namespace boost {

class in_place_init_t { /* see below */ } ;
const in_place_init_t in_place_init ( /* see below */ ) ;

class in_place_init_if_t { /*see below*/ } ;
const in_place_init_if_t in_place_init_if ( /*see below*/ ) ;

}

Classes in_place_init_t and in_place_init_if_t are empty clsses. Their purpose is to control overload resolution in the initialization of optional objects. They are empty, trivially copyable classes with disabled default constructor.