Before pushing I'll fix the summary to say "LWG" instead of "LGW" (the
airport code for London Gatwick!)
On Thu, 30 Nov 2023 at 15:51, Jonathan Wakely wrote:
>
> This hasn't been finally approved by LWG yet, but everybody seems to be
> in favour of it. I think I'll push this soon.
>
> Tested x86_64-linux.
>
> -- >8 --
>
> This implements the proposed resolution of LWG 4016, so that
> std::ranges::to does not use std::back_inserter and std::inserter.
> Instead it inserts at the back of the container directly, using
> the first supported one of emplace_back, push_back, emplace, and insert.
>
> Using emplace avoids creating a temporary that has to be moved into the
> container, for cases where the source range and the destination
> container do not have the same value type.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/ranges (__detail::__container_insertable): Remove.
> (__detail::__container_inserter): Remove.
> (ranges::to): Use emplace_back or emplace, as per LWG 4016.
> * testsuite/std/ranges/conv/1.cc (Cont4, test_2_1_4): Check for
> use of emplace_back and emplace.
> ---
> libstdc++-v3/include/std/ranges | 50 +++----
> libstdc++-v3/testsuite/std/ranges/conv/1.cc | 149 ++++++++++++++++----
> 2 files changed, 144 insertions(+), 55 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 9d4c2e01c4d..afd0a38e0cf 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -9229,26 +9229,6 @@ namespace __detail
> { __c.max_size() } -> same_as<decltype(__n)>;
> };
>
> - template<typename _Container, typename _Ref>
> - constexpr bool __container_insertable
> - = requires(_Container& __c, _Ref&& __ref) {
> - typename _Container::value_type;
> - requires (
> - requires { __c.push_back(std::forward<_Ref>(__ref)); }
> - || requires { __c.insert(__c.end(), std::forward<_Ref>(__ref)); }
> - );
> - };
> -
> - template<typename _Ref, typename _Container>
> - constexpr auto
> - __container_inserter(_Container& __c)
> - {
> - if constexpr (requires { __c.push_back(std::declval<_Ref>()); })
> - return std::back_inserter(__c);
> - else
> - return std::inserter(__c, __c.end());
> - }
> -
> template<typename _Cont, typename _Range>
> constexpr bool __toable = requires {
> requires (!input_range<_Cont>
> @@ -9301,17 +9281,33 @@ namespace __detail
> std::forward<_Args>(__args)...);
> else
> {
> - using __detail::__container_insertable;
> - using __detail::__reservable_container;
> using _RefT = range_reference_t<_Rg>;
> static_assert(constructible_from<_Cont, _Args...>);
> - static_assert(__container_insertable<_Cont, _RefT>);
> _Cont __c(std::forward<_Args>(__args)...);
> - if constexpr (sized_range<_Rg> && __reservable_container<_Cont>)
> + if constexpr (sized_range<_Rg>
> + && __detail::__reservable_container<_Cont>)
>
> __c.reserve(static_cast<range_size_t<_Cont>>(ranges::size(__r)));
> - auto __ins = __detail::__container_inserter<_RefT>(__c);
> - for (auto&& __e : __r)
> - *__ins++ = std::forward<decltype(__e)>(__e);
> + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> + // 4016. container-insertable checks do not match what
> + // container-inserter does
> + auto __it = ranges::begin(__r);
> + const auto __sent = ranges::end(__r);
> + while (__it != __sent)
> + {
> + if constexpr (requires { __c.emplace_back(*__it); })
> + __c.emplace_back(*__it);
> + else if constexpr (requires { __c.push_back(*__it); })
> + __c.push_back(*__it);
> + else
> + {
> + auto __end = __c.end();
> + if constexpr (requires { __c.emplace(__end, *__it); })
> + __end = __c.emplace(__end, *__it);
> + else
> + __end = __c.insert(__end, *__it);
> + }
> + ++__it;
> + }
> return __c;
> }
> }
> diff --git a/libstdc++-v3/testsuite/std/ranges/conv/1.cc
> b/libstdc++-v3/testsuite/std/ranges/conv/1.cc
> index 4b6814b1add..b5f861dedb3 100644
> --- a/libstdc++-v3/testsuite/std/ranges/conv/1.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/conv/1.cc
> @@ -203,33 +203,51 @@ test_2_1_3()
> VERIFY( c2.c.get_allocator() == Alloc(78) );
> }
>
> -template<typename C, bool UsePushBack = true>
> +enum AppendKind { None, EmplaceBack, PushBack, Emplace, Insert };
> +
> +template<typename C, AppendKind Kind>
> struct Cont4
> {
> - using value_type = typename C::value_type;
> -
> // Only support construction with no args or an allocator.
