[Boost-users] boost solution for transforming stl algorithms on proxy iterators?

Subject: [Boost-users] boost solution for transforming stl algorithms on proxy iterators?
From: Oswin Krause (Oswin.Krause_at_[hidden])
Date: 2012-10-16 06:25:56

Hi,

I often have problems which look a bit like this:

     //example code, only for problem statement
     //std::vector<bool> chosen only because it is no container :). If 
this is too silly for you,
     //think about a matrix interpreted as range of row-vectors.
     std::vector<bool> myContainer;//does only have proxy iterators.
     std::vector<double> myDistances; //stores for every element in 
myContainer a distance

     //now i want to to sort the elements of both ranges in place
     //as if they were in a hypothetic range std::vector<std::pair<bool, 
double> >
     //and sort them by the second part:
     boost::sort(make_key_value_range(myContainer,myDistances));//bam, 
not allowed

I omited the definition of make_key_value_range but it should be clear 
that this returns a proxy range with writable iterators and reference 
type similar to std::pair<std::vector<bool>::reference,double&> for 
which swap is overloaded to do the right thing.

This is not allowed by the standard as boost::sort calls std::sort 
which in turn requires random_access_iterators - and they disallow proxy 
references. This issue is AFAIK resolved with C++11 but some of us still 
need to support C++03 - and I am really annoyed by this problem.
The thing is: this will compile on most compilers. One could just lie 
about the iterator category, saying it is a random_access_iterator and 
even VC would be happy with it. However it could just break randomly on 
a new compiler without warning or has severe performance issus (for 
example gcc works, but completely ignores my nicely defined swap and 
instead does a triangle copy :-( ).

So how can one resolve this issue? I have the problem, that i need to 
do this in-place, since copying would result in std::bad_alloc almost 
surely. Also the above single line of code just looks that nice, that i 
just want to have it this way.

So, how do you resolve this issue? is there maybe some boost library 
capable of getting this done? e.g. boost.betterSTL? :-).

Greetings,
Oswin