Returning C struct array

Question

Hello stackoverflow fellow members?

Struct Declaration in class A

struct PointSprite 
{
    GLfloat x;
    GLfloat y;
    GLfloat size;
    Color4f color;
} ParticleSystems[MAXIMUM_PARTICLES_ON_SCREEN];
// I generally put some stuffs in ParticleSystem array. 
// for ex) struct PointSprite *ps = &ParticleSystems[index];
// and it works well on the class A, but I want to get class B to access this array.

My question is, how am I suppose be return the array of 'ParticlelSystems' array so that other class can access to it? I have tried below code to return the pointer, but compiler gives me a warning.

- (struct ParticleSystems *) commitParticles
{
    struct ParticleSystems *ptr = &ParticleSystems; // it said, assigning incompatible pointer type
    
    return ptr;
}

Or should I need to allocate the 'ParticleSystems' array? Please help ! Thanks

The correct answer is different in C, C++, and objective-c. Please pick one language for your question. — Robᵩ
– Robᵩ, Commented Aug 5, 2011 at 16:49

Alok Save · Accepted Answer · 2011-08-05 16:41:09Z

2

If you are creating the array inside the function then you should dynamically allocate it using new and then return a pointer to it.

You cannot return arrays from a function, you will have to return a pointer to it.

Sample Code:

ParticleSystems* doSomethingInteresting()
{
    ParticleSystems *ptr = new ParticleSystems[MAXIMUM_PARTICLES_ON_SCREEN];

    //do the processing

    return ptr;

}

The caller takes the ownership of the returned dynamically allocated array and needs to deallocate it to avoid memory leaks:

delete []ptr;

edited Aug 5, 2011 at 16:41

answered Aug 5, 2011 at 16:29

Alok Save

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4 Comments

datenwolf Over a year ago

While you're technically correct, in C++ I'd use a std::vector instead of a regular array. If you want to avoid the overhead of copying the vector data around, instead of using a plain pointer I recommend using a boost::shared_ptr, i.e. shared_ptr<vector<PointSprite>> ptr(new vector<PointSprite>>.

Alok Save Over a year ago

@datenwolf: Absolutely, One really needs to use RAII and almost never use raw pointers in C++, but again if the OP already knows the number of elements of his array, then it would make sense to use a array rather than a std::vector to avoid the overhead of extensive allocations a vector would make.

datenwolf Over a year ago

You can initialize a std::vector with a size, then stay within the bounds of that size. That way no reallocation will occcur:

typedef std::vector<PointSprite> PointSprite_vector; boost::shared_ptr<PointSprite_vector> ptr(new PointSprite_vector(size))

.

Nicol Bolas Over a year ago

Or you can use vector::reserve to pre-allocate the memory; then you can do all the push-backs you want up to that pre-allocated limit. I do this a lot to prevent extra allocations, while not having to run the default constructor alot.

Rudy Velthuis · Accepted Answer · 2011-08-05 17:05:58Z

2

You can either return it, after allocating one, or you can fill one passed to you by the user. The latter leaves the responsibility to the user to provide a ParticleSystem to the method which receives the data. It can be a local array, or a malloced one.

- (void) commitParticles: (ParticleSystems *) sprites
{
    // set members of the structs here
}

I prefer this kind of passing to returning a malloced array. Your mileage may vary.

edited Aug 5, 2011 at 17:05

answered Aug 5, 2011 at 16:40

Rudy Velthuis

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Comments

Matt · Accepted Answer · 2011-08-05 16:35:38Z

0

You're getting the assigning incompatible pointer type compiler warning because your ptr declaration should be of type PointSprite *, not ParticleSystems *

answered Aug 5, 2011 at 16:35

Matt

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