Re: [PATCH 2/2] libstdc++: Add pretty printer for std::stringstream

[Jonathan Wakely <jwakely@redhat.com>]

Thanks for the patch. Comments below.


On Sun, 4 Sept 2022 at 19:48, Philipp Fent via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Signed-off-by: Philipp Fent <fent@in.tum.de>
> ---
>  libstdc++-v3/python/libstdcxx/v6/printers.py  | 37 +++++++++++++++++++
>  .../libstdc++-prettyprinters/debug.cc         |  5 +++
>  .../libstdc++-prettyprinters/simple.cc        |  5 +++
>  .../libstdc++-prettyprinters/simple11.cc      |  5 +++
>  4 files changed, 52 insertions(+)
>
> diff --git a/libstdc++-v3/python/libstdcxx/v6/printers.py b/libstdc++-v3/python/libstdcxx/v6/printers.py
> index d70c8d5d616..5083f693387 100644
> --- a/libstdc++-v3/python/libstdcxx/v6/printers.py
> +++ b/libstdc++-v3/python/libstdcxx/v6/printers.py
> @@ -969,6 +969,39 @@ class StdStringPrinter:
>      def display_hint (self):
>          return 'string'
>
> +class StdStringBufPrinter:
> +    "Print a std::basic_stringbuf"
> +
> +    def __init__(self, _, val):
> +        self.val = val
> +
> +    def to_string(self):
> +        pbase = self.val['_M_out_beg']
> +        pptr = self.val['_M_out_cur']
> +        egptr = self.val['_M_in_end']

It would be nice to encapsulate this into a generic helper for all
streambufs, as this part isn't specific to basic_stringbuf. We could
then reuse it for printing spanbufs and other types in future. It
could be a function that takes a streambuf and returns a tuple of
(pbase, pptr, egptr).

> +        # Logic from basic_stringbuf::_M_high_mark()
> +        if pptr:
> +            if not egptr or pptr > egptr:
> +                return pbase.string(length = pptr - pbase)
> +            else:
> +                return pbase.string(length = pptr - egptr)

Shouldn't this be egptr - pbase?

This suggests the tests are inadequate. I think this bug would have
been found by a test something like:

std::stringstream ssin("input", std::ios::in);
// { dg-final { note-test ssin "\"input\"" } }

The (egptr - pbase) case is also needed for printing an istringstream.

> +        return self.val['_M_string']
> +
> +    def display_hint(self):
> +        return 'string'
> +
> +class StdStringStreamPrinter:
> +    "Print a std::basic_stringstream"
> +
> +    def __init__(self, _, val):
> +        self.val = val
> +
> +    def to_string(self):
> +        return self.val['_M_stringbuf']

This assumes that the stream is still using its stringbuf, which is
probably true 99% of the time, but isn't guaranteed. In the following
situation, the printer would give potentially misleading output:

std::stringstream ss("xxx");
ss.rdbuf(std::cin.rdbuf());

I wonder if we want to have a check that self.val['_M_streambuf'] ==
self.val['_M_stringbuf'].address

Alternatively, don't provide a printer for the stringstream at all,
just for the stringbuf, and then when GDB uses its default display for
the stringstream it will show that member.


> +
> +    def display_hint(self):
> +        return 'string'
> +
>  class Tr1HashtableIterator(Iterator):
>      def __init__ (self, hashtable):
>          self.buckets = hashtable['_M_buckets']
> @@ -2232,6 +2265,10 @@ def build_libstdcxx_dictionary ():
>      libstdcxx_printer.add_version('std::', 'initializer_list',
>                                    StdInitializerListPrinter)
>      libstdcxx_printer.add_version('std::', 'atomic', StdAtomicPrinter)
> +    libstdcxx_printer.add_version('std::', 'basic_stringbuf', StdStringBufPrinter)
> +    libstdcxx_printer.add_version('std::__cxx11::', 'basic_stringbuf', StdStringBufPrinter)
> +    libstdcxx_printer.add_version('std::', 'basic_stringstream', StdStringStreamPrinter)
> +    libstdcxx_printer.add_version('std::__cxx11::', 'basic_stringstream', StdStringStreamPrinter)

Wouldn't we want it for ostringstream and istringstream too?