Re: Are throwing default constructors bad style, and if so, why?
Eugene Gershnik wrote:
On Sep 19, 11:23 pm, Andrei Alexandrescu
<SeeWebsiteForEm...@erdani.org> wrote:
The advantage of separating teardown from deallocation is that there are
no more dangling pointers,
They can still happen if they point to external resources.
and consequently even incorrect programs
could achieve predictable, reproducible, and consequently debuggable
behavior.
This assumes that the zombie state and the entire state machine it
brings to every object are implemented correctly. Which from my
experience with GC languages is as hard to do as to correctly use
manual memory management. If there is a mistake in zombification or
checking for a zombie-ness you might still have crashes or other forms
of UB when accessing an object in this state. Also note that usually
zombie state will be a rarely used and debugged one. It is also often
only accessed under specific timing conditions which makes the
'reproducible' argument weak.
Au contraire, the fact that zombie state is the default-constructed
state makes it very easy to both attain and check. It only requires
careful writing of the destructor (it would be nicer still if the
compiler planted the appropriate code itself). Also, since all member
functions must check for the default-constructed state to start with,
they won't have any trouble in dealing with zombie objects.
Once we accept the reality of "destroyed but not deallocated" objects,
we need to define that state appropriately. That state claims no more
resources than the bits of Foo alone, and must be detected by all member
functions of the object such that they have defined behavior for that state.
Yep. And this is hard to get right (and maintain being right).
Well if you implemented std::vector, wouldn't you check for the empty
vector (begin == end == store_end) anyway? Would you argue that empty
states bring trouble to implementing std::vector?
Andrei
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