How to use a C++20 istream_view?

Question

I cannot find a single example on the web or in C++20 books that shows how the results of a std::ranges::istream_view can be used with follow-up operations. Every follow-up operation that I try to use results in compiler errors that are too difficult for me to figure out alone.

For example, how should I split a lazy stream of characters that comes in through an istream_view? The following code does not work, but hopefully communicates what I am trying to achieve:

#include <cstdlib>
#include <ranges>
#include <sstream>
#include <fmt/core.h>

auto main() -> int
{
  std::istringstream data{"a,b"};
  for (const auto& item:
      std::views::istream<std::string>(data)
    | std::views::split(",")
  ) {
    fmt::print("{}\n", item);
  }
  return EXIT_SUCCESS;
}

^ I expect this to write "a" on the first line, and "b" on the second line.

GCC 12.2 emits an intimidating error message:

Could not execute the program
Compiler returned: 1
Compiler stderr
<source>: In function 'int main()':
<source>:11:5: error: no match for 'operator|' (operand types are 'std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >' and 'std::ranges::views::__adaptor::_Partial<std::ranges::views::_Split, const char*>')
   10 |       std::views::istream<std::string>(data)
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                       |
      |                                       std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >
   11 |     | std::views::split(",")
      |     ^ ~~~~~~~~~~~~~~~~~~~~~~
      |                        |
      |                        std::ranges::views::__adaptor::_Partial<std::ranges::views::_Split, const char*>
In file included from <source>:2:
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:868:7: note: candidate: 'template<class _Lhs, class _Rhs>  requires (derived_from<_Lhs, std::ranges::views::__adaptor::_RangeAdaptorClosure>) && (derived_from<_Rhs, std::ranges::views::__adaptor::_RangeAdaptorClosure>) constexpr auto std::ranges::views::__adaptor::operator|(_Lhs, _Rhs)'
  868 |       operator|(_Lhs __lhs, _Rhs __rhs)
      |       ^~~~~~~~
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:868:7: note:   template argument deduction/substitution failed:
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:868:7: note: constraints not satisfied
In file included from /opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:37:
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/concepts: In substitution of 'template<class _Lhs, class _Rhs>  requires (derived_from<_Lhs, std::ranges::views::__adaptor::_RangeAdaptorClosure>) && (derived_from<_Rhs, std::ranges::views::__adaptor::_RangeAdaptorClosure>) constexpr auto std::ranges::views::__adaptor::operator|(_Lhs, _Rhs) [with _Lhs = std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >; _Rhs = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Split, const char*>]':
<source>:11:28:   required from here
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/concepts:67:13:   required for the satisfaction of 'derived_from<_Lhs, std::ranges::views::__adaptor::_RangeAdaptorClosure>' [with _Lhs = std::ranges::basic_istream_view<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, char, std::char_traits<char> >]
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/concepts:67:28: note:   'std::ranges::views::__adaptor::_RangeAdaptorClosure' is not a base of 'std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >'
   67 |     concept derived_from = __is_base_of(_Base, _Derived)
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:859:7: note: candidate: 'template<class _Self, class _Range>  requires (derived_from<typename std::remove_cvref<_Tp>::type, std::ranges::views::__adaptor::_RangeAdaptorClosure>) && (__adaptor_invocable<_Self, _Range>) constexpr auto std::ranges::views::__adaptor::operator|(_Range&&, _Self&&)'
  859 |       operator|(_Range&& __r, _Self&& __self)
      |       ^~~~~~~~
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:859:7: note:   template argument deduction/substitution failed:
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:859:7: note: constraints not satisfied
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges: In substitution of 'template<class _Self, class _Range>  requires (derived_from<typename std::remove_cvref<_Tp>::type, std::ranges::views::__adaptor::_RangeAdaptorClosure>) && (__adaptor_invocable<_Self, _Range>) constexpr auto std::ranges::views::__adaptor::operator|(_Range&&, _Self&&) [with _Self = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Split, const char*>; _Range = std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >]':
<source>:11:28:   required from here
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:831:13:   required for the satisfaction of '__adaptor_invocable<_Self, _Range>' [with _Self = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Split, const char*>; _Range = std::ranges::basic_istream_view<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, char, std::char_traits<char> >]
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:832:9:   in requirements  [with _Args = {std::ranges::basic_istream_view<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, char, std::char_traits<char> >}; _Adaptor = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Split, const char*>]
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:832:44: note: the required expression 'declval<_Adaptor>()((declval<_Args>)()...)' is invalid
  832 |       = requires { std::declval<_Adaptor>()(declval<_Args>()...); };
      |                    ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
cc1plus: note: set '-fconcepts-diagnostics-depth=' to at least 2 for more detail
In file included from /opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/streambuf:41,
                 from /opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/streambuf_iterator.h:35,
                 from /opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/iterator:66,
                 from /opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:43:
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/ios_base.h:87:3: note: candidate: 'constexpr std::_Ios_Fmtflags std::operator|(_Ios_Fmtflags, _Ios_Fmtflags)'
   87 |   operator|(_Ios_Fmtflags __a, _Ios_Fmtflags __b)
      |   ^~~~~~~~
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/ios_base.h:87:27: note:   no known conversion for argument 1 from 'std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >' to 'std::_Ios_Fmtflags'
   87 |   operator|(_Ios_Fmtflags __a, _Ios_Fmtflags __b)
      |             ~~~~~~~~~~~~~~^~~
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/ios_base.h:130:3: note: candidate: 'constexpr std::_Ios_Openmode std::operator|(_Ios_Openmode, _Ios_Openmode)'
  130 |   operator|(_Ios_Openmode __a, _Ios_Openmode __b)
      |   ^~~~~~~~
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/ios_base.h:130:27: note:   no known conversion for argument 1 from 'std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >' to 'std::_Ios_Openmode'
  130 |   operator|(_Ios_Openmode __a, _Ios_Openmode __b)
      |             ~~~~~~~~~~~~~~^~~
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/ios_base.h:170:3: note: candidate: 'constexpr std::_Ios_Iostate std::operator|(_Ios_Iostate, _Ios_Iostate)'
  170 |   operator|(_Ios_Iostate __a, _Ios_Iostate __b)
      |   ^~~~~~~~
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/ios_base.h:170:26: note:   no known conversion for argument 1 from 'std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >' to 'std::_Ios_Iostate'
  170 |   operator|(_Ios_Iostate __a, _Ios_Iostate __b)
      |             ~~~~~~~~~~~~~^~~
In file included from /opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/span:42,
                 from /opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/ranges:45:
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/cstddef:132:3: note: candidate: 'constexpr std::byte std::operator|(byte, byte)'
  132 |   operator|(byte __l, byte __r) noexcept
      |   ^~~~~~~~
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/cstddef:132:18: note:   no known conversion for argument 1 from 'std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >' to 'std::byte'
  132 |   operator|(byte __l, byte __r) noexcept
      |             ~~~~~^~~

Answer 1

views::istream is an input range. So any adaptor that you pipe that into has to work on input ranges. views::split requires forward-or-better, which is why you can't split an istream.

But there is also a views::lazy_split that provides a slight differently interface such that it does work for input ranges (see P2210 for added context for why we wanted the forward+ split adaptor).

However, then you have the issue that views::istream<string> is a range of string that you're trying split on a comma. But that's not quite right, what you want you want is a range of char so that you end up with a range of range of char. What you mean is views::istream<char>.

Fixing that, this works:

auto main() -> int
{
  std::istringstream data{"a,b"};
  fmt::print("{}\n",
      std::views::istream<char>(data)
    | std::views::lazy_split(','));
}

That prints [['a'], ['b']]