One issue I'm having with these smart pointers is the following. Suppose I have some asset manager class like a texture manager. I want this manager to be responsible to load textures on demand when needed, and also responsible for freeing the memory. So it seems like unique_ptr makes sense. However, many other classes in the application will need a texture pointer at some time. For example, a RenderItem might need to know the textures to bind for a draw call, or a Sprite might need to know its texture.

I can't give RenderItem or Sprite a copy of unique_ptr, so this means I have to use shared_ptr. But what I don't like about shared pointer in this case is that I really only want the manager to create or delete textures. If I have shared_ptrs around, I can't be sure when all of them get destroyed--it won't necessarily be when the manager gets destoyed.

I could use shared_ptr in the manager and then weak_ptr on RenderItem and Sprite, but that feels like a workaround. I don't really want reference counting at all.

I also though about just using naked pointers in RenderItem and Sprite, with the understanding that the manager owns them. But this seems to break one of the purposes of smart pointers, which is that the code makes ownership clear rather than having to have assumptions or knowledge about the system.

What do you think is the best approach for modern C++?

If I have shared_ptrs around, I can't be sure when all of them get destroyed--it won't necessarily be when the manager gets destoyed.

That sounds backwards to me - The reference counting nature of shared_ptrs means you can know when all other references have been lost: Once your manager holds the only shared_ptr then the reference count will equal 1. (See the use_count() function).

You could also have the manager hold a weak_ptr but have it return shared_ptrs. If you use a custom-deleter then once all the external shared_ptr references are dead then your custom-deleter can signal back to the manager to release the resource.


There might also be a case for passing around references or raw-pointers outside the manager, since these indicate non-ownership semantics which is exactly what you're trying to achieve. The downside is that the lack of reference-counting means the manager can't track external usage; even if your manager isn't going to immediately delete unused textures and only remove them when the manger itself dies it can be extremely useful if it can indicate whether some textures have not been correctly released by the rest of the system.

Seems to me like the objects using the textures should have a smaller lifetime than the texture manager.

In that case, the manager will/should be destroyed after all of the objects have been destroyed, and thus, you know that nobody is using the textures anymore.

This way, you could store unique_ptrs in a vector, and simply hand out integer handles (index of the texture in the vector) or naked pointers to users. It takes a bit more thought into structuring the data and code, but IMO that's a good thing.

Once upon a time there was an L. Spiro.
One day this little L. Spiro was walking harmlessly through the office, when suddenly, sneak attack!
“You WILL optimize Phantasy Star Nova,” a voice commanded.
The poor helpless L. Spiro replied, “B-b-but, it looks haaaard… Look at how complex some of the scenes are…”
“SILENCE! YOU WILL DOUBLE ITS FRAMERATE OR YOU WILL NEVER SEE YOUR FAVORITE STUFFED BUNNY AGAIN!!”

The poor defenseless and innocent L. Spiro sat at his desk and looked for things to optimize. “It looks pretty fast already, what can I do?”, he thought.

But profiling showed massive uses of memory allocations and freeing.
Like on this line:

LookTarget_ptr pLookAt = CUtil::TargetListCurrent().IsNull() ? NULL : CUtil::TargetListCurrent().GetSharedPtr();

And this one:

LookTarget_ptr pNextTarget;

L. Spiro was puzzled but then he remembered stories from his grandmother who told of the evils of using std::vector, std::shared_ptr, etc., in games. “Beware the shared_ptr that points to nothing, for it too must allocate a reference counter.”

“That’s right! Grandmother was preparing me for this day all along!”, he exclaimed.
“When these shared pointers get assigned to either default constructor or NULL, they allocate a reference counter and then free it when scope ends since they are just temporaries.”


And with memory usage being the biggest bottleneck in the entire game, by fixing all of the shared_ptr usages, L. Spiro was able to improve the performance significantly, and all rejoiced. But it was only 196% of its original performance and L. Spiro’s bunny was destroyed.

The End.

What do you think is the best approach for modern C++?

If we are talking about games, my advice would be: Don’t use std::shared_ptr. The only good thing about std::shared_ptr in games is that when you hand your project off to the optimizer guy, he’s going to look like a hero with all the performance he will be able to get back.

In your situation, you don’t even necessarily need shared pointers anyway. We have a texture manager and objects hold references to textures. If the object is not deleted before the texture manager, crash and fix it. When it’s assumed the manager itself must outlive objects that hold references to pointers to textures, it is not your problem if others try to break that rule. Besides, in our case the texture manager lasts the lifetime of the game so it really can’t be outlived by any objects that use textures.

If you ever do need shared pointers, there is no harm in rolling your own.

To avoid unexpected allocations mine uses an intrusive model in which no allocations are made until you call CSharedPtr<T>::New(). The shared pointer template class will allocate both the object and the reference counter together, at a predictable time, and with only 1 allocation instead of 2.

And that’s not even mentioning all the atomic increments and decrements needed by std::shared_ptr for thread safety.

In my world, there is a shared pointer and then there is a thread-safe shared pointer. Use as necessary.

I would avoid std::vector for similar “not made for games” reasons.

L. Spiro

Smart pointers are not an excuse to be ignorant about memory ownership issues.

In this case, you say up from that there is to be a single, unique owner of the allocated memory: the texture manager. That's a strong design, and the way that should be done in C++ is std::unique_ptr (notice the way the name matches the purpose). The texture manager owns the memory using a std::unique_ptr, and allows weak references to the textures using raw pointers.

There's nothing wrong with raw pointers in C++ as long as you've thought out the ownership issues. Seeing new and delete in code (outside of the std::unique_ptr constructor, at least until std::make_unique() appears in the standard) is a sign you may not have though through ownership.

L. Spiro was puzzled but then he remembered stories from his grandmother who told of the evils of using std::vector, std::shared_ptr, etc., in games. “Beware the shared_ptr that points to nothing, for it too must allocate a reference counter.”

+1 because your post is quite similar to what I was going to write. Don't use shared_ptr.

Just wanted to note that this particular quoted line is not true of any good std::shared_ptr implementation (I don't know about the one used in the Vita toolchain); a null shared_ptr can and should have both a null pointer to the data and a null pointer to its reference count (or a single null pointer if enable_shared_from_this or the like is used). Memory is only allocated when needed and never during default construction; true for std::vector and the other standard containers, too, at least in every standard implementation I've seen in recent years. There are a lot of custom types often called things like shared_ptr that behave quite oddly, though. The engine we've been using on our current project has heavily used (unfortunately) shared_ptr and weak_ptr types that act _almost_ like but not identically to the C++11 std:: versions (and this confuses some of our junior devs who are familiar with C++11 and makes it hard to teach C++11 stuff to some of the more veteran developers since practice and theory aren't matching up).

I'd go further to mention that smart pointers of any type are a huge pain when debugging. Since most compilers don't have something like GCC's -Og (optimize with debugging in mind) you are often stuck choosing between easy-to-debug builds with zero optimization or nightmare-to-debug builds with high optimization. Since every smart pointer dereference is a function call you end up taking huge performance hits and suffer other annoyances (like when trying to step into code) in those non-optimized debug builds.

std::unique_ptr is a fantastic idea but those debugging issues bog it down, too. It's unfortunate the C++ committee didn't add an owning pointer to the core language, a la Rust; maybe in the future (I'd wager it has a higher likelihood of happening sooner than Visual C++ gaining something like -Og does).