Re: [Boost-users] boost::range RangeConcept problem.

Subject: Re: [Boost-users] boost::range RangeConcept problem.
From: Akira Takahashi (faithandbrave_at_[hidden])
Date: 2013-01-16 00:35:15

Hi Hurcan,

2013/1/16 Hurcan Solter <hsolter_at_[hidden]>

>
> The following snippet is the mock-up of the problem I am having in order
> to be able to reproduce.
>
>
> --------------------------------------------------------------------------------
> #include <iostream>
> #include <boost/range.hpp>
> #include <boost/range/adaptors.hpp>
> #include <boost/range/algorithm.hpp>
> #include <boost/bind.hpp>
> #include <boost/assign.hpp>
>
>
> using namespace boost::assign;
> struct wxSize
> {
>     wxSize(int height=0,int width=0):m_height(height),m_width(width)
>     { }
>     int GetWidth() const {return m_width;}
>     int m_height;
>     int m_width;
> };
> class wxWindowBase
> {
> public:
>     wxSize GetTextExtent(const std::string val)
>     {
>         return wxSize(0,val.length());
>     }
>     void DoLayout(std::vector<std::string> m_FilteredItems)
>     {
>
>         using boost::adaptors::transformed;
>         wxSize longest = *boost::max_element(m_FilteredItems |
> transformed(boost::bind(&wxWindowBase::GetTextExtent,this,_1)),
>
> boost::bind(std::less<int>(),boost::bind(&wxSize::GetWidth,_1),boost::bind(&wxSize::GetWidth,_2)));
>
>         std::cout <<longest.GetWidth();
>     }
> };
> int main()
> {
>     std::vector<std::string> input;
>     input += "o","two","threexxx","four";
>     wxWindowBase window;
>     window.DoLayout(input);
> }
>
> -----------------------------------------------------------------------------------------
> The above code fails to compile on GCC4.7.2 and MSVC10 although
> it compiles and runs fine on clang3.3
>
> From what I understand ,transformed returns a Single Pass Range and
> max_element expects a ForwardRange
> and that's the gist of the problem.But it works for something like
>
>          max_element(someint_vector |
> transformed(doubleit()),std::less<int>()); without problems.
>
> Is there a elegant way to achieve what I am trying to without using a
> intermediate container?
> Thanks in advance..
>

This problem's some solution:

1. Don't use member function version `bind` in `transformed`.
the `bind`'s result functor is not DefaultConstructible.
An iterator holding such a functor can't conform to even Input Iterator.
(`max_element` need default construct operation for iterator.)

2. Use `regular` function.
`regular` function is Boost.Range extension library's feature.
`regular` function is convert functor to DefaultConstructible and
CopyAssignable.

http://dl.dropbox.com/u/1682460/git/OvenToBoost/libs/range/doc/html/range_extension/reference/range_utilities/regular.html
http://dl.dropbox.com/u/1682460/git/OvenToBoost/libs/range/doc/html/range_extension/reference/regular_extension/transformed.html

#include <boost/range/adaptor/regular_extension/transformed.hpp>

wxSize longest = *boost::max_element(m_FilteredItems |+
transformed(boost::bind(&wxWindowBase::GetTextExtent,this,_1)),

boost::bind(std::less<int>(),boost::bind(&wxSize::GetWidth,_1),boost::bind(&wxSize::GetWidth,_2)));

You can get here:
https://github.com/faithandbrave/OvenToBoost

This extension is now reviewing by Boost.Range developers.

Thanks,
Akira

>>========================
Akira Takahashi
mailto:faithandbrave_at_[hidden]
https://sites.google.com/site/faithandbrave/about/en