[boost] Re: About the "base_from_member" template in boost lib

From: Daryle Walker (darylew_at_[hidden])
Date: 2002-08-24 23:44:35

on 8/23/02 4:50 PM, Xiaoming Shi at polyrandom_at_[hidden] wrote:

> As for the base_from_member template
> http://www.boost.org/libs/utility/base_from_member.html the typical usage is
> as follow:
> ----------------------------------------------------------------------------
> ---------------------------------------------------
> class someclass
> : private boost::base_from_member<int>
> , private boost::base_from_member<long>
> , private boost::base_from_member<string,1>
> , private boost::base_from_member<string,2>
> {
> // Helper typedef's
> typedef boost::base_from_member<int>        pbase0_type;
> typedef boost::base_from_member<long>       pbase1_type;
> typedef boost::base_from_member<string, 1>  pbase2_type;
> typedef boost::base_from_member<string, 2>  pbase3_type;
> 
> void some_func()
> {
> pbase0_type::member=0;
> pbase3_type::member="aaa";
> .............
> ----------------------------------------------------------------------------
> ---------------------------------------------------
[TRUNCATE complicated extension with typelists and without constructors]

That is not a typical usage.  I meant for that template to be used in
situations like:

class fdostream : public std::ostream {
  protected:
    fdoutbuf buf;
  public:
    fdostream (int fd) : buf(fd), std::ostream(&buf) {
    }
};

(It's from "The C++ Standard Library" book by Nicolai M. Josuttis.)  The
author wants the initialization to happen in the order given.  C++'s rules
will make the "std::ostream" base get initialized before the "buf" member,
so the base will use a pointer to unprepared memory.  This template is
supposed to help get around the problem.

If you do not need a data member to be part of an initialization of a base
class, then you do not need to use "base_from_member" at all.  There is no
need to extend the template with type-lists, especially if you drop the
pass-through constructors.

-- 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT mac DOT com