Re: [Boost-users] boost range question

Subject: Re: [Boost-users] boost range question
From: Neil Groves (neil_at_[hidden])
Date: 2012-07-12 05:20:03

On 12/07/12 09:15, tolik levchik wrote:
> Why min_element give me errors 
> without BOOST_RANGE_ENABLE_CONCEPT_ASSERT defined 0? Is it uses only 
> for makeerror messages more readable? Is it  safe to define it to 0?

Firstly, it is definitely safe to consistently define 
BOOST_RANGE_ENABLE_CONCEPT_ASSERT as 0. This will disable concept 
checking within Boost.Range. The intent of the concept checking is 
two-fold. It will detect usage errors at compile-time that in some cases 
may become run-time errors without the checks. It also provides better 
error messages close to the location of the error.

> #include <vector>
> //#define BOOST_RANGE_ENABLE_CONCEPT_ASSERT 0
> #include <boost/range.hpp>
> #include <boost/range/adaptors.hpp>
> #include <boost/range/algorithm.hpp>
> #include <boost/range/numeric.hpp>
> #include <boost/bind.hpp>
>
> struct foo{
> int x;
> };
>
> int main()
> {
> using namespace boost::adaptors; using boost::bind;
>
> std::vector<foo> vec;
> auto rng = vec | transformed(bind(&foo::x, _1));
> boost::accumulate(rng, 0); // ok
> boost::min_element(rng); // error
> }

I suspect that the underlying transform_iterator alters the return type 
of the dereference operator that technically demotes the iterator category.

If this is the case then this is a defect in the Boost.Range library 
that I shall need to address. I should have used the Boost iterator 
traversal categories.

I shall look into this. Thank you for the report.

Regards,
Neil Groves