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

[Philipp Fent <fent@in.tum.de>]

Hi Jonathan,

I've sent an updated patch addressing your comments here:
https://gcc.gnu.org/pipermail/libstdc++/2022-September/054587.html

Details below.

On 06.09.22 13:27, Jonathan Wakely wrote:
>> +        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).

I factored this into a reusable access_streambuf_ptrs().

>> +        # 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.

Good catch! I fixed that bug and added a test for this in the testsuite.

> 
>> +        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.

I didn't know one could redirect a stringstream, since its rdbuf() 
method hides the basic_ios rdbuf() methods. But of course, that's still 
possible via the base class...
The Patch v2 checks that case in the StdStringStreamPrinter constructor, 
and follows that redirect in to_string().

> 
>> +
>> +    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?

Indeed. The updated patch registers the pretty printer for all flavors.