Why does boost::ptr_list uses underlying void *?

Question

The boost ptr_list documentation states that the container uses an underlying std::list<void*>.

Why are they using this type instead of a more specialized std::list<T*>?

Answer 1

It's probably to cut down on the number of template instantiations. If it uses a std::list<T*>, then every use of ptr_list<T> would also instantiate std::list<T*>. That's a lot of instantiations if you use ptr_list a lot.

Answer 2

This makes it easy to share almost all the code regardless of the type(s) over which you instantiate it. Nearly all the code is in the single std::list<void *>. Each instantiation only adds code to cast between T * and void * where needed.

Of course, modern compilers/linkers can do a fair amount of this without such help, but that hasn't always been the case (and some people still use older tool chains, for various reasons).