> - // This forces the use of either push_back or insert to fill the container.
> + // This forces the use of either emplace_back, push_back, emplace or
> insert.
> Cont4() { }
> Cont4(typename C::allocator_type a) : c(a) { }
>
> - // Satisfying container-insertable requires either this ...
> template<typename T>
> - requires UsePushBack
> + requires (Kind <= EmplaceBack)
> + && requires(C& c, T&& t) { c.emplace_back(std::forward<T>(t)); }
> + void
> + emplace_back(T&& t)
> + {
> + kind = EmplaceBack;
> + c.emplace_back(std::forward<T>(t));
> + }
> +
> + template<typename T>
> + requires (Kind <= PushBack)
> && requires(C& c, T&& t) { c.push_back(std::forward<T>(t)); }
> void
> push_back(T&& t)
> {
> + kind = PushBack;
> c.push_back(std::forward<T>(t));
> - used_push_back = true;
> }
>
> - // ... or this:
> + template<typename T>
> + requires (Kind <= Emplace)
> + && requires(C& c, T&& t) { c.emplace(c.end(), std::forward<T>(t)); }
> + typename C::iterator
> + emplace(typename C::iterator, T&& t)
> + {
> + kind = Emplace;
> + return c.emplace(c.end(), std::forward<T>(t));
> + }
> +
> template<typename T>
> typename C::iterator
> insert(typename C::iterator, T&& t)
> {
> - used_push_back = false;
> + kind = Insert;
> return c.insert(c.end(), std::forward<T>(t));
> }
>
> @@ -254,7 +272,7 @@ struct Cont4
> auto max_size() const { return c.max_size(); }
>
> C c;
> - bool used_push_back = false;
> + AppendKind kind{};
> bool used_reserve = false;
> };
>
> @@ -265,38 +283,113 @@ test_2_1_4()
> // container-insertable<C, range_reference_t<R>>
>
> using Alloc = __gnu_test::uneq_allocator<int>;
> + using Alloc2 = __gnu_test::uneq_allocator<short>;
> using V = std::vector<int, Alloc>;
> + using List = std::list<short, Alloc2>;
>
> std::list<unsigned> l{1u, 2u, 3u};
> - auto c = std::ranges::to<Cont4<V>>(l);
> - static_assert(std::is_same_v<decltype(c), Cont4<V>>);
> + std::list<long> l2{4l, 5l, 6l};
> +
> + // use vector::emplace_back and vector::reserve
> + auto c = std::ranges::to<Cont4<V, EmplaceBack>>(l);
> + static_assert(std::is_same_v<decltype(c), Cont4<V, EmplaceBack>>);
> VERIFY( c.c == V(l.begin(), l.end()) );
> - VERIFY( c.used_push_back );
> + VERIFY( c.kind == EmplaceBack );
> VERIFY( c.used_reserve );
>
> - std::list<long> l2{4l, 5l, 6l};
> - auto c2 = std::ranges::to<Cont4<V>>(l2, Alloc(78));
> - static_assert(std::is_same_v<decltype(c2), Cont4<V>>);
> + // use vector::emplace_back and vector::reserve
> + auto c2 = std::ranges::to<Cont4<V, EmplaceBack>>(l2, Alloc(78));
> + static_assert(std::is_same_v<decltype(c2), Cont4<V, EmplaceBack>>);
> VERIFY( c2.c == V(l2.begin(), l2.end()) );
> VERIFY( c2.c.get_allocator() == Alloc(78) );
> - VERIFY( c2.used_push_back );
> + VERIFY( c2.kind == EmplaceBack );
> VERIFY( c2.used_reserve );
>
> - using Alloc2 = __gnu_test::uneq_allocator<short>;
> - using List = std::list<short, Alloc2>;
> - auto c3 = std::ranges::to<Cont4<List>>(c.c, Alloc2(99));
> - static_assert(std::is_same_v<decltype(c3), Cont4<List>>);
> + // use list::emplace_back
> + auto c3 = std::ranges::to<Cont4<List, EmplaceBack>>(c.c, Alloc2(99));
> + static_assert(std::is_same_v<decltype(c3), Cont4<List, EmplaceBack>>);
> VERIFY( c3.c == List(l.begin(), l.end()) );
> VERIFY( c3.c.get_allocator() == Alloc(99) );
> - VERIFY( c3.used_push_back );
> + VERIFY( c3.kind == EmplaceBack );
> VERIFY( ! c3.used_reserve );
>
> - auto c4 = std::ranges::to<Cont4<List, false>>(c.c, Alloc2(111));
