> On Dec 2, 2015, at 21:28, Jens Alfke <[email protected]> wrote: > > >> On Dec 2, 2015, at 5:18 PM, Doug Hill <[email protected] >> <mailto:[email protected]>> wrote: >> >> "Certain other operations are described in this International Standard as >> undefined (for example, the effect of dereferencing the null pointer). >> [Note: this International Standard imposes no requirements on the behavior >> of programs that contain undefined behavior. ]” > > Not relevant; I already explained in a previous reply that this does not > dereference a null pointer. You can call nonvirtual methods on NULL pointers > just fine without any runtime issues … just as long as you don’t access any > member variables, of course. > >> The problem seems to be that you can make up C++ code that constructs a NULL >> pointer and then crash when you dereference it. My understanding is that the >> C++ spec unequivocally defines this as an error and therefor it shouldn’t be >> happening at all. > > It’s not a weird situation at all. Something returns you an object pointer. > You call a method on that object. But the pointer you got happens to be NULL. > Ergo, you’ve just called a method on a NULL pointer. > Widget *w = factory.getCurrentWidget(); > Paint *p = w->getPaint(); > If the currentWidget is NULL, the second line does this.
That counts as a pointer dereference (the special case of &*foo is just that, a special case). > In some circumstances it’s useful to be able to handle this — heck, we should > be able to agree on this, since we use this handy behavior of messaging nil > in Obj-C all the time. So Widget::getPaint() could check for this==nil and if > so return NULL instead of crashing. > > It’s not a big deal; I’ve adjusted my code not to do this. I was just curious > where this tightening of the standard came from. I don’t think this was a result of a tightening of the standard, IIRC, this was *always* undefined behavior.
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