Question

I’ve got a small problem. I’ve got an array and its size changes when passing to a c-function from an objective-c function.

void test(game_touch *tempTouches)
{
    printf("sizeof(array): %d", sizeof(tempTouches) );
}

-(void)touchesEnded: (NSSet *)touches withEvent: (UIEvent *)event
{
    game_touch tempTouches[ [touches count] ];
    int i = 0;

    for (UITouch *touch in touches)
    {
        tempTouches[i].tapCount = [touch tapCount];

        tempTouches[i].touchLocation[0] = (GLfloat)[touch locationInView: self].x;
        tempTouches[i].touchLocation[1] = (GLfloat)[touch locationInView: self].y;

        i++;
    }

    printf("sizeof(array): %d", sizeof(tempTouches) );

    test(tempTouches);
}

The console log is:

[touchesEnded] sizeof(array): 12
[test] sizeof(array): 4

Why is the size different in 2 of the methods?

In the [test] method the returning size is always 4, not depending on the original size of an array.

Thanks.

Answer 1

In C arrays decay into pointers when they are passed as parameters.
The sizeof operator has no way of knowing the size of the array passed to void test(game_touch *tempTouches), from it’s perspective it’s merely a pointer whose size is 4.

When declaring arrays using this syntax int arr[20], the size is known at compile-time therefore sizeof can return it’s true size.