mem_fun doesn't track object lifetime (original) (raw)

Re: [sigc] C++11 'auto' with sigc::mem_fun doesn't track object lifetime (potential crash)

Hi Paul,

On 03/02/2016 03:10 PM, Paul Davis wrote:

On Wed, Mar 2, 2016 at 7:45 AM, Andrejs Hanins <andrejs hanins ubnt com <mailto:andrejs hanins 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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