Re: [boost] [GSoC][Phoenix3] Regarding mutability of arguments passed to phoenix expressions

Subject: Re: [boost] [GSoC][Phoenix3] Regarding mutability of arguments passed to phoenix expressions
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-04-30 07:17:36

Thomas Heller wrote:
> On Wednesday 28 April 2010 19:43:43 Stewart, Robert wrote:
> > Eric Niebler wrote:
> > > On 4/28/2010 10:19 AM, Mathias Gaunard wrote:
> > > > Thomas Heller wrote:
>
> > > >> The first is: Arguments are mutable. The current phoenix is
> > > >> implemented to be able to this.
> > >
> > > We now have lambdas in C++0x. IMO, we should be paying
> > > attention to the default semantics of lambdas which,
> > > IIRC, accept their arguments by value(?), and have
> > > special syntax for accepting their arguments by
> > > reference. Someone should correct me if I got that
> > > backwards. In the
> >
> > The introducer uses & before names to get references.  By
> > value is the default.
> >
> > > long haul, I think this will satisfy the Principle of
> > > Least Surprise.
>
> giving that pure functional phoenix stuff a little more thought.
> Consider the following code (Reminder, this is just an example):
>
> template< typename Container >
> Container & push_back_impure( Container & c, typename
> Container::value_type
> const & v )
> {
>    c.push_back( v );
>    return c;
> }
>
> template< typename Container >
> Container push_back_pure( Container c,
>    typename Container::value_type const & v )
> {
>    c.push_back( v );
>    return c;
> }

Why not pass the container by reference to const?  You could even use Boost.CallTraits to manage by value versus reference to const.

> int main()
> {
>     std::vector<int> v;
>
>     // this could be equivalent to a phoenix expression like
>     // ( phoenix::push_back( phoenix::_1, 4 ) )( v );
>     push_back_impure( v, 4 );
>
>     // whereas this needs to be written as (to get the same effect):
>     // v = ( phoenix::push_back( phoenix::_1, 5 ) )( v );
>     // looks like a performance hog
>     v = push_back_pure( v, 5 );

The suggestion was that the latter could be handled by explicitly making _1 pass by reference to non-const using ref() or something.  Thus, if writing code like you've shown is a performance problem, write the impure version using ref().

_____
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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