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Re: Status of the PPH implementation

On 5/25/12, Jonathan Wakely <> wrote:
> On 25 May 2012 21:26, Diego Novillo wrote:
> > - Some files give syntax errors when compiled in isolation
> > because they are missing symbols. for example, ext/algorithm
> > needs __gnu_cxx::is_sorted.
> I'm not sure what this refers to (<ext/algorithm> either
> defines __gnu_cxx::is_sorted or has a using declaration for
> std::is_sorted in C++11 mode) but I'm very much in favour of
> anything that moves towards support for modules, so if there's
> anything libstdc++-related I can do to help please let me know.

The main thing you can do is try to make sure that all headers
are independently compilable.  If that isn't possible, then those
headers that cannot be independently compilable should be identified
and/or segregated.

As an example, there is on header that expect a type parameter
in a preprocessor symbol.  That header is included several times
with different #defines ahead of it.  Clearly that structure is
there to reduce the redundancy in the headers.  However, it's
not friendly to interpreting the headers as modules.  There are
two techniques that are friendlier.  First, define a macro that
produces the declarations.  Second, have an external script generate
the headers.  The redundancy will appear in the user-facing headers,
but not in the sources programmed.

The second thing you can do is minimize the number of behaviors
that depend on command-line arguments.  These bloat the number of
compiled forms of any given header.

Lawrence Crowl

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