Re: [Boost-Users] Re: Newbie lambda question

From: Peter Dimov (pdimov_at_[hidden])
Date: 2003-06-25 06:44:05

Ian wrote:
> Markus
>
> Thanks for the reply, but no dice. The idea was to use the address of
> the current item as a quick and dirty counter modular counter,
> However this code:
>
>   std::for_each(begin, end,
>               l::_1 = l::ll_static_cast<int>(_1) % 3);
>
> does not take the address but the items value, and unfortunately this:
>
>   std::for_each(begin, end,
>               l::_1 = l::ll_static_cast<int>(&_1) % 3);
>
> does not compile. Now I could get a bit more realistic and just code
> the thing a bit cleaner like this:
>
>   int x = 0;
>   for_each( lookup_, lookup_ + (sizeof(lookup_) / sizeof(unsigned
>       int)), _1 = ++var(x) % 3
>   );
>
> But that's kinda skirting around the real issue which is, if I can't
> use lambda to *directly* replace code that is trivial with a hand
> written loop, what chance when the complexity is raised a bit?

It is a feature that your original (nonportable) code cannot be replaced by
a lambda expression. Why not just use the correct version? Or even better:

template<class T, size_t N> size_t array_size(T (&a) [N]) { return N; }

int x = 0;
std::generate_n(lookup_, array_size(lookup_), ++ll::var(x) % 3);

You can even get rid of x by using

std::generate_n(lookup_, array_size(lookup_), boost::bind<int>(++ll::_1 % 3,
0));

if you are using the CVS version of boost::bind; 1.30 doesn't approximate

http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1438.htm

well.