Re: [iostreams] Custom allocator support for compression filters

From: Peter Dimov (pdimov_at_[hidden])
Date: 2024-09-04 16:15:19

Daniel Boehme wrote:
> Hello everyone,
> 
> I'm currently looking into custom allocators with the
> boost::iostreams::gzip_compressor and now need some help understanding
> why the code is the way it is.
> 
> My problem is that I need to control how zlib internally de-/allocates memory.
> 
> It seems to have support for using custom de-/allocation functions. And
> iostreams seems to have used it some time ago.
> However, passing the custom allocator to zlib has been disabled in this
> commit:
> https://github.com/boostorg/iostreams/commit/95baf782098f0dee00cc7ca6
> f62b9b105ca6573f
> 
> Does anyone happen to know what has been non-conforming and why it was
> disabled? I could not find any information about that in the code, on the
> mailing lists, or anywhere else in the internet.
> 
> I'm tempted to submit a patch that enables it, but for that I need to
> understand the reasons behind that deliberate decision.
> 
> Any hint is welcome.

My _guess_ is that the non-conformity refers to the fact that pointers
to C functions (extern "C") and pointers to C++ functions are not guaranteed
to be compatible. Since zlib accepts pointers to C functions, and the allocator
wrapper class defines C++ functions, assigning one to the other is not
guaranteed to work.

However... in practice, this works essentially everywhere. EDG-based compilers
used to reject this (many years ago), but I don't know whether this remains
a problem today on any platform we support.