Returning a Pointer (beginner question)

The variable myImg is of type (SDL_Surface *) a pointer to a memory address that marks the beginning of the type SDL_Surface. So, when you are loading your image, you want to use the new keyword.

For example:



That code creates a new memory allocation of an SDL_Surface inside inside the heap, which is not local to any function or method. The only way to free this memory up, once the new keyword is used, is to use the delete keyword.

For example:



Since you are holding a pointer to the object, which is just a memory address where the actual object is located, when you return the pointer value you are only returning the memory address. There are no ties to any variables inside of the load_image() function now. You've just copied the address to a new pointer, which now points to the SDL_Surface object that you've just created. Hope that helps to clear things up a bit.

Daishim, that code you put there is exactly what I was thinking. That what I thought was the right way. Thanks.

However, I'm not sure I understood your last sentence. Since i created the myImg within Load_img(), doesn't myImg get destroyed when the function is done thus retuning a dangling pointer? A pointer to nothing if you will.

Quote:

Original post by radioact1ve Daishim, that code you put there is exactly what I was thinking. That what I thought was the right way. Thanks. However, I'm not sure I understood your last sentence. Since i created the myImg within Load_img(), doesn't myImg get destroyed when the function is done thus retuning a dangling pointer? A pointer to nothing if you will.

Not if Load_img() uses new to assign the block of memory - which, as you can see, it does. [smile]
However this also means you'll have to free the memory later; if you lose track of the pointer which had dynamically allocated memory you get a memory leak.

Quote:

Original post by Frequency Not if Load_img() uses new to assign the block of memory - which, as you can see, it does. [smile] However this also means you'll have to free the memory later; if you lose track of the pointer which had dynamically allocated memory you get a memory leak.

Exactly, so if you look at my original code at the top, I don't use new. My question is, if I DON'T use new, how come the function DOES work as intended? [smile]

The actual values in memory may still be there, since your program may not have requested any new memory or written over top of it yet. So, you're pointer points to that location in memory, but that location is not actually tied to a variable, so it can be reassigned and changed at any time without your knowledge. That may explain the lingering appearance of that code still working, however, at some point in time, it will fail.

When dealing with other libraries, it's a bad idea to call delete on any object that you didn't create yourself. For example, when you're finished with a surface created by SDL_CreateRGBSurface(), you should call SDL_FreeSurface() on it. An SDL_Surface may contain pointers to other objects (like SDL_PixelFormat), and only SDL knows how to free everything properly.

Also, C libraries use malloc/free instead of C++'s new/delete, and mixing them is not guaranteed to work. However when using the memory it doesn't matter how or where it is allocated. As long as you control its lifetime properly.

I hope that answers everything :)

Quote:

Original post by Daishim That may explain the lingering appearance of that code still working, however, at some point in time, it will fail.

Well said. I'm pretty sure I understand this now. Thanks everyone!!

Quote:

Original post by hh10k When dealing with other libraries, it's a bad idea to call delete on any object that you didn't create yourself. For example, when you're finished with a surface created by SDL_CreateRGBSurface(), you should call SDL_FreeSurface() on it. An SDL_Surface may contain pointers to other objects (like SDL_PixelFormat), and only SDL knows how to free everything properly. Also, C libraries use malloc/free instead of C++'s new/delete, and mixing them is not guaranteed to work. However when using the memory it doesn't matter how or where it is allocated. As long as you control its lifetime properly. I hope that answers everything :)

Ah yes, I should have mentioned that I'm not familiar with SDL. But it looks like that's the missing piece. :)