Re: [boost] [lambda] Can't see the wood for the trees"

Subject: Re: [boost] [lambda] Can't see the wood for the trees"
From: Neil Groves (neil_at_[hidden])
Date: 2008-11-21 08:16:03

Comments are inline...

On Fri, Nov 21, 2008 at 1:01 PM, Robert Jones <robertgbjones_at_[hidden]>wrote:

> I have a little code snippet...
>
> #include <iostream>
> #include "boost/range.hpp"
> #include "boost/function.hpp"
> #include "boost/lambda/lambda.hpp"
>

#include <boost/lambda/lambda.hpp>

>
> int main( )
> {
>  using namespace boost::lambda;
>
>  typedef int block[2][4];
>
>  boost::function< int(boost::range_value< block >::type) > third = _1[2];
>
>  block values = {{0,1,2,3},{4,5,6,7}};
>
>  std::for_each(boost::begin(values), boost::end(values),
>      std::cout << third << "\n" );

If you replace the above line with ...

    std::for_each(boost::begin(values), boost::end(values),
        std::cout << boost::lambda::bind(third, _1) << "\n");

 ... it all appears to work.

>
> }

Although, it clearly isn't optimal in terms of performance. Perhaps
accessing the third element could be better achieved with a functor
templatised on the index value?

>
>
> which I want to print the third element of each row of values. I can see
> that
> what I've written is not quite right, since the compiler quite reasonably
> complains
> that it doesn't kknow how to print a function object, but I can't seem to
> write the correct incantation! The error message, if it helps is...
>
> $ g++ -o junk.exe junk.cpp
> junk.cpp: In copy constructor `std::basic_ios<char, std::char_traits<char>
> >::basic_ios(const std::basic_ios<char, std::
> char_traits<char> >&)':
> /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ios_base.h:781: error:
> `std::ios_base::ios_base(const std::ios_base&)
> ' is private
> junk.cpp:17: error: within this context
> junk.cpp: In function `int main()':
> junk.cpp:17: error:   initializing argument 3 of `_Function
> std::for_each(_InputIterator, _InputIterator, _Function) [wi
> th _InputIterator = int (*)[4], _Function = std::basic_ostream<char,
> std::char_traits<char> >]'
> /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_algo.h: In function
> `_Function std::for_each(_InputIterator, _Inp
> utIterator, _Function) [with _InputIterator = int (*)[4], _Function =
> std::basic_ostream<char, std::char_traits<char> >]
> ':
> junk.cpp:17:   instantiated from here
> /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_algo.h:158: error:
> no
> match for call to `(std::basic_ostream<char
> , std::char_traits<char> >) (int[4])'
>
> Thank Folks!
>
> - Rob.

I hope this all helps.

Regards,
Neil Groves

>
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