On Fri, Oct 30, 2009 at 3:54 PM, Jeremy Orlow <jor...@chromium.org> wrote:
> On Fri, Oct 30, 2009 at 3:46 PM, Scott Hess <sh...@chromium.org> wrote: > >> Just to be clear for those of us who are wobbly on C++, this is >> because during the constructor or destructor, your object is of the >> class in question, NOT of the class it will finally be, because in the >> constructor the subclass has not been constructed, yet, and in the >> destructor the subclass was already destructed. So calling to the >> subclass virtual implementation would be bad. >> >> Scott Meyers says: http://www.artima.com/cppsource/nevercall.html >> >> Is there any way we could modify an object to assert that it can't >> happen in development? Like scoped_vtable_killer declared in the >> constructor and destructor which makes calling a virtual method on >> that object fatal? >> > > Or is there any sort of built in compiler warning that we could turn on? I > did a bit of searching and was really surprised that I couldn't find any > mention of such a thing. > The compiler could find if it's called directly from the destructor, but there's no way it'd find your case ! The virtual call happens on another thread. To Scott's question: you can blit NULL into the vtable field, if you know where it is (it's not too hard, but depends on the compiler). You'll know if you call it - you'll die. Better yet, you could have a static table of functions that print a message before dying and blit that one, but that gets trickier. Antoine > > On Fri, Oct 30, 2009 at 3:12 PM, Jeremy Orlow <jor...@chromium.org> wrote: >> > I've spent a good deal of this week trying to track down what turned out >> to >> > be a simple but fairly common problem: I forgot virtual dispatch only >> > partially works in destructors. There have been several email threads >> about >> > this, but it still bites us form time to time, so I thought it was worth >> > another reminder. >> > >> > Details: >> > I subclassed ChromeThread which subclasses base::Thread. base::Thread >> calls >> > CleanUp on the thread right before termination. CleanUp is virtual. >> Both >> > ChromeThread and my class override CleanUp(). base::Thread calls Stop() >> in >> > its destructor to stop the thread (if it hasn't already been stopped). >> But >> > by the time you hit destruction, the vtable is no longer available and >> thus >> > the destructor of base::Thread (and anything it calls) does NOT have >> access >> > to the vtable of ChromeThread (or my class). So, if you don't >> explicitly >> > call Stop(), your subclass's CleanUp method will NOT be called. Thus >> the >> > thread was going away without my CleanUp method ever being called. >> > Obviously this affects more than just base::Thread. And this is also >> how >> > you can hit errors with pure virtual methods being called. >> > J >> > > >> > >> > > > > > --~--~---------~--~----~------------~-------~--~----~ Chromium Developers mailing list: chromium-dev@googlegroups.com View archives, change email options, or unsubscribe: http://groups.google.com/group/chromium-dev -~----------~----~----~----~------~----~------~--~---
