I want to search an input range for an element that has a certain value in a member via an accessor.
range-v3 documentation is... thin, but there are sources such as this answer by AFAIK the 2 main range-v3 developers indicates this kind of thing should Just Work, albeit with sort not find.
Given the code, compiled with g++ -std=c++17 against ericniebler/range-v3 release range-v3-0.10.0, using g++.exe (Rev2, Built by MSYS2 project) 9.3.0:
#include <range/v3/algorithm/find.hpp>
#include <vector>
auto
main() -> int
{
struct S {
int i{};
auto get_i() const { return i; }
};
auto const ss = std::vector<S>(10);
ranges::find(ss, 1, &S::get_i);
return 0;
}
I get a spew of errors:
test.cpp: In function 'int main()':
test.cpp:14:31: error: no match for call to '(const ranges::find_fn) (const std::vector<main()::S>&, int, int (main()::S::*)() const)'
14 | ranges::find(ss, 1, &S::get_i);
| ^
In file included from FOO/include/range-v3/range/v3/range_fwd.hpp:24,
from FOO/include/range-v3/range/v3/algorithm/find.hpp:18,
from test.cpp:1:
FOO/include/range-v3/range/v3/detail/config.hpp:618:27: note: candidate: 'template<class I, class S, class V, class P> constexpr concepts::return_t<I, typename std::enable_if<(((input_iterator<I> && sentinel_for<S, I>) && indirect_relation<ranges::equal_to, typename ranges::detail::select_projected_<P>::apply<I>, const V*>) && concepts::detail::CPP_true(concepts::detail::Nil{})), void>::type> ranges::find_fn::operator()(I, S, const V&, P) const'
618 | #define RANGES_FUNC(NAME) operator() RANGES_FUNC_CONST_ /**/
| ^~~~~~~~
FOO/include/range-v3/range/v3/algorithm/find.hpp:47:24: note: in expansion of macro 'RANGES_FUNC'
47 | constexpr auto RANGES_FUNC(find)(I first, S last, V const & val, P proj = P{})
| ^~~~~~~~~~~
FOO/include/range-v3/range/v3/detail/config.hpp:618:27: note: template argument deduction/substitution failed:
618 | #define RANGES_FUNC(NAME) operator() RANGES_FUNC_CONST_ /**/
| ^~~~~~~~
FOO/include/range-v3/range/v3/algorithm/find.hpp:47:24: note: in expansion of macro 'RANGES_FUNC'
47 | constexpr auto RANGES_FUNC(find)(I first, S last, V const & val, P proj = P{})
| ^~~~~~~~~~~
In file included from FOO/include/range-v3/range/v3/iterator/access.hpp:22,
from FOO/include/range-v3/range/v3/iterator/concepts.hpp:30,
from FOO/include/range-v3/range/v3/algorithm/find.hpp:22,
from test.cpp:1:
FOO/include/range-v3/std/detail/associated_types.hpp: In substitution of 'template<bool B, class T> using enable_if_t = typename ranges::detail::enable_if::apply<T> [with bool B = ranges::readable<std::vector<main()::S> >; T = std::vector<main()::S>]':
FOO/include/range-v3/range/v3/iterator/concepts.hpp:561:19: required by substitution of 'template<class I> using apply = ranges::detail::enable_if_t<(bool)(readable<I>), I> [with I = std::vector<main()::S>]'
FOO/include/range-v3/range/v3/algorithm/find.hpp:48:15: required by substitution of 'template<class I, class S, class V, class P> constexpr concepts::return_t<I, typename std::enable_if<(((input_iterator<I> && sentinel_for<S, I>) && indirect_relation<ranges::equal_to, typename ranges::detail::select_projected_<P>::apply<I>, const V*>) && concepts::detail::CPP_true(concepts::detail::Nil{})), void>::type> ranges::find_fn::operator()(I, S, const V&, P) const [with I = std::vector<main()::S>; S = int; V = int (main()::S::*)() const; P = ranges::identity]'
test.cpp:14:31: required from here
FOO/include/range-v3/std/detail/associated_types.hpp:73:15: error: no class template named 'apply' in 'struct ranges::detail::enable_if<false>'
73 | using enable_if_t = typename enable_if<B>::template apply<T>;
| ^~~~~~~~~~~
In file included from FOO/include/range-v3/range/v3/range_fwd.hpp:24,
from FOO/include/range-v3/range/v3/algorithm/find.hpp:18,
from test.cpp:1:
FOO/include/range-v3/range/v3/detail/config.hpp:618:27: note: candidate: 'template<class Rng, class V, class P> constexpr concepts::return_t<typename ranges::detail::if_then<forwarding_range_<R> >::apply<decltype (ranges::_::begin(declval<Rng&>())), ranges::dangling>, typename std::enable_if<((input_range<Rng> && indirect_relation<ranges::equal_to, typename ranges::detail::select_projected_<P1>::apply<decltype (ranges::_::begin(declval<Rng&>()))>, const V*>) && concepts::detail::CPP_true(concepts::detail::Nil{})), void>::type> ranges::find_fn::operator()(Rng&&, const V&, P) const'
