How to return a C-array from method in Objective-C?

Question

I have a function that returns a variable and I want to know how to return an array the issue is it isn't an NSArray it is just an average C array like this...

-(b2Fixture*) addFixturesToBody:(b2Body*)body forShapeName:(NSString*)shape
{
    BodyDef *so = [shapeObjects objectForKey:shape];
    assert(so);
       FixtureDef *fix = so->fixtures;
    int count = -1;
    b2Fixture *Fixi[4];
    while(fix)
    {

        count++;
        NSLog(@"count = %d",count);
        Fixi[count]= body->CreateFixture(&fix->fixture);
        if (Fixi[count]!=0) {
            NSLog(@"Fixi %d is not 0",count);
        }
        if (body->CreateFixture(&fix->fixture)!=0) {
            NSLog(@"body %d is not 0",count);
        }

        fix = fix->next;

    }

    return *Fixi;

}

If you see some variable types you don't know it's because I'm using cocos2d framework to make a game but I'm returning a variable of b2Fixture... This code compiles however only saves the value of the first block of the array "fixi[0]" not the whole array like I want to pass

anyhelp :) thankyou

If you're going to return a C array (of anything), where do you intend to allocate it? (Hint: It can't be on the stack of the called function.) — Greg Hewgill
– Greg Hewgill, Commented Mar 11, 2012 at 0:45

Carl Norum · Accepted Answer · 2012-03-11 00:52:20Z

2

You can't return a local array. You'll need to do some kind of dynamic allocation or pull a trick like having the array inside a structure.

Here is a link to an in-depth article that should help you out.

edited Mar 11, 2012 at 0:52

answered Mar 11, 2012 at 0:45

Carl Norum

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3 Comments

King Popsicle Over a year ago

And how would I go about doing that

Carl Norum Over a year ago

malloc(3), perhaps?

user166390 Over a year ago

A very true answer but ... should be expanded or reference one of the myriad of other answers.

CRD · Accepted Answer · 2012-03-11 01:07:20Z

2

In general returning C arrays by value is a bad idea, as arrays can be very large. Objective-C arrays are by-reference types - they are dynamically allocated and a reference, which is small, is what is passed around. You can dynamically allocate C arrays as well, using one of the malloc family for allocation and free for deallocation.

You can pass C structures around by value, and this is common, as in general structures tend to be small (or smallish anyway).

Now in your case you are using a small array, it has just 4 elements. If you consider passing these 4 values around by value is reasonable and a good fit for your design then you can do so simply by embedding the C array in a C structure:

typedef struct
{
    b2Fixture *elements[4];
} b2FixtureArray;

...

-(b2FixtureArray) addFixturesToBody:(b2Body*)body forShapeName:(NSString*)shape
{
    BodyDef *so = [shapeObjects objectForKey:shape];
    assert(so);
    FixtureDef *fix = so->fixtures;
    int count = -1;
    b2FixtureArray Fixi;
    while(fix)
    {   
        count++;
        NSLog(@"count = %d", count);
        Fixi.elements[count]= body->CreateFixture(&fix->fixture);
        if (Fixi.elements[count] != 0)
        {
            NSLog(@"Fixi %d is not 0",count);
        }
        if (body->CreateFixture(&fix->fixture) != 0)
        {
            NSLog(@"body %d is not 0", count);
        }

        fix = fix->next;    
    }    
    return Fixi;    
}

...

// sample call outline
b2FixtureArray result = [self addFixturesToBody...]

Whether this standard C "trick" for passing arrays by value is appropriate for your case you'll have to decide.

Note: If b2fixture is an Objective-C object make sure you understand the memory management implications of having a C array of objects references depending on the memory management model (MRC, ARC, GC) you are using.

answered Mar 11, 2012 at 1:07

CRD

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3 Comments

King Popsicle Over a year ago

how would I retrieve the struct that it's returning i.e. before I would write b2fixture = function now what? since its a struct and all... do I need to make a new struct of the same type

King Popsicle Over a year ago

I get an error something along the lines of b2Fixture can't be saved as b2FixtureArray incompatible type

CRD Over a year ago

@KingPopsicle - the last line shows how it can be called. What this standard C technique does is wrap an array as the single member of a structure. So instead of array[index] you use structure.array[index] everywhere. The error you're getting sounds like you're trying to assign a b2Fixture to the structure instead of to an element of the array within the structure.

hotpaw2 · Accepted Answer · 2012-03-11 06:16:57Z

0

If you need to design function or method that has to return a fixed or limited size array, one possibility is to pass a pointer to the result array to the function or method as a parameter. Then the caller can take care of allocating space, or just use a local or instance variable array. You might want the called function to sanity check that the array parameter isn't NULL before using the array.

answered Mar 11, 2012 at 6:16

hotpaw2

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