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Re: [PATCH] P0356R5 Simplified partial function application
- From: Jonathan Wakely <jwakely at redhat dot com>
- To: libstdc++ at gcc dot gnu dot org, gcc-patches at gcc dot gnu dot org
- Date: Thu, 7 Mar 2019 14:38:37 +0000
- Subject: Re: [PATCH] P0356R5 Simplified partial function application
- References: <20190307141544.GA13593@redhat.com>
On 07/03/19 14:15 +0000, Jonathan Wakely wrote:
* include/std/functional [C++20] (_Bind_front, _Bind_front_t): Define
helpers for bind_front.
(bind_front, __cpp_lib_bind_front): Define.
* testsuite/20_util/function_objects/bind_front/1.cc: New test.
I'm considering something like the attached patch (but with better
names for __tag1 and __tag2 obviously). With this change bind_front
would unwrap nested binders, so that bind_front(bind_front(f, 1), 2)
would create a _Bind_front<F, int, int> instead of
_Bind_front<_Bind_front<F, int>, int>.
That would make the call go straight to the target object, instead of
through an extra layer of wrapper (which should improve compile times,
and also unoptimized runtime performance).
diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
index 8cf2c670648..da61b00bd15 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -839,6 +839,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cplusplus > 201703L
#define __cpp_lib_bind_front 201902L
+ struct __tag1;
+ struct __tag2;
+
template<typename _Fd, typename... _BoundArgs>
struct _Bind_front
{
@@ -849,13 +852,31 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// instead of the copy/move constructor.
template<typename _Fn, typename... _Args>
explicit constexpr
- _Bind_front(int, _Fn&& __fn, _Args&&... __args)
+ _Bind_front(__tag1*, _Fn&& __fn, _Args&&... __args)
noexcept(__and_<is_nothrow_constructible<_Fd, _Fn>,
is_nothrow_constructible<_BoundArgs, _Args>...>::value)
: _M_fd(std::forward<_Fn>(__fn)),
_M_bound_args(std::forward<_Args>(__args)...)
{ static_assert(sizeof...(_Args) == sizeof...(_BoundArgs)); }
+ template<typename... _BArgs, typename... _Args>
+ explicit constexpr
+ _Bind_front(__tag2*, const _Bind_front<_Fd, _BArgs...>& __fn,
+ _Args&&... __args)
+ : _M_fd(__fn._M_fd),
+ _M_bound_args(std::tuple_cat(__fn._M_bound_args,
+ std::make_tuple(std::forward<_Args>(__args)...)))
+ { }
+
+ template<typename... _BArgs, typename... _Args>
+ explicit constexpr
+ _Bind_front(__tag2*, _Bind_front<_Fd, _BArgs...>&& __fn,
+ _Args&&... __args)
+ : _M_fd(std::move(__fn._M_fd)),
+ _M_bound_args(std::tuple_cat(std::move(__fn._M_bound_args),
+ std::make_tuple(std::forward<_Args>(__args)...)))
+ { }
+
_Bind_front(const _Bind_front&) = default;
_Bind_front(_Bind_front&&) = default;
_Bind_front& operator=(const _Bind_front&) = default;
@@ -919,19 +940,42 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_Fd _M_fd;
std::tuple<_BoundArgs...> _M_bound_args;
+
+ template<typename _Fd2, typename... _BArgs>
+ friend class _Bind_front;
};
template<typename _Fn, typename... _Args>
- using _Bind_front_t
- = _Bind_front<decay_t<_Fn>, unwrap_ref_decay_t<_Args>...>;
+ struct _Bind_front_helper
+ {
+ using type = _Bind_front<_Fn, _Args...>;
+ using __tag = __tag1*;
+ };
+
+ template<typename _Fn, typename... _BoundArgs, typename... _Args>
+ struct _Bind_front_helper<_Bind_front<_Fn, _BoundArgs...>, _Args...>
+ {
+ using type = _Bind_front<_Fn, _BoundArgs..., _Args...>;
+ using __tag = __tag2*;
+ };
+
+ template<typename _Fn, typename... _Args>
+ using _Bind_front_t = typename
+ _Bind_front_helper<decay_t<_Fn>, unwrap_ref_decay_t<_Args>...>::type;
+
+ template<typename _Fn, typename... _Args>
+ using _Bind_front_tag = typename
+ _Bind_front_helper<decay_t<_Fn>, unwrap_ref_decay_t<_Args>...>::__tag;
template<typename _Fn, typename... _Args>
_Bind_front_t<_Fn, _Args...>
bind_front(_Fn&& __fn, _Args&&... __args)
- noexcept(is_nothrow_constructible_v<int, _Bind_front_t<_Fn, _Args...>,
+ noexcept(is_nothrow_constructible_v<_Bind_front_tag<_Fn, _Args...>,
+ _Bind_front_t<_Fn, _Args...>,
_Fn, _Args...>)
{
- return _Bind_front_t<_Fn, _Args...>(0, std::forward<_Fn>(__fn),
+ return _Bind_front_t<_Fn, _Args...>(_Bind_front_tag<_Fn, _Args...>(),
+ std::forward<_Fn>(__fn),
std::forward<_Args>(__args)...);
}
#endif