[boost] Re: reference binding

From: David Abrahams (dave_at_[hidden])
Date: 2004-04-15 11:26:47

"Dill, John" <john-dill_at_[hidden]> writes:

>> > While having pass-by-value may be more convenient and occur more
>> > often, the counter-intuitiveness of the semantics will lead to
>> > subtle bugs in people's code who misunderstand or forget bind's
>> > behavior in this area.
>>
>>  A bold prediction in the face of contradictory evidence.
>
> Can you elaborate?  

I can't recall ever having seen a complaint from a user who had a
"subtle bug" because something was copied rather than taken by
reference, especially in the bind library.  The opposite, however, is
not true ;-)

> You may have no problems understanding the
> library, but the first time I tried to bind with a reference, I ran
> into the same confusion.  I am speaking from my own limited experience
> with the library.  I've read the documentation:
>
> // Start of docs The arguments that bind takes are copied and held
> internally by the returned function object. For example, in the
> following code:
>
> int i = 5;
>
> bind(f, i, _1);
>
> a copy of the value of i is stored into the function
> object. boost::ref and boost::cref can be used to make the function
> object store a reference to an object, rather than a copy:
>
> int i = 5;
>
> bind(f, ref(i), _1); // End of docs
>
> I know this makes it explicit that you need a ref or cref to have bind
> store a reference to an object, but there is no reasoning behind this
> design decision in the documentation, or that I can find in the
> implementation and I've not stumbled across it in the mail archives.

So in summary, you weren't confused about how the library works; you
just were unhappy that you had no justification for how it works.
That lends no credence to your claim that "the counter-intuitiveness
of the semantics will lead to subtle bugs in people's code who
misunderstand or forget bind's behavior in this area."

P.S. if you want to understand the justification, see
http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1385.htm

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com