Why do C++ libraries and frameworks never use smart pointers?

Question

I read in a few articles that raw pointers should almost never be used. Instead they should always be wrapped inside smart pointers, whether it\'s scoped or shared pointers.

Answer 1

There are also other types of smart pointers. You might want a specialized smart pointer for something like network replication (one that detects if it's accessed and sends any modifications to the server or some such), keeps a history of changes, marks the fact that it was accessed so it can be investigated when you save data to disk and so on. Not sure if doing that in the pointer is the best solution but using the built in smart pointer types in libraries could result in people being locked into them and loosing the flexibility.

People can have all kinds of different memory management requirements and solutions beyond smart pointers. I might want to manage memory myself, I could be allocating space for things in a memory pool so it's allocated in advance and not at runtime (useful for games). I might be using a garbage collected implementation of C++ (C++11 makes this possible although none exist yet). Or maybe I'm just not doing anything advanced enough to worry about bothering with them, I can know that I'm not going to forget to uninitialized objects and so on. Maybe I'm just confident in my ability to manage memory without the pointer crutch.

Integration with C is another issue too.

Another issue is smart pointers are part of the STL. C++ is designed to be usable without the STL.

Answer 2

Apart from the fact that many libraries were written before the advent of standard smart pointers, the biggest reason is probably the lack of a standard C++ Application Binary Interface (ABI).

If you’re writing a header-only library, you can pass around smart pointers and standard containers to your heart’s content. Their source is available to your library at compile time, so you rely on the stability of their interfaces alone, not of their implementations.

But because of the lack of standard ABI, you generally cannot pass these objects safely across module boundaries. A GCC shared_ptr is probably different from an MSVC shared_ptr, which too can differ from an Intel shared_ptr. Even with the same compiler, these classes are not guaranteed to be binary compatible between versions.

The bottom line is that if you want to distribute a prebuilt version of your library, you need a standard ABI on which to rely. C doesn’t have one, but compiler vendors are very good about interoperability between C libraries for a given platform—there are de facto standards.

The situation is not as good for C++. Individual compilers can handle interoperation between their own binaries, so you have the option of distributing a version for every supported compiler, often GCC and MSVC. But in light of this, most libraries just export a C interface—and that means raw pointers.

Non-library code should, however, generally prefer smart pointers over raw.

Answer 3

Qt pointlessly re-invented many parts of the Standard library in an attempt to become Java. I believe that it does actually have its own smart pointers now, but in general, it is hardly a pinnacle of design. wxWidgets, as far as I'm aware, was designed long before usable smart pointers were written.

As for Boost, I fully expect that they use smart pointers wherever appropriate. You might have to be more specific.

In addition, don't forget that smart pointers exist to enforce ownership. If the API has no ownership semantics, then why use a smart pointer?

Answer 4

You shouldn't stay away from smart pointers, they have their use especially in applications where you have to pass a object around.

Libraries tend to either just return a value or populate a object. They don't usually have objects that need to be used in a lot of places, so there is no need for them to use smart pointers (at least not in their interface, they may use them internally).

I could take as example a library we have been working on, where after a few months of development I realized we only used pointers and smart pointers in a few classes (3-5% of all classes).

Passing variables by reference was enough in most places, we used smart pointers whenever we had a object that could be null, and raw pointers when a library that we used forced us to.

Edit (I can't comment because of my reputation): passing variables by reference is very flexible: if you want the object to be readonly you can use a const reference (you can still do some nasty casts to be able to write the object) but you get the maximum of protection possible (it's the same with smart pointers). But I do agree that it's much nicer to just return the object.

Answer 5

It also depends on what domain you work in. I write game engines for a living, we avoid boost like the plague, in games the overhead of boost isn't acceptable. In our core engine we ended up writing our own version of stl (Much like the ea stl).

If i was to write a forms application, i might consider using smart pointers; but once memory management is second nature not having granular control over memory becomes quiet annoying.

Answer 6

There are two issues with smart pointers (pre C++11):

• non-standards, so each library tend to reinvent its own (NIH syndrom & dependencies issues)
• potential cost

The default smart pointer, in that it is cost-free, is unique_ptr. Unfortunately it requires C++11 move semantics, which only appeared recently. All other smart pointers have a cost (shared_ptr, intrusive_ptr) or have less than ideal semantics (auto_ptr).

With C++11 around the corner, bringing a std::unique_ptr, one would be tempted to think that it is finally over... I am not so optimistic.

Only a few major compilers implement most of C++11, and only in their recent versions. We can expect major libraries such as QT and Boost to be willing to retain compatibility with C++03 for a while, which somewhat precludes the wide adoption of the new and shiny smart pointers.