On Wed, Mar 2, 2016 at 7:45 AM, Andrejs Hanins <andrejs.han...@ubnt.com>
wrote:
> Hi,
>
> Consider simple code below which results in Foo::Call method to be
> actually called for _deleted instance_:
>
> class Foo : public sigc::trackable {
> public:
> void Call() { printf("Foo call\n"); }
> };
>
> Foo* inst = new Foo();
> auto fun = sigc::mem_fun(inst, &Foo::Call); // <-- bad
> //sigc::slot<void> fun = sigc::mem_fun(inst, &Foo::Call); // <-- good
> delete inst;
> fun(); // <--- ooops!
>
>
> The documentation for mem_fun says that "...only if the object type
> inherits from sigc::trackable <
> https://developer.gnome.org/libsigc++/stable/structsigc_1_1trackable.html>
> is the slot automatically cleared...", but the problem is that result of
> mem_fun is not a slot, but a bound_mem_fun which for some reason doesn't
> track referenced object lifetime even if it inherits from trackable. If
> result of mem_fun is put explicitly into a slot, then everything works fine
> and Call() method is not called after instance is freed.
>
> For me it looks like a typical "shoot in the foot" for C++11 programs
> and should be indeed fixed. Just in case - issue is reproducible in current
> master too.
>
your expectations are too high. the behaviour of sigc::trackable as
documented applies ONLY to a slot.
a sigc::mem_fun is just a closure, aka a functor aka a functor object aka
an invokable object. it has not semantics beyond operator(). In C terms, it
is really nothing more than:
struct {
SomeThing* instance_of_thing;
ReturnType (*function_taking_something)(SomeThing*);
};
and operator() is just a call to function_taking_something
(instance_of_thing); there are no magic semantics here, no lifetime
tracking. this is intentional (and correct).
if you want the behaviour of sigc::trackable, then you MUST use
sigc::slot. there's no "shoot in the foot" issue here.
the same behaviour applies to boost::bind() for example, which also returns
a functor/closure.
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