Re: [boost] [xint] Boost.XInt formal review

Subject: Re: [boost] [xint] Boost.XInt formal review
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-03-05 14:34:49

Chad Nelson wrote:
> DE <satan66613_at_[hidden]> wrote:
>
> > for what reason integer_t inherits its implementation
> > *virtualy* ? you know that virtual inheritance involves
> > (possibly unneeded) overhead
>
> Because many (all?) of the classes it inherits from have to
> have access to the same data, so they all inherit from the
> same base class. The "virtual" there tells the compiler that
> all of those inheritances come from a single instance of the
> base class, rather than a separate one in every case.

One uses virtual inheritance when there is state that would be duplicated in the face of multiple inheritance via the dread diamond inheritance pattern.  If your policy classes have state and there is any chance of a diamond inheritance pattern, then virtual inheritance is not unreasonable.

By now, of course, you know that CRTP is the better solution in your case.

> I believe this use of virtual results only in compile-time
> overhead. Logically there shouldn't be any need for the
> run-time code to even notice that virtual was used there. But
> there may be more going on behind the scenes than I'm aware of.

Virtual function or data member access, in the face of virtual inheritance, incurs runtime overhead to determine the base subobject offset, if the particular base subobject is not first.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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