Re: [PATCH] PR59170 make pretty printers check for singular iterators

[Jonathan Wakely <jwakely at redhat dot com>]

On 16/12/16 14:06 +0100, Jan Kratochvil wrote:
> On Fri, 16 Dec 2016 13:33:52 +0100, Jonathan Wakely wrote:
> > We don't do auto-deref for std::shared_ptr or std::unique_ptr, even
> > though we know the object they point to definitely is live and safe to
> > access, and that's because those types have pointer semantics not
> > reference semantics.
>
> This is wrong std::shared_ptr or std::unique_ptr is not auto-dereferenced for
> GDB printing.  But it may be more a GDB problem, not libstdc++ pretty printers
> problem.
>
> For example glib pretty printers already auto-dereference data structures:
> 	5  GList* list = NULL;
> 	(gdb) p/r list
> 	$1 = (GList *) 0x607a00
> 	(gdb) p list
> 	$2 = 0x607a00 = {0x400810}
> 	/usr/share/glib-2.0/gdb/glib.py
> 	    if type.code == gdb.TYPE_CODE_PTR:
> 		type = type.target().unqualified()
> 		t = str(type)
> 		if t == "GList":
> 		    return GListPrinter(val, "GList")
>
> But that is more a GDB bug that should be solved even for generic pointers.
> Currently while traversing through data structures one has to randomly add or
> remove '*' from the beginning of the GDB print expression:
>
> 	1	class E { int a[1000]; int i=42; } ee;
> 	2	class D { E *e=ⅇ } dd;
> 	3	class C { D &d=dd; } cc;
> 	4	class B { C *c=&cc; } bb;
> 	5	int main() {}
> 	(gdb) p bb
> 	$1 = {c = 0x601030 <cc>}
> 	(gdb) p bb.c
> 	$2 = (C *) 0x601030 <cc>
> Oops, I need to add a dereference:
> 	(gdb) p *bb.c
> 	$3 = {d = @0x601028}
> 	(gdb) p *bb.c.d
> 	No symbol "operator*" in current context.
> Oops, I need to remove a dereference:
> 	(gdb) p bb.c.d
> 	$4 = (D &) @0x601028: {e = 0x601060 <ee>}

Wat?

bb.c.d is not a valid expression.

B::c is a pointer, it should be bb.c->d

Is it GDB policy to make invalid expressions like that "work"?

Because I'm not comfortable emulating that in the libstdc++ printers.


> 	(gdb) p bb.c.d.e
> 	$5 = (E *) 0x601060 <ee>
> Oops, I need to add a dereference:
> 	(gdb) p *bb.c.d.e
> 	$6 = {a = {0 <repeats 1000 times>}, i = 42}
> 	(gdb) p *bb.c.d.e.i
> 	Cannot access memory at address 0x2a
> Oops, I need to remove a dereference:
> 	(gdb) p bb.c.d.e.i
> 	$7 = 42

I had no idea this even worked, I'd have used bb.c->d.e->i because
that's the correct expression for accessing that variable.