Getting the length of a static C array in Objective-C

Question

I have a static array of NSString defined in my implementation file like so:

NSString * const knames[] = { @"", @"", @"", ..., @"" };

and what I'd like to do is get the length dynamically. Since this is a C array, length and count are not valid, so I've tried using sizeof:

int count = (sizeof knames) / (sizeof knames[0]);

However, it results in the error message:

Invalid application of 'sizeof' to an incomplete type 'NSString *const _strong[]'

I'm not familiar with objective C only C. But would this work sizeof(*knames) since knames is a pointer to an array? — Tobias Johansson
– Tobias Johansson, Commented May 30, 2016 at 20:49
@TobiasJohansson great suggestion! That actually makes the error message go away, however yields incorrect results :) — smaili
– smaili, Commented May 30, 2016 at 21:12

trojanfoe · Accepted Answer · 2016-05-30 20:48:31Z

0

Small typo; sizeof requires brackets:

/*const?*/ int count = sizeof(knames) / sizeof(knames[0]);

You could create a macro somewhere globally:

#define ARRAY_SIZE(a) (sizeof(a) / sizeof(a[0]))
...

const int count = ARRAY_SIZE(knames);

edited May 30, 2016 at 20:48

answered May 30, 2016 at 20:46

trojanfoe

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7 Comments

trojanfoe Over a year ago

@smaili I don't see why you are still getting an error. Please check you have typed it correctly. However the "incomplete type" bit of the error message is a clue. Are you sure it's in scope?

Duncan C Over a year ago

This answers the question that was posted, but beware. ARC will not memory-manage objects that are saved into a C array. The code @smaili posted will work because all of the strings are statically defined. If you try to put a dynamically created string into a C array the reference in the array will be __unsafe_unretained, meaning that it will not keep ownership of the object, nor will it get set to nil if the object gets deallocated. You'd be much better off using an NSArray of string objects.

trojanfoe Over a year ago

@DuncanC True. I think the compiler will throw some sort of ... is not a compile-time constant error in that case anyway.

Duncan C Over a year ago

Yes, if you try to create a compile-time array that includes dynamically created objects, it would throw an error. There would be nothing stopping you from replacing an entry in the C array with a different object once it was created however, and that would be bad. Same issue if you malloc() a C array of pointers and try to save objects into it, but for malloc'ed C arrays sizeof doesn't work. You have to remember the size of the array and pass it around, as @dashblinkenlight says in his answer. The long and the short of it is that C arrays are not a good place to store NSObjects.

smaili Over a year ago

@trojanfoe I'm not sure why, but it's definitely in scope.

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