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Re: breakpoints in constructors
Daniel Jacobowitz wrote:
> On Tue, Apr 29, 2003 at 04:45:06PM -0400, Paul Koning wrote:
> > >>>>> "Daniel" == Daniel Jacobowitz <email@example.com> writes:
> > Daniel> On Fri, Apr 18, 2003 at 01:04:46PM -0700, David Carlton
> > Daniel> wrote:
> > >> I might have some time over the next few weeks (/months) to work
> > >> on the "breakpoints in constructors" issue. Daniel: clearly
> > >> you've thought about this already, so if you happen to have time
> > >> to do a bit of a brain dump on the issue at some point, I'd
> > >> appreciate it.
> > Daniel> Sure. First of all, a rough overview of the problem; might
> > Daniel> as well keep everything in one place.
> > Daniel> With the new GCC 3.x multi-vendor C++ ABI, constructors are
> > Daniel> implemented as multiple functions: C1, the complete object
> > Daniel> constructor [in-charge] C2, the base object constructor
> > Daniel> [not-in-charge] C3, the allocating constructor [not currently
> > Daniel> used]
> > Daniel> Similarly for destructors - most of the rest of this message
> > Daniel> applies to destructors too. The base constructor is
> > Daniel> generally called for the base objects of a derived class,
> > Daniel> esp. with virtual inheritance; it's been a while since I
> > Daniel> looked at exactly when.
> > Daniel> GCC has chosen to implement this by duplicating the function,
> > Daniel> including any user-provided code and any compiler-added code.
> > Daniel> A better implementation would have one copy and labels for
> > Daniel> multiple entry points, on systems where that is supported;
> > Daniel> that's temporarily tabled pending a better description of the
> > Daniel> GCC tree structure to describe multiple entry points.
> > I looked at a few examples to see how they differ. Didn't see any
> > where the two constructors that gcc generates differ at all. Ditto
> > for the two (in charge vs. not in charge) destructors.
> > The "deleting" constructor does what the name suggests, it frees the
> > item at the end. Since the difference is at the end, that doesn't
> > sound like a case where multiple entry points can help.
> > Couldn't one constructor/destructor call another, so that there one
> > "bottom level" constructor or destructor where all three variants
> > eventually end up? Then that would be the one you'd want to match
> > when you set a breakpoint by name or by line.
> > The only drawback I can see is that you'd see an extraneous frame in
> > the callstack.
> Wow, Paul, you're really on top of this one. Yes, that's what Apple
> implemented, and I'm looking over their patches right now :) There are
> some quirks in the implementation which are throwing me for a loop.
> The constructors will differ in some cases involving virtual bases;
> that's what they're for.
I think that you can generate a jump rather than a call, so there is no
extraneous call frame on the stack. (I think that's what the Apple patch
Michael Eager Eager Consulting firstname.lastname@example.org
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