Re: Pretty-printed MI vars with children get "{...}" instead of the wanted string

[Vladimir Prus <vladimir at codesourcery dot com>]

Noam Yorav-Raphael wrote:

> Hello,
> 
> I sent this as a bug (
> http://sourceware.org/bugzilla/show_bug.cgi?id=10616 ), but as this is
> an intended behavior, Tom asked me to raise the subject on the mailing
> list.
> 
> The current situation in the python branch is that if a pretty-printer
> has both a to_string() method and a children() method, in MI the
> to_string method is ignored and instead "{...}" is used. The result is
> that in a GUI which uses gdb as a backend, a variable gets no
> meaningful text near it; it looks like this:
> 
> [+] x   {...}
> [+] y   {...}
> 
> Only if I open the "x" node, I see meaningful stuff:
> 
> [-] x    {...}
>  |---a   1
>  |---b   2
>  |---c   3
>  ... more fields
> [+] y   {...}
> 
> I have structs with quite a few number of fields, but only a few
> usually change during the running of the program and should be
> constantly viewed. I wrote a pretty-printer for the struct which
> returns the important fields in the to_string() method, so now the GUI
> should look like this:
> 
> [+] x   a=1,b=2
> [+] y   {...}
> 
> And of course I can open the "x" node if I'm interested in some other
> fields. This saves a lot of screen space and lets the user focus on
> the important stuff.
> 
> I also wrote a pretty-printer to display 64-bit machine words, which
> are treated as a vector of 8 bytes. Thus I see in the GUI:
> 
> [+] x   1,2,3,4,5,6,7,8
> [+] y   {...}
> 
> I might have to make the "locals" pane (which is located on the left
> side of the screen) a bit wider, but since today's screens are quite
> wide, it's not a problem.
> If the to_string() method is ignored, I have to open the variable view
> in order to see the bytes:
> 
> [+] x   {...}
>  |---[0]   1
>  |---[1]   2
>  |---[2]   3
>  ... 5 more lines
> [+] y   {...}
> 
> Since I only have about 30 rows to view locals, using 9 of them just
> to view the value of x is quite wasteful.
> 
> So, can this be changed? I'm sure there are reasons why the current
> behavior was chosen, but as you see, using the to_string() method can
> improve the debugging experience significantly in some situations.

Unfortunately, the to_string implementation for std::vector produces
huge string that makes sense for CLI, but makes no sense for MI. So, for
now we went with backward-compatible behaviour that composite values in
MI are shown as "{...}" on top-level.

While this might be possible to extend in future, I think we should stop
for 7.0. Reliable pretty-printing in frontend is hard, let's get basics
working first.

- Volodya