> - static_assert(std::is_same_v<decltype(c4), Cont4<List, false>>);
> - VERIFY( c4.c == List(l.begin(), l.end()) );
> - VERIFY( c4.c.get_allocator() == Alloc(111) );
> - VERIFY( ! c4.used_push_back );
> - VERIFY( ! c4.used_reserve );
> + // use vector::push_back and vector::reserve
> + auto c4 = std::ranges::to<Cont4<V, PushBack>>(l);
> + static_assert(std::is_same_v<decltype(c4), Cont4<V, PushBack>>);
> + VERIFY( c4.c == V(l.begin(), l.end()) );
> + VERIFY( c4.kind == PushBack );
> + VERIFY( c4.used_reserve );
> +
> + // use vector::push_back and vector::reserve
> + auto c5 = std::ranges::to<Cont4<V, PushBack>>(l2, Alloc(78));
> + static_assert(std::is_same_v<decltype(c5), Cont4<V, PushBack>>);
> + VERIFY( c5.c == V(l2.begin(), l2.end()) );
> + VERIFY( c5.c.get_allocator() == Alloc(78) );
> + VERIFY( c5.kind == PushBack );
> + VERIFY( c5.used_reserve );
> +
> + // use list::push_back
> + auto c6 = std::ranges::to<Cont4<List, PushBack>>(c.c, Alloc2(99));
> + static_assert(std::is_same_v<decltype(c6), Cont4<List, PushBack>>);
> + VERIFY( c6.c == List(l.begin(), l.end()) );
> + VERIFY( c6.c.get_allocator() == Alloc(99) );
> + VERIFY( c6.kind == PushBack );
> + VERIFY( ! c6.used_reserve );
> +
> + // use vector::emplace and vector::reserve
> + auto c7 = std::ranges::to<Cont4<V, Emplace>>(l);
> + static_assert(std::is_same_v<decltype(c7), Cont4<V, Emplace>>);
> + VERIFY( c7.c == V(l.begin(), l.end()) );
> + VERIFY( c7.kind == Emplace );
> + VERIFY( c7.used_reserve );
> +
> + // use vector::emplace and vector::reserve
> + auto c8 = std::ranges::to<Cont4<V, Emplace>>(l2, Alloc(78));
> + static_assert(std::is_same_v<decltype(c8), Cont4<V, Emplace>>);
> + VERIFY( c8.c == V(l2.begin(), l2.end()) );
> + VERIFY( c8.c.get_allocator() == Alloc(78) );
> + VERIFY( c8.kind == Emplace );
> + VERIFY( c8.used_reserve );
> +
> + // use list::emplace
> + auto c9 = std::ranges::to<Cont4<List, Emplace>>(c.c, Alloc2(99));
> + static_assert(std::is_same_v<decltype(c9), Cont4<List, Emplace>>);
> + VERIFY( c9.c == List(l.begin(), l.end()) );
> + VERIFY( c9.c.get_allocator() == Alloc(99) );
> + VERIFY( c9.kind == Emplace );
> + VERIFY( ! c9.used_reserve );
> +
> + // use vector::insert and vector::reserve
> + auto c10 = std::ranges::to<Cont4<V, Insert>>(l);
> + static_assert(std::is_same_v<decltype(c10), Cont4<V, Insert>>);
> + VERIFY( c10.c == V(l.begin(), l.end()) );
> + VERIFY( c10.kind == Insert );
> + VERIFY( c10.used_reserve );
> +
> + // use vector::insert and vector::reserve
> + auto c11 = std::ranges::to<Cont4<V, Insert>>(l2, Alloc(78));
> + static_assert(std::is_same_v<decltype(c11), Cont4<V, Insert>>);
> + VERIFY( c11.c == V(l2.begin(), l2.end()) );
> + VERIFY( c11.c.get_allocator() == Alloc(78) );
> + VERIFY( c11.kind == Insert );
> + VERIFY( c11.used_reserve );
> +
> + // use list::insert
> + auto c12 = std::ranges::to<Cont4<List, Insert>>(c.c, Alloc2(99));
> + static_assert(std::is_same_v<decltype(c12), Cont4<List, Insert>>);
> + VERIFY( c12.c == List(l.begin(), l.end()) );
> + VERIFY( c12.c.get_allocator() == Alloc(99) );
> + VERIFY( c12.kind == Insert );
> + VERIFY( ! c12.used_reserve );
> +
> + struct NoCopyPls
> + {
> + NoCopyPls(int) { }
> + NoCopyPls(const NoCopyPls&) { throw; }
> + };
> +
> + // Uses list::emplace_back(const int&) not list::push_back(NoCopyPls&&).
> + (void) std::ranges::to<std::list<NoCopyPls>>(l);
> }
>
> void
> --
> 2.43.0
>
