...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::container::allocator_arg_t
// In header: <boost/container/scoped_allocator_fwd.hpp> struct allocator_arg_t { };
The allocator_arg_t struct is an empty structure type used as a unique type to disambiguate constructor and function overloading. Specifically, several types have constructors with allocator_arg_t as the first argument, immediately followed by an argument of a type that satisfies the Allocator requirements