in C following the pattern:

typedef struct
{
   // some fields
   int data[0];
} someStruct;

Such structures are dynamically allocated based on their size (sizeof(someStruct)) plus enough additional space to store sufficient integers (e.g. n * sizeof(int)).

In other words, specifying an array as the last field of a structure does not enforce in anyway that there is space for exactly that number of integers; there may be space for more, or fewer…