Re: Improved pretty printing for smart pointers

[Tom Tromey <tromey at redhat dot com>]

>>>>> "Jonathan" == Jonathan Wakely <jwakely.gcc@gmail.com> writes:

Michael> I know that there are ways to dereference a smart pointer on
Michael> the gdb command line. But when using an IDE (like Eclipse CTD,
Michael> Qt Creator or kdevelop), the pretty printer, in its current
Michael> state, is in the way: The variable view of the debugger will
Michael> show a message like

Michael> "std::shared_ptr (count 1, weak 0) 0x61e1a8"

Jonathan> This sounds like a deficiency of those debugger GUIs.

The fundamental problem is probably in gdb.

The current SharedPointerPrinter just renders the pointer value into a
string:

        return '%s (%s) %s' % (self.typename, state, self.val['_M_ptr'])

But this strips any identifying info that the MI client might use to
dereference this.  It isn't obviously a pointer and any type information
isn't present in a usable (by the MI client) form.

Having 'children' would solve this.


There's another issue in gdb where if a printer has children, then the
to_string result is replace with "{...}" for MI.  This means that
returning the various counts in to_string but the pointer in children
will make the counts invisible to the MI client :(

There's a patch for this pending... maybe we should put it in.


I think the downside of moving all the fields to 'children' is just that
the CLI will be more verbose.  I'm not sure this is so bad.  What this
would mean for the patch is simplifying the to_string method to just
show the type name; then I think the output would be like:

$1 = std::shared_ptr = {Use count = 1, Weak count = 2, Managed value = 0x605028}


Maybe there is some other gdb change that would make this better.
I haven't thought of one but I'm open to suggestions.

Jonathan> Would it work if we changed the output to this?
Jonathan> $1 = std::shared_ptr<int> (usecount 1, weakcount 2) { 0x605028 }
Jonathan> $2 = std::weak_ptr<int> (usecount 1, weakcount 2) { 0x605028 }
Jonathan> $4 = std::unique_ptr<double> { 0x605040 }

This runs into the MI issue.


In the patch itself, creating an iterator is overkill.  That approach is
mostly useful for containers, not something with a fixed number of
fields.  For fixed fields, just return a tuple or a list from
'children'.

Tom