c++range-v3

Why can't I reverse a split-range using range-v3?

I want to split, reverse, and then join a string using range-v3. However, code below won't compile.

#include <range/v3/all.hpp>
#include <iostream>

using namespace ranges;

int main(int argc, char *argv[])
{
    auto str = std::string("abc.def.ghi");
    auto sv = str
              | view::split('.')
              | view::reverse
              | view::join('.');
    std::cout<<sv;
    return 0;
}

Compiler output:

error: invalid operands to binary expression ('decltype(pipeable_access::impl<view<reverse_fn> >::pipe(static_cast<ranges::v3::split_view<ranges::v3::iterator_range<std::_String_iterator<std::_String_val<std::_Simple_types<char> > >, std::_String_iterator<std::_String_val<std::_Simple_types<char> > > >, ranges::v3::view::split_fn::element_pred<std::basic_string<char, std::char_traits<char>, std::allocator<char> > &> > &&>(arg), pipe))' (aka 'void') and 'decltype(make_view(view_access::impl<join_fn>::bind(this->view_, static_cast<char &&>(ts))))' (aka 'view<ranges::v3::detail::pipeable_binder<std::_Binder<std::_Unforced, ranges::v3::view::join_fn &, const std::_Ph<1> &, char> > >'))
range\v3\view\any_view.hpp:60: candidate function not viable: cannot convert argument of incomplete type 'decltype(pipeable_access::impl<view<reverse_fn> >::pipe(static_cast<ranges::v3::split_view<ranges::v3::iterator_range<std::_String_iterator<std::_String_val<std::_Simple_types<char> > >, std::_String_iterator<std::_String_val<std::_Simple_types<char> > > >, ranges::v3::view::split_fn::element_pred<std::basic_string<char, std::char_traits<char>, std::allocator<char> > &> > &&>(arg), pipe))' (aka 'void') to 'ranges::v3::category' for 1st argument
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.21.27702\include\regex:1219: candidate function not viable: cannot convert argument of incomplete type 'decltype(pipeable_access::impl<view<reverse_fn> >::pipe(static_cast<ranges::v3::split_view<ranges::v3::iterator_range<std::_String_iterator<std::_String_val<std::_Simple_types<char> > >, std::_String_iterator<std::_String_val<std::_Simple_types<char> > > >, ranges::v3::view::split_fn::element_pred<std::basic_string<char, std::char_traits<char>, std::allocator<char> > &> > &&>(arg), pipe))' (aka 'void') to 'std::_Node_flags' for 1st argument
range\v3\utility\functional.hpp:725: candidate template ignored: substitution failure [with Arg = void, Pipe = ranges::v3::view::view<ranges::v3::detail::pipeable_binder<std::_Binder<std::_Unforced, ranges::v3::view::join_fn &, const std::_Ph<1> &, char> > >, _concept_requires_724 = false, $3 = nullptr]: cannot form a reference to 'void'
range\v3\utility\functional.hpp:734: candidate template ignored: requirement 'false || (is_pipeable<void>() && is_pipeable<ranges::v3::view::view<ranges::v3::detail::pipeable_binder<std::_Binder<std::_Unforced, ranges::v3::view::join_fn &, const std::_Ph<1> &, char> > > >())' was not satisfied [with Pipe0 = void, Pipe1 = ranges::v3::view::view<ranges::v3::detail::pipeable_binder<std::_Binder<std::_Unforced, ranges::v3::view::join_fn &, const std::_Ph<1> &, char> > >, _concept_requires_733 = false]

Found a way of doing this:

    auto temp_container = str
              | view::split('.')
              | ::ranges::to_vector
              | action::reverse
              ;
    std::string output = temp_container 
              | view::all
              | view::join('.')
              ;

Any better ideas?

Solution

  • You can achieve that, by (a) reversing the string_view, then (b) splitting, then (c) reversing the inner sub-strings, then (d) joining them all back.

    This may be achieved with ranges-v3, but here is the C++23 version:

    auto sv = "abc.def.ghi"sv;
auto str = sv
          | views::reverse
          | views::split('.')
          | views::transform(views::reverse)
          | views::join_with('.')
          | to<std::string>();
std::cout << str;

    https://gcc.godbolt.org/z/qo6qf8WxG

    Note that in the more general case, where the delimiter may have more than a single char, when splitting on it we need to reverse the delimiter also. Thus the code would be:

    auto sv = "abc->def->ghi"sv;
auto delim = "->"sv;
auto str = sv
          | views::reverse
          | views::split(views::reverse(delim)) // <== here
          | views::transform(views::reverse)
          | views::join_with(delim)
          | to<std::string>();
std::cout << str;

    https://gcc.godbolt.org/z/M7xqjcMMc

    To compare above approach, with another approach of splitting into a vector of words then reversing the vector (not seen here, you can see the code in the benchmark link below) -- intuitively and by watching generated assembly it seems that the approach that doesn't need the vector would have better performance. But to get the notion of how better, potentially, here is a benchmark result:

    enter image description here