Question

I’m attempting to simplify my code with a function that generates an array of CGPoints and returns a pointer to the first element in this array.

CGPoint* getVerticesFromRect(const CGRect rect) {
    CGPoint vertexArray[] = 
    {  ccp(rect.origin.x, rect.origin.y),
        ccp(rect.origin.x + rect.size.width, rect.origin.y),
        ccp(rect.origin.x + rect.size.width, rect.origin.y - rect.size.height),
        ccp(rect.origin.x, rect.origin.y - rect.size.height) };
    CGPoint *vertices = &vertexArray[0];
    return vertices;
}

This work correctly, but the problem arises when I attempt to use this CGPoint* in:

void ccDrawPoly( const CGPoint *poli, NSUInteger numberOfPoints, BOOL closePolygon )

Like so:

void ccDrawRect( const CGRect rect ) {
    CGPoint *vertices = getVerticesFromRect(rect);
    // Makes an array from the pointed CGPoints
    CGPoint vertexArray[] = {vertices[0], vertices[1], vertices[2], vertices[3]};
    ccDrawPoly(vertexArray, 4, YES); // This works
    ccDrawPoly(vertices, 4, YES); // This doesn't
}

I have found that attempting to read the *vertices variable in ccDrawRect gives the correct values, but I’m getting garbage values on the other side when passing it into ccDrawPoly. What’s the reason behind this? From what I know, CGPoint *poli and CGPoint poli[] should be identical. Is it the const that’s causing the issue?

EDIT

Following the advice here I changed my function and passing the pointer works fine now. The problem was returning a pointer to a local variable which didn’t stick around for long. Instead, I’ve used malloc and I free the memory in the caller function once it’s no longer needed.

CGPoint* getVerticesFromRect(const CGRect rect) {
    CGPoint *vertices = (CGPoint*) malloc(sizeof(CGPoint) * 4);
    assert(vertices);

    vertices[0] = ccp(rect.origin.x, rect.origin.y);
    vertices[1] = ccp(rect.origin.x + rect.size.width, rect.origin.y);
    vertices[2] = ccp(rect.origin.x + rect.size.width, rect.origin.y - rect.size.height);
    vertices[3] = ccp(rect.origin.x, rect.origin.y - rect.size.height);

    return vertices;
}

void ccDrawRect( const CGRect rect ) {
    CGPoint *vertices = getVerticesFromRect(rect);
    ccDrawPoly(&vertices[0], 4, YES);
    free(vertices);
    vertices = NULL;
}

Answer 1

You’re using a local variable outside of its scope.

Instead of allocating array on stack as you do:

CGPoint vertexArray[] = ...

You need to use the malloc function and create a dynamically allocated array. Don’t forget to free it of course.

(Is it really C?)