Re: [boost] Giving names to formal parameters of lambda expressions: a named placeholder trick.

29 Mar 2008


      AMDG

Giovanni Piero Deretta wrote:
...
That is, something like this is actually possible:
void foo  () {
    int i =
     lambda
      // introduce formal parameter names
      [  _<class left>(),
         _<class right>()
      ]
      // now use them!
      [_<left>() + _<right>()]
      // actual parameters values
     ( 10,  20 );
  }
The _<...>() looks a bit clumsy, but it might be actually usable.
I'd rather not use _.  arg sound better.
lambda<class left, class right>(arg<left>() + arg<right>())
...
Still the non-descriptive _N placeholders bother me.
Using the named placeholder trick it would become:
typedef   compose<
    // introduce formal args names
    args<
      class range
    , class init
    , class op
    >
    // expression
  , fold(reverse(range), init, op)
    // function name
...
reverse_fold;
With complex expressions, this could be quite a readability improvement.
Except for confirming that the above expression does actually compile
(at least with gcc), I have yet to try to implement it,
but I think it should be fairly easy.
Comments
Wow.  This is awesome.  You'd probably better reference the standard 
(3.3.1/5)
since most people will look at this and be surprised to find that
it's legal.  (I certainly was)

In Christ,
Steven Watanabe