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Date: Fri, 01 May 2015 18:22:00 -0000
From: Jonathan Wakely <jwakely@redhat.com>
To: Daniel =?iso-8859-1?Q?Kr=FCgler?= <daniel.kruegler@gmail.com>
Cc: libstdc++ <libstdc++@gcc.gnu.org>,        gcc-patches List <gcc-patches@gcc.gnu.org>
Subject: Re: [patch] Implement ISO/IEC TS 18822 C++ File system TS
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References: <20150430173236.GT3618@redhat.com> <CAGNvRgAzHFMbsw3PLzEb4zprmGnZiLJux+vJCxY9kC-gCOhvLg@mail.gmail.com>
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On 01/05/15 19:03 +0200, Daniel Krügler wrote:
>b/libstdc++-v3/src/filesystem/path.cc:
>
>- path::compare(const path& p) const noexcept:
>
>Shouldn't the implementation of this noexcept function not try to
>create copies of path objects? Couldn't _Cmpt just hold references to
>_M_pathname?

All your other comments are definitely correct and I'll make the
relevant fixes soon, but I'm not sure what you mean here, could you
clarify?

I think it would be possible to implement a path like:

  class path
  {
    using value_type = ...;
    using string_type = basic_string<value_type>;
    struct _Cmpt;
    using composite_path = pair<string_type, list<_Cmpt>>;
    using sub_path = basic_string_view<value_type>;

    variant<composite_path, sub_path> _M_data;
    enum _Type { ... } _M_type;
  };

  struct _Cmpt : path
  {
    // ...
  };

With the invariant that for all objects of type _Cmpt the variant
member holds a string_view that refers to the string_type object in
some other path.

This would reduce the memory footprint of a path object (but for the
new std::string ABI, only if there are path components longer than the
SSO small string buffer). Even with that design, paths would still
be sizeof(string_type) + sizeof(std::list<_Cmpt>) + sizeof(_Type).

Alternatively, it could be done without a variant, just adding a
string_view member to the path type, which would make it even larger,
but would simplify the implementation.