618 | #define RANGES_FUNC(NAME) operator() RANGES_FUNC_CONST_ /**/
| ^~~~~~~~
FOO/include/range-v3/range/v3/algorithm/find.hpp:60:24: note: in expansion of macro 'RANGES_FUNC'
60 | constexpr auto RANGES_FUNC(find)(Rng && rng, V const & val, P proj = P{})
| ^~~~~~~~~~~
FOO/include/range-v3/range/v3/detail/config.hpp:618:27: note: template argument deduction/substitution failed:
618 | #define RANGES_FUNC(NAME) operator() RANGES_FUNC_CONST_ /**/
| ^~~~~~~~
FOO/include/range-v3/range/v3/algorithm/find.hpp:60:24: note: in expansion of macro 'RANGES_FUNC'
60 | constexpr auto RANGES_FUNC(find)(Rng && rng, V const & val, P proj = P{})
| ^~~~~~~~~~~
In file included from FOO/include/range-v3/range/v3/iterator/access.hpp:22,
from FOO/include/range-v3/range/v3/iterator/concepts.hpp:30,
from FOO/include/range-v3/range/v3/algorithm/find.hpp:22,
from test.cpp:1:
FOO/include/range-v3/std/detail/associated_types.hpp: In substitution of 'template<bool B, class T> using enable_if_t = typename ranges::detail::enable_if::apply<T> [with bool B = ranges::indirectly_regular_unary_invocable<int (main()::S::*)() const, __gnu_cxx::__normal_iterator<const main()::S*, std::vector<main()::S> > >; T = ranges::detail::projected_<__gnu_cxx::__normal_iterator<const main()::S*, std::vector<main()::S> >, int (main()::S::*)() const>]':
FOO/include/range-v3/range/v3/iterator/concepts.hpp:552:19: required by substitution of 'template<class Proj> template<class I> using apply = ranges::detail::enable_if_t<(bool)(indirectly_regular_unary_invocable<Proj, I>), ranges::detail::projected_<I, Proj> > [with I = __gnu_cxx::__normal_iterator<const main()::S*, std::vector<main()::S> >; Proj = int (main()::S::*)() const]'
FOO/include/range-v3/range/v3/algorithm/find.hpp:61:15: required by substitution of 'template<class Rng, class V, class P> constexpr concepts::return_t<typename ranges::detail::if_then<forwarding_range_<R> >::apply<decltype (ranges::_::begin(declval<Rng&>())), ranges::dangling>, typename std::enable_if<((input_range<Rng> && indirect_relation<ranges::equal_to, typename ranges::detail::select_projected_<P1>::apply<decltype (ranges::_::begin(declval<Rng&>()))>, const V*>) && concepts::detail::CPP_true(concepts::detail::Nil{})), void>::type> ranges::find_fn::operator()(Rng&&, const V&, P) const [with Rng = const std::vector<main()::S>&; V = int; P = int (main()::S::*)() const]'
test.cpp:14:31: required from here
FOO/include/range-v3/std/detail/associated_types.hpp:73:15: error: no class template named 'apply' in 'struct ranges::detail::enable_if<false>'
73 | using enable_if_t = typename enable_if<B>::template apply<T>;
| ^~~~~~~~~~~
shell returned 1
Making the projection a lambda...
ranges::find( ss, 1, [](auto const& s){ return s.get_i(); } );
...works but seems wasted typing.
Referring directly to the data member...
ranges::find(ss, 1, &S::i);
...works but is not possible if it should be encapsulated behind a const getter, transformer, etc.
What am I doing wrong? Can I not use a pointer-to-member-function as projection? Is it intended?
Edit: clang++ (also on MSYS2) does work here. So I guess this must be a bug in g++. Off to Bugzilla I go... edit: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94973
This is caused by GCC on Windows defaulting to -fms-extensions for compatibility with Microsoft compilers, and that pulling in a non-standard extension that introduces ambiguity between pointers-to-member-data vs -functions written as instance.*member.
We can pass -fno-ms-extensions to stop this and compile such code succesfully, until GCC remove that particular extension, which seems unnecessary and unhelpful nowadays.
Jonathan answered this on my GCC bug 94973:
Jonathan Wakely 2020-05-06 16:26:58 UTC
Aha, the same problem happens on linux if I compile with -fms-extensions
This is the old MS extension that causes x.*f to be accepted when f is a pointer to member function, which should only be valid when used as (x.*f)().
That causes ranges::invoke to think that the projection is a pointer to data member, when actually it's a pointer to member function.
See also PR 94771 comment 4.
Jason, do we want to disable that extension in SFINAE contexts?
Jonathan Wakely 2020-05-06 16:47:42 UTC
They're on by default for mingw, for compatibility with the MS compiler (but in this case it seems the relevant extension is ancient history).