如何解决无法在 Clang 上将 std::reference<T> 转换为 std::reference<const T>
我希望 std::reference_wrapper 可以作为将非常量转换为常量的参考,例如:
int a = 10;
int& refA = a;
const int& constRefA = refA;
以下代码在 MSVC 和 GCC 中编译并正常工作,但在 Clang 上则不行。我只是不明白为什么,它是 UB,还是 Clang 编译器的实际问题?
#include <functional>
#include <optional>
int main()
{
int a = 10;
std::reference_wrapper<int> ref = a;
std::reference_wrapper<const int> constRef = ref;
std::optional<std::reference_wrapper<int>> optRef = a;
std::optional<std::reference_wrapper<const int>> optConstRef = optRef;
return 0;
}
仅在 Clang 上,显示以下错误:
prog.cc:13:39: error: no viable conversion from 'reference_wrapper<int>' to 'reference_wrapper<const int>'
std::reference_wrapper<const int> constRef = ref;
https://wandbox.org/permlink/FSY4tCvE9B17hbVn
prog.cc:13:39: error: no viable conversion from 'reference_wrapper<int>' to 'reference_wrapper<const int>'
std::reference_wrapper<const int> constRef = ref;
^ ~~~
/opt/wandbox/clang-head/include/c++/v1/__functional_base:374:28: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'std::reference_wrapper<int>' to 'const std::reference_wrapper<const int> &' for 1st argument
class _LIBCPP_TEMPLATE_VIS reference_wrapper
^
/opt/wandbox/