On 15/10/10 10:41, Gerardo Hernández wrote:
Scott McMurray <me22.ca+boost@gmail.com> wrote:
The problem is that the underlying type of boost::ptr_vector<T> is always vector<void*>, no matter T is const.
Since the point of this is to only ever instantiate one version of the underlying container, wouldn't it be better to always use underlying_container<void const*>, then const_cast when returning to the user?
My point was to allow ptr containers of <const T> to work, maintaining const-correctness. I believe that requires different underlying container types and not only one as you suggest. So in this code snippet:
boost::ptr_vector<const int> v;
v.push_back(new int); v[0] = 3;
the assignment shall produce a compiler error as v[0] would be const, to maintain const-correctness.
With the underlying type defined as mpl::if_<boost::is_const<T>, const void*, void*>::type this is accomplished. But using void* const always as the underlying type and using const_cast during returning (and inserting) elements, would not const-correctness be lost?
I presume that Scott only intended to suggest abandoning const-correctness in the implementation, not the interface. Thus the above assignment would still cause an error. Since type safety has already been abandoned by making all pointers void*, const-correctness doesn't seem much of an additional sacrifice. John Bytheway