Hi Paul, On 03/02/2016 03:10 PM, Paul Davis wrote: > > > On Wed, Mar 2, 2016 at 7:45 AM, Andrejs Hanins <andrejs.han...@ubnt.com > <mailto: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. That makes sense, but isn't documentation still confusing, as it mentions 'slot' when speaking about mem_fun. Slot is indeed present in the example code below mem_fun description, but it is not obvious which 'slot' the sentence is talking about. I'm not insisting on this, just a space for a potential improvement. Maybe change the docs to "Note that _in the example below_ only if the object..." > > the same behaviour applies to boost::bind() for example, which also returns a > functor/closure.
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