>
> `volatile std::reference_wrapper<S>` will cause `_Ur __uref = __ref;` hard
>> error. But I'm not sure we really need to care about it.
>>
>>> result_of (where __inv_unwrap is sourced from), does not care about it.
>>> So I would say, we do not need to.
>>
>>
Thanks for the explanation. Should I file a new bug regarding this? Only
libstdc++ fails below:
#include <functional>
int main() {
struct S {
void f() { };
};
S s;
volatile auto sr = std::ref(s);
static_assert(!std::is_invocable_v<decltype(&S::f), decltype(sr)>);
}
https://godbolt.org/z/b5oP75613
Hewill Kang <[email protected]> 於 2025年10月25日 週六 上午12:15寫道：
> Here are some examples:
>
> struct S {
> void f() const;
> };
>
> int main() {
> S s;
> volatile auto sr = std::ref(s);
> std::function_ref<void() const> fr(std::nontype<&S::f>, sr);
> fr();
> }
>
>
> Hewill Kang <[email protected]> 於 2025年10月25日 週六 上午12:10寫道：
>
>> `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 <[email protected]> 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
>>>
>>>
