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convert SDL_Rect' to SDL_Rect*'

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Started by c4c0d3m0n February 02, 2008 10:47 AM

5 comments, last by speciesUnknown 16 years, 2 months ago

c4c0d3m0n

100

Author

February 02, 2008 10:47 AM

Hey all I am trying to fill a rectangle on a surface with SDL_FillRect(). I get the error: error: cannot convert `SDL_Rect' to `SDL_Rect*' for argument `2' to `int SDL_FillRect(SDL_Surface*, SDL_Rect*, Uint32)' To make the rectangle, I use:

    SDL_Rect playfieldrect;
    playfieldrect.w = 24 * 10;
    playfieldrect.h = 24 * 20;
    playfieldrect.x = (screen->w / 2) - (24 * 5);
    playfieldrect.y = (screen->h / 2) - (24 * 10);

To draw the rectangle, I use:

        SDL_FillRect( screen, playfieldrect, SDL_MapColor( screen, color[whitea] ) );

Can I convert the SDL_Rect to *SDL_Rect? Cos if I define playfieldrect as a pointer, I can't edit its properties anymore... I must admit, I don't know much about pointers/references...

SiCrane

11,840

February 02, 2008 10:50 AM

&playfieldrect

c4c0d3m0n

100

Author

February 02, 2008 10:54 AM

That was too easy... I wish I understood what this whole pointer-thing was about. Can't find any places that explained it in layman's terms, even my own books are too complicated.

Ezbez

1,164

February 02, 2008 12:14 PM

The important thing to note here is that '&' is the reference operator, just as '*' is the dereference operator. &something returns the address to something. *something returns what something is pointing to (assuming that something is a pointer). Because the function is expecting a pointer and you have a plain old SDL_Rect, you get the address to the SDL_Rect by using the reference operator.

Now this all gets especially confusing since * and & are used when declaring pointer and reference variables, respectively.

int integer = 5;
int* pointer = &integer
int& reference = integer;
int anotherInt = *pointer;

At the end of this, integer is a plain old integer value. Pointer is the address to integer. Reference *is* integer. anotherInt is a separate integer which has the same value as what pointer is pointing to (which is integer). integer, *pointer, reference, and anotherInt all evaluate to 5.

Hopefully that helps. I used to have a link to a website that explained pointers nicely, but I have lost it. :(

c4c0d3m0n

100

Author

February 02, 2008 01:59 PM

Alright, this clears up a few things... I've used &reference before already, to make some TicTacToe code more readable. I don't quite see what would be the advantage of sing pointers instead of the real thing though. Why doesn't SDL_FillRect() just eat a normal SDL_Rect instead of a pointer? Thanks for all the help!

rip-off

11,000

February 02, 2008 02:20 PM

1) Efficiency. In general, most non-primitive types are expensive enough that it pays to avoid making copies of them.

2) Reference. SDL_FillRect modifies the rectangle passed to it to reflect the area that was actually filled. I'm not entirely sure why, but perhaps this has its uses. SDL_BlitSurface does this too.

void example( SDL_Surface *surface ){    SDL_Rect rect = { 0, 0, surface->w * 2, surface->h * 2 };    SDL_FillRect(surface,rect,/* some colour */);        // now, rect.w == surface->w and rect.h == surface->h}

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speciesUnknown

527

February 05, 2008 07:06 PM

Quote:Original post by c4c0d3m0n
That was too easy... I wish I understood what this whole pointer-thing was about. Can't find any places that explained it in layman's terms, even my own books are too complicated.

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convert SDL_Rect' to SDL_Rect*'

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