Re: [Boost-users] shared_ptr as reference argument

Subject: Re: [Boost-users] shared_ptr as reference argument
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2009-03-20 18:50:10

On Fri, Mar 20, 2009 at 2:18 PM, Steven Watanabe <watanabesj_at_[hidden]> wrote:
> AMDG
>
> Ovanes Markarian wrote:
>>
>> On Fri, Mar 20, 2009 at 5:00 PM, Steven Watanabe
>> <watanabesj_at_[hidden]>wrote:
>>
>>>
>>> If you have a reference to const, the compiler cannot make
>>> any assumptions about whether it is modified or not, because
>>> of const_cast.  Objects declared const are another matter,
>>> because it is undefined behavior to modify them.
>>>
>>>
>>
>> Ok, I don't want to stick on this. In my initial post, I wrote that const
>> provides a compiler an additional information which !!!might!!! be used
>> usefully for optimization (!!!big doubt!!!). Emil writes that this info
>> for
>> sure is neglected, but I still don't think so.
>
> It is theoretically possible for the compiler to use the const to optimize,
> but to do so it would have to prove that the functions which
> are called
> a) do not use a non-const reference to the same object.
> b) do not cast away the const.

Since b) is a subset of a), only a) needs to be proven. But const *can
not* possibly help prove a).

> (a) is independent of the presence or absence of const.
> Determining (b) is not very different from determining
> from scratch whether a variable is modified.

Exactly. Therefore, const in references does *not* provide anything
the compiler can use for optimization purposes. If we're talking about
an actual object, not a reference, that's a different story
altogether. Since we keep discussing this, I went ahead and explained
it all here: http://www.revergestudios.com/reblog/index.php?n=ReCode.Const.

Emil Dotchevski
Reverge Studios, Inc.
http://www.revergestudios.com/reblog/index.php?n=ReCode