Re: libstdc++ pretty-printers vs. inspecting objects

[Avi Kivity <avi at cloudius-systems dot com>]

On 07/26/2015 10:45 PM, Jonathan Wakely wrote:
> On 26 July 2015 at 16:06, Avi Kivity wrote:
> > libstdc++ pretty-printers are a life-saver when inspecting containers.
> > However, they also hide information: the type of the element being inspected
> > (can often be inferred by the user) and the address of the element (which
> > cannot).
> >
> > Is there a way to add this information to the pretty-printers output?
> In principle we can add anything, the difficulty is figuring out how
> to present it clearly without cluttering or obscuring the output.
>
> Suggestions for improving the printers are very welcome.

Well, with gdb support, I can go wild:

(gdb) print /addresses $container
$5 = std::map<std::string, myvalue> with 5 elements {
      (std::pair<const std::string, myvalue>*)0x46374672 { "foo": 
whatever },
      ...
}

(the address printout is designed for easy cut'n'paste)

or even

(gdb) print $container
$5 = std::map<std::string, myvalue> with 5 elements {
      $6 = { "foo": whatever },
      $7 = { "bar": another one },
      ...
}

(gdb) print &$6
$8 = (std::pair<const std::string, myvalue>*)0x46374672

Without gdb support, we could have a user-defined flag to toggle address 
printing

(gdb) glibcxx set print addresses on
(gdb) print $container
$5 = std::map<std::string, myvalue> with 5 elements {
      (std::pair<const std::string, myvalue>*)0x46374672 { "foo": 
whatever },
      ...
}

xmethods for operator[] would also help, but these would require the 
user to write xmethods for user-defined constructors if used as a 
container key.

I realize this is a lot of work, but as it is, debugging modern C++ 
programs is quite a headache with g++/gdb.  gdb won't even load binaries 
compiled with -O0, and with -O2, you're either in the maze that is 
libstdc++, or you see nice output from the prettyprinters but you can't 
inspect the data further.