[PATCH] libstdc++: Fix construction function_ref from nontype<&S::x> and reference_wrapper [PR121858]
Hewill Kang
hewillk@gmail.com
Fri Oct 24 16:10:32 GMT 2025
`volatile std::reference_wrapper<S>` will cause `_Ur __uref = __ref;` hard
error. But I'm not sure we really need to care about it.
Tomasz Kamiński <tkaminsk@redhat.com> 於 2025年10月24日 週五 下午11:33寫道：
> To reduce instantiation count, function_ref(nontype<&S::x>, r) previously
> reused the invoker from function_ref(nontype<&S::x>, &r). This assumed r
> was
> always a reference to S or a derived class. However, this constructor is
> also
> valid for lvalues (but not rvalues) of reference_wrapper specializations.
>
> This patch fixes this by unwrapping the reference_wrapper<T> within the
> nontype<f> constructor when f is a member pointer. This is achieved by
> casting
> the argument to typename __inv_unwrap<_Td>::type _GLIBCXX_MOF_CV&. This
> results
> in U& if _Td is a (possibly-const) reference_wrapper<U>, and _Td
> _GLIBCXX_MOF_CV&
> otherwise. We use __inv_unwrap because unwrap_reference_t does not handle
> cv-qualified types.
>
> Consequently, a function_ref constructed from nontype<&S::x> and a
> reference_wrapper (rw) now refers to rw.get() instead of the rw object
> itself,
> mitigating the risk of dangling references.
>
> Note that reference_wrapper arguments cannot be unwrapped in the general
> case,
> s the functor f referenced by nontype<f> may explicitly accept
> reference_wrapper<T>&
> as a parameter to observe identity or modify the argument.
>
> PR libstdc++/121858
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/funcref_impl.h
> (function_ref::function_ref(nontype<__fn>, _Up&&)): Unwrap
> reference_wrapper.
> * testsuite/20_util/function_ref/call.cc: Call and update
> test05(). Add new test06() for reference_wrapper.
> ---
> Testing on x88_64-linux. OK for trunk?
>
> libstdc++-v3/include/bits/funcref_impl.h | 21 ++++--
> .../testsuite/20_util/function_ref/call.cc | 75 ++++++++++++++-----
> 2 files changed, 69 insertions(+), 27 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/funcref_impl.h
> b/libstdc++-v3/include/bits/funcref_impl.h
> index 44c992281be..cc510aba3a6 100644
> --- a/libstdc++-v3/include/bits/funcref_impl.h
> +++ b/libstdc++-v3/include/bits/funcref_impl.h
> @@ -138,14 +138,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> if constexpr (is_pointer_v<_Fn> || is_member_pointer_v<_Fn>)
> static_assert(__fn != nullptr);
>
> - using _Tr = _Td _GLIBCXX_MOF_CV&;
> - if constexpr (is_member_pointer_v<_Fn> &&
> is_lvalue_reference_v<_Tr>)
> - // N.B. invoking member pointer on lvalue produces the same
> effects,
> - // as invoking it on pointer to that lvalue.
> - _M_invoke = &_Invoker::template _S_bind_ptr<__fn, _Td
> _GLIBCXX_MOF_CV>;
> + if constexpr (is_member_pointer_v<_Fn>)
> + {
> + // is_invocable_v with member pointer is only true for
> reference
> + // to class (or derived from it) or reference wrapper to
> such,
> + // we instead store pointer to referenced object
> + using _Ur = typename __inv_unwrap<_Td>::type
> _GLIBCXX_MOF_CV&;
> + _Ur __uref = __ref;
> + _M_invoke = &_Invoker::template _S_bind_ptr<__fn,
> remove_reference_t<_Ur>>;
> + _M_init(std::addressof(__uref));
> + }
> else
> - _M_invoke = &_Invoker::template _S_bind_ref<__fn, _Tr>;
> - _M_init(std::addressof(__ref));
> + {
> + _M_invoke = &_Invoker::template _S_bind_ref<__fn, _Td
> _GLIBCXX_MOF_CV&>;
> + _M_init(std::addressof(__ref));
> + }
> }
>
> /// Target object is equivalent to std::bind_front<_fn>(__ptr).
