I am going through 'Functional Programming in C++' (yes, it is getting a bit long in the tooth, but it is what I have right now), and I am trying to convert some of the examples in it to C++20 (g++ 13.3.0) and I have run into a roadblock with the writeup on ranges.
The example I am struggling with has to do with reading text from std::cin and converting this to individual words, lowercasing the letters, removing any non-alphanumerics from this set, then sorting it. The original writeup used the ranges/v3 library and I am having trouble converting this properly to C++20. Specifically the initial part dealing with creating a range (or view) on stdin and then passing this to the next transformation. I am trying to use something like the following:
const std::vector<std::string> words =
// Getting a range of words (tokens) from cin
std::views::istream<std::string>(std::cin)
| views::transform(tolower) // Converting all words to lower-case
| views::transform(isalnum) // Removing non alphanumeric characters from the words
| views::filter([] (const std::string &s) {
return !s.empty();
})
| std::ranges::sort;
This is failing on the first transform with the error:
main.cpp: In function ‘int main(int, char**)’:
main.cpp:38:9: 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::_Transform, int (*)(int) noexcept>’)
37 | std::views::istream<std::string>(std::cin)
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >
38 | | views::transform(tolower) // Converting all words to lower-case
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| std::ranges::views::__adaptor::_Partial<std::ranges::views::_Transform, int (*)(int) noexcept>
In file included from main.cpp:7:
/usr/include/c++/13/ranges:896:7: 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)’
896 | operator|(_Lhs __lhs, _Rhs __rhs)
| ^~~~~~~~
/usr/include/c++/13/ranges:896:7: note: template argument deduction/substitution failed:
/usr/include/c++/13/ranges:896:7: note: constraints not satisfied
In file included from /usr/include/c++/13/compare:37,
from /usr/include/c++/13/bits/char_traits.h:56,
from /usr/include/c++/13/ios:42,
from /usr/include/c++/13/ostream:40,
from /usr/include/c++/13/iostream:41,
from main.cpp:1:
/usr/include/c++/13/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::_Transform, int (*)(int) noexcept>]’:
main.cpp:38:35: required from here
/usr/include/c++/13/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> >]
/usr/include/c++/13/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)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/13/ranges:887:7: 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&&)’
887 | operator|(_Range&& __r, _Self&& __self)
| ^~~~~~~~
/usr/include/c++/13/ranges:887:7: note: template argument deduction/substitution failed:
/usr/include/c++/13/ranges:887:7: note: constraints not satisfied
/usr/include/c++/13/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::_Transform, int (*)(int) noexcept>; _Range = std::ranges::basic_istream_view<std::__cxx11::basic_string<char>, char, std::char_traits<char> >]’:
main.cpp:38:35: required from here
/usr/include/c++/13/ranges:859:13: required for the satisfaction of ‘__adaptor_invocable<_Self, _Range>’ [with _Self = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Transform, int (*)(int)noexcept (true)>; _Range = std::ranges::basic_istream_view<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, char, std::char_traits<char> >]
/usr/include/c++/13/ranges:860:9: in requirements [with _Adaptor = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Transform, int (*)(int)noexcept (true)>; _Args = {std::ranges::basic_istream_view<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, char, std::char_traits<char> >}]
/usr/include/c++/13/ranges:860:44: note: the required expression ‘declval<_Adaptor>()((declval<_Args>)()...)’ is invalid
860 | = requires { std::declval<_Adaptor>()(declval<_Args>()...); };
| ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
cc1plus: note: set ‘-fconcepts-diagnostics-depth=’ to at least 2 for more detail
In file included from /usr/include/c++/13/bits/memory_resource.h:38,
from /usr/include/c++/13/string:58,
from /usr/include/c++/13/bits/locale_classes.h:40,
from /usr/include/c++/13/bits/ios_base.h:41,
from /usr/include/c++/13/ios:44:
/usr/include/c++/13/cstddef:135:3: note: candidate: ‘constexpr std::byte std::operator|(byte, byte)’
135 | operator|(byte __l, byte __r) noexcept
| ^~~~~~~~
/usr/include/c++/13/cstddef:135:18: 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’
135 | operator|(byte __l, byte __r) noexcept
| ~~~~~^~~
/usr/include/c++/13/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)
| ^~~~~~~~
/usr/include/c++/13/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)
| ~~~~~~~~~~~~~~^~~
/usr/include/c++/13/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)
| ^~~~~~~~
/usr/include/c++/13/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)
| ~~~~~~~~~~~~~~^~~
/usr/include/c++/13/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)
| ^~~~~~~~
/usr/include/c++/13/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 typical C++ fashion, this is very precise and completely unintelligible (to me). I have tried creating the initial range a variety of ways but, apparently, they all resolve to the same underlying std::ranges::basic_istream_view so the result is the same.
Apparently this approach worked in the original ranges/v3 library (with istream_range<std::string>(std::cin)), but doesn't work in the current std library.
Is there a way to get this to work?
There's a lot to change in your code:
namespace cpo = std::ranges;
const auto words = []{
using namespace std::views;
auto init
// istream_view:
= istream<std::string>(std::cin)
// take_wile_view:
| take_while(std::not_fn(cpo::empty))
// filter_view:
| filter(std::bind_back(cpo::all_of, std::isalnum, std::identity<>{}))
//transform_view:
| transform([](auto&& s){
auto ret { std::move(s) };
for(char& c : ret)
c = char(std::tolower(c));
return ret; })
//Now pull with 'ranges::to':
| cpo::to<std::vector>();
cpo::sort(init);
return init;
}(/*init*/); // words defined
for is the best you can get.namespace std::ranges that uses pipe operator is to.words is lazily(after sort) initialized from init via NRVO+direct initialization.