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

From: David Abrahams (dave_at_[hidden])
Date: 2008-03-30 17:28:32

on Fri Mar 28 2008, Steven Watanabe <watanabesj-AT-gmail.com> wrote:

> 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>())

  lambda<args(class left, class right)>(arg<left>() + arg<right>())

I actually like the short nondescriptive names.  It's very rare that the
semantic information in a descriptive name is more valuable than the
syntactic economy of the existing syntax.  Just compare the above to:

  _1 + _2.

Which do you find clearer?  When the lambda expression is complicated
enough that having real names is actually a win, I am usually inclined
to write a separate, named function (object), anyway.

> 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)

Not so fast.  It's legal outside a function, but inside a function

    "class A"

refers to a local class of the function, which isn't a legal template
parameter.  You might try the following at
http://www.comeaucomputing.com/tryitout/

  template <class T>
  struct x{};

  template <class T>
  void f()
  {
     x<class A> y;
  }

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