> diff --git a/libstdc++-v3/testsuite/20_util/function_ref/call.cc
> b/libstdc++-v3/testsuite/20_util/function_ref/call.cc
> index 23253c33ee3..d986e94b7bf 100644
> --- a/libstdc++-v3/testsuite/20_util/function_ref/call.cc
> +++ b/libstdc++-v3/testsuite/20_util/function_ref/call.cc
> @@ -130,7 +130,6 @@ int callback_ref(ftype& f, int x) { return f(x); }
> void
> test05()
> {
> -
> function_ref<int(int)> r1(nontype<&callback_ptr>, &twice);
> VERIFY( r1(2) == 4 );
> function_ref<int(int)> r2(nontype<&callback_ptr>, cube);
> @@ -140,27 +139,61 @@ test05()
> VERIFY( r3(3) == 6 );
> function_ref<int(int)> r4(nontype<&callback_ref>, cube);
> VERIFY( r4(3) == 27 );
> +}
>
> - // Checks if distinction between reference and pointer
> - // is preserved.
> - struct F
> - {
> - static
> - int operator()(ftype* f, int x)
> - { return f(x) + 1000; }
> -
> - static
> - int operator()(ftype& f, int x)
> - { return f(x) + 2000; }
> +void
> +test06()
> +{
> + struct S {
> + int v;
> + int& m() { return v; }
> + const int& c() const { return v; }
> };
> - function_ref<int(int)> r5(nontype<F{}>, &twice);
> - VERIFY( r5(2) == 1004 );
> - function_ref<int(int)> r6(nontype<F{}>, twice);
> - VERIFY( r6(2) == 2008 );
> - function_ref<int(int)> r7(nontype<F{}>, &cube);
> - VERIFY( r7(3) == 1006 );
> - function_ref<int(int)> r8(nontype<F{}>, cube);
> - VERIFY( r8(3) == 2027 );
> + S s{10};
> + std::reference_wrapper<S> sr(s);
> + std::reference_wrapper<S> csr(s);
> +
> + std::function_ref<int&()> f1(std::nontype<&S::v>, sr);
> + VERIFY( &f1() == &s.v );
> + std::function_ref<const int&()> f2(std::nontype<&S::v>, sr);
> + VERIFY( &f2() == &s.v );
> + std::function_ref<int&()> f3(std::nontype<&S::m>, sr);
> + VERIFY( &f3() == &s.v );
> + std::function_ref<const int&()> f4(std::nontype<&S::c>, sr);
> + VERIFY( &f4() == &s.v );
> +
> + std::function_ref<const int&()> f5(std::nontype<&S::v>, csr);
> + VERIFY( &f5() == &s.v );
> + std::function_ref<const int&()> f6(std::nontype<&S::c>, sr);
> + VERIFY( &f6() == &s.v );
> + static_assert( !std::is_constructible_v<
> + std::function_ref<int&()>,
> + std::nontype_t<&S::c>, std::reference_wrapper<S>&>
> + );
> +
> + std::function_ref<int&()> f7(std::nontype<&S::v>, std::as_const(sr));
> + VERIFY( &f7() == &s.v );
> + std::function_ref<const int&()> f8(std::nontype<&S::m>,
> std::as_const(sr));
> + VERIFY( &f8() == &s.v );
> +
> + // No rvalue reference wrapepr support
> + static_assert( !std::is_constructible_v<
> + std::function_ref<int&()>,
> + std::nontype_t<&S::v>, std::reference_wrapper<S>>
> + );
> + static_assert( !std::is_constructible_v<
> + std::function_ref<int&()>,
> + std::nontype_t<&S::v>, std::reference_wrapper<const S>>
> + );
> +
> + constexpr auto id = [](const std::reference_wrapper<S>& x)
> + { return &x; };
> +
> + // identity of reference_wrapper is preserved
> + std::function_ref<const std::reference_wrapper<S>*()>
> f9(std::nontype<id>, sr);
> + VERIFY( f9() == &sr );
> + std::function_ref<const std::reference_wrapper<S>*()>
> f10(std::nontype<id>, csr);
> + VERIFY( f10() == &csr );
> }
>
> struct Incomplete;
> @@ -182,5 +215,7 @@ int main()
> test02();
> test03();
> test04();
> + test05();
> + test06();
> test_params();
> }
> --
> 2.51.0
>
>
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