Type requirements for std::list

Question

I've got a type that can't be moved or copied (by making the necessary constructors and operators private). But when I tried to compile a std::list of them, the operation failed with a very strange error (class name snipped for brevity).

1>C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xmemory(202)
: error C2248: 'T::T' : cannot access private member declared in class 'T'

Surely it's not incumbent of a type in a linked list to be movable or copyable.

When these members are made public, the code compiles fine- even though, if the std::list had tried to access them, it would be an unresolved external, since they're only declared private. Makes no sense :(

Answer 1

As of C++03, elements must be copy constructible and copy assignable. §23.1/3:

The type of objects stored in these components must meet the requirements of CopyConstructible types (20.1.3), and the additional requirements of Assignable types.

In C++0x, requirements are put on a per-operation basis, but in general it's safe to say elements must be move constructible and move assignable. (Though some operations require copy constructibility and assign-ability, etc.)

A typical solution to your problem is to store pointers to objects, via shared_ptr or some other smart pointer.

Answer 2

standard containers need their element type to be copyable, if you have noncopyable types, use (smart) pointers as the element type instead (like: std::list<boost::shared_ptr<YourType> >.

template class/functions often generate errors unless you use them explicitly, so it could be that you havent got an error.