Pointers and containers

Question

We all know that RAW pointers need to be wrapped in some form of smart pointer to get Exception safe memory management. But when it comes to containers of pointers the issue becomes more thorny.

The std containers insist on the contained object being copyable so this rules out the use of std::auto_ptr, though you can still use boost::shared_ptr etc.

But there are also some boost containers designed explicitly to hold pointers safely:
See Pointer Container Library

The question is: Under what conditions should I prefer to use the ptr_containers over a container of smart_pointers?

boost::ptr_vector<X>

or

std::vector<boost::shared_ptr<X> >

Answer 1

Well, overhead is one case.

A vector of shared pointers will do a lot of extraneous copying that involves creating a new smart pointer, incrementing a reference, decrementing a reference, etc on a resize. All of this is avoided with a pointer container.

Requires profiling to ensure the container operations are the bottleneck though :)

Answer 2

Boost pointer containers have strict ownership over the resources they hold. A std::vector<boost::shared_ptr<X>> has shared ownership. There are reasons why that may be necessary, but in case it isn't, I would default to boost::ptr_vector<X>. YMMV.

Answer 3

Steady on: smart pointers are a very good method of handling resource management, but not the only one. I agree you will see very few raw pointers in well-written C++ code, but in my experience you don't see that many smart pointers either. There are plenty of perfectly exception-safe classes implemented using containers of raw pointers.