Telling a shared_ptr not to delete object?

Hello!

Is there any way to tell a shared_ptr not to delete the object it's pointing to if the shared_ptr is destroyed and it is the only shared_ptr pointing to that object?

For example: I have a class 'image' containing a shared_ptr object, pointing to the place where the pixel data is stored. But sometimes that data is created to be used by a SDL_Surface in the first place, not by an image object, but is later pointed to from such an object. Thus, it shouldn't be destroyed by such an object either. I have to this point had a flag set in the object to tell if it is responsible for deleting[] the data or not when it's destroyed. Hm, I should probably use shared_array in this case.

If I set the shared_ptr to point to NULL, will it still keep count of the number of shared_ptr:s pointing to that place and try to delete the object at that position when it's the last shared_ptr being destroyed?

You could create a dummy shared_ptr that you aren't really using so that there is still one pointing to it when all of the shared_ptr's you're actually using are destroyed.

Doesn't that defeat the purpose of a shared_ptr though?

Quote:

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Originally Posted by Kernel Sanders

Doesn't that defeat the purpose of a shared_ptr though?

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That's an excellent question. If you want an object to exist after the last reference to it has gone away, then it doesn't sound like shared_ptr is the type to be using for it.

I guess I could always create my own class, or even just add a counter directly into the image class. shared_ptr seems to be a class of quite low weight complexity, so it wouldn't be hard to reproduce it.

Quote:

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Originally Posted by TriKri

I guess I could always create my own class, or even just add a counter directly into the image class. shared_ptr seems to be a class of quite low weight complexity, so it wouldn't be hard to reproduce.

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What about auto_ptr? I believe it has a release member function that basically releases the memory from automatic destruction. You could carry it around in the auto_ptr until you hand it off, "release it," to the SDL system for management.

Well, I see two solutions here.

One, and conceptually the better one, is to associate a shared_ptr with the SDL_Surface. If the SDL_Surface references the image data, the associated shared_ptr does so, too. Thus, you keep the reference counting correct.

Two, when you create the shared_ptr and an SDL_Surface references the image data, you give it a noop deleter.

Code:

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struct noop_deleter { void operator ()(void *) {} };
shared_ptr<Whatever> sp(data, noop_deleter());

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However, I can't remember if shared_ptr supports set_deleter, and if it doesn't, this means that you cannot change whether an SDL_Surface refers to the data during the lifetime of the data.