Hi All, I am in the middle of writing a large body of numerical code for data analysis and have found the boost library very useful for a number of things. I do a lot of work with std::map and with containers of std::pair objects, and one thing I've always felt annoyed with is the difficulty of interfacing a value_type of std::pair with the STL algorithms. Frequently, for instance, it is nice to represent data as a std::list< std::pair<double,double> > and then to be able to work with the first and second coordinates independently and pass them to std::accumulate() or whatever. The SGI extensions select1st() and select2nd() are somewhat useful but also non-standard, and they frequently require memory-wasting buffers to store only the first or second elements. I have a relatively simple system for solving this problem and it strikes me as the kind of quick thing that might be worth including in boost. The idea is to define a template class, template<typename Iterator> class first_iterator_t { Iterator pair_iter; public: first_iterator_t(Iterator const&); /*...*/ }; which overloads operator*() and operator->() to access the element named "first" of the object returned by Iterator::operator*(). (With something analagous for second_iterator_t.) A function named first_iterator() provides a convenient creation function a-la std::make_pair(). That's it for the most part, the class only needs to provide the required iterator interface which mostly consists of just calls to Iterator::operator functions. There is also the need to define the appropriate iterator_traits typedefs. There is a small issue of deducing whether the iterator needs to be a const iterator or not, which can be solved with partial specialization of a second template class containing nested typedefs. The end result is that something like this is now conveniently available: map<int, double> m; m[1] = 0.1; m[2]=0.2; m[5]=0.5; //prints: 1 2 5 copy(first_iterator(m.begin()), first_iterator(m.end()), ostream_iterator<int>(cout, "\t")); //prints: 0.1 0.2 0.5 copy(second_iterator(m.begin()), second_iterator(m.end()), ostream_iterator<double>(cout, "\t")); I don't see a way to do this otherwise without resorting to something like select1st(), and that also requires the use of std::transform(), which I find distasteful since the required operation is really just a simple copy. So anyway do you think this is useful enough to possibly include in boost somewhere, or is it too trivial? I have what I believe to be correct code, which could be made boost-ready with little effort. Thanks for your advice! -Lewis