Custom range for boost::range library - c++

Question

I’m writing filter and map algorithms using boost::range library:
template <class Range> struct Converter
{
Converter(const Range& p_range) : m_range(p_range) {}
template<class OutContainer> operator OutContainer() const
{
return {m_range.begin(), m_range.end()};
}
private:
Range m_range;
};
template<class Range> Converter<Range> convert(const Range& p_range) { return {p_range}; }
template<class Range, class Fun> auto map(Range&& p_range, Fun&& p_fun)
{
return convert(p_range | boost::adaptors::transformed(p_fun));
}
template<class Range, class Pred> auto filter(Range&& p_range, Pred&& p_pred)
{
return convert(p_range | boost::adaptors::filtered(p_pred));
}
Right now I can use them like this:
std::vector<int> l_in = {1, 2, 3, 4, 5};
std::vector<int> l_tmp_out = filter(l_in, [](int p){ return p < 4; });
std::vector<int> l_out = map(l_tmp_out, [](int p){ return p + 5; });
I would also like to write code this way:
map(filter(l_in, [](int p){ return p < 4; }), [](int p){ return p + 5; });
Unfortunately my Converter class does not compose with boost::range algorithms so this example does not compile. I'm looking for a proper way to change that.
UPDATE
I followed #sehe link and it turned out that all I had to do was to add this four lines to Converter class:
using iterator = typename Range::iterator;
using const_iterator = typename Range::const_iterator;
auto begin() const { return m_range.begin(); }
auto end() const { return m_range.end(); }

Answer 1

Here's my take on things:
Live On Coliru
#include <boost/range.hpp>
#include <boost/range/adaptors.hpp>
#include <boost/range/algorithm.hpp>
#include <iostream>
#include <vector>
namespace MyRange {
template <typename R> struct Proxy {
Proxy(R&& r) : _r(std::move(r)) {}
Proxy(R const& r) : _r(r) {}
template <typename OutContainer> operator OutContainer() const {
return boost::copy_range<OutContainer>(_r);
}
using iterator = typename boost::range_mutable_iterator<R>::type;
using const_iterator = typename boost::range_const_iterator<R>::type;
auto begin() const { return range_begin(_r); }
auto end() const { return range_end(_r); }
auto begin() { return range_begin(_r); }
auto end() { return range_end(_r); }
private:
R _r;
};
template <typename R> auto make_proxy(R&& r) { return Proxy<R>(std::forward<R>(r)); }
template <typename Range, typename Fun> auto map(Range&& p_range, Fun&& p_fun) {
return make_proxy(std::forward<Range>(p_range) | boost::adaptors::transformed(std::forward<Fun>(p_fun)));
}
template <typename Range, typename Pred> auto filter(Range&& p_range, Pred&& p_pred) {
return make_proxy(std::forward<Range>(p_range) | boost::adaptors::filtered(std::forward<Pred>(p_pred)));
}
}
int main() {
using namespace MyRange;
{
std::vector<int> l_in = {1, 2, 3, 4, 5};
std::vector<int> l_tmp_out = filter(l_in, [](int p){ return p < 4; });
std::vector<int> l_out = map(l_tmp_out, [](int p){ return p + 5; });
boost::copy(l_out, std::ostream_iterator<int>(std::cout << "\nfirst:\t", "; "));
}
{
boost::copy(
map(
filter(
std::vector<int> { 1,2,3,4,5 },
[](int p){ return p < 4; }),
[](int p){ return p + 5; }),
std::ostream_iterator<int>(std::cout << "\nsecond:\t", "; "));
}
}
Prints
first: 6; 7; 8;
second: 6; 7; 8;
NOTES
it uses std::forward<> more accurately
it uses const/non-const iterators
it uses Boost Range traits (range_mutable_iterator<> etc.) instead of hardcoding assuming nested typedefs. This allows things to work with other ranges (e.g. std::array<> or even int (&)[]).
the user-defined converson operator uses boost::copy_range<> for similar reasons