Re: [PATCH] Slightly better way to __USE_MALLOC

[Brad Spencer <spencer at infointeractive dot com>]

On Thu, Oct 10, 2002 at 06:33:42PM -0400, Phil Edwards wrote:
> On Thu, Oct 10, 2002 at 07:22:36PM -0300, Brad Spencer wrote:
> > 
> > The symbols I'm talking about aren't new.  __default_alloc_template
> > and __malloc_alloc_template already exist and are already both in the
> > GLIBCPP_3.2 version.  I think it would make sense to rename __default
> > to __pool, but it's probably too late for that now.
> 
> Too late for the 3.2 series, but nobody's expecting the library to have
> the same ABI for 3.[34].  There're still too many useful changes to make.
> Like this one.  :-)

Ok!  I've changed __default_alloc_template to __pool_alloc_template
and instantiated both.

> On my whiteboard I have a Grand Master Plan to reorganize the stl_alloc.h
> header, making things more generic, and allowing multiple allocation
> schemes to be instantiated in the library.  For it, I was freely changing
> the names of the internal classes.

Hopefully what I am playing with here is working towards that goal and
not against it.

> Then you can in theory write
> 
>     std::vector<foo,the_malloc_allocator>     x;
>     std::vector<foo,the_default_allocator>    y;
> 
> without needing to carry around instantiations of one of those allocators
> in your code, which is wasteful at best and could lead to multiple symbol
> definitions at worst.

Mmmm.  Yes.  If everything uses rebind (I think it does), then the
following should do the trick:

#ifdef _GLIBCPP_USE_MALLOC
  typedef allocator<void> __malloc_allocator;
  typedef __allocator<void, __pool_alloc_template<true, 0> >
    __pool_allocator;
#else /* _GLIBCPP_USE_MALLOC */
  typedef __allocator<void, __malloc_alloc_template<0> >
    __malloc_allocator;
  typedef allocator<void> __pool_allocator;
#endif /* _GLIBCPP_USE_MALLOC */


But that might wastefully instantiate __allocator<void, ...>.  If
that's problematic, that might mean something like:

  template<typename _Tp>
  struct __allocator_select
  {
#ifdef _GLIBCPP_USE_MALLOC
    typedef __allocator<_Tp, __pool_alloc_template<true, 0> >
      pool_allocator_type;
    typedef allocator<_Tp> malloc_allocator_type;
#else /* _GLIBCPP_USE_MALLOC */
    typedef allocator<_Tp> pool_allocator_type;
    typedef __allocator<_Tp, __malloc_alloc_template<0> >
      malloc_allocator_type;
#endif /* _GLIBCPP_USE_MALLOC */
  };

-- 
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Brad Spencer - spencer@infointeractive.com - "It's quite nice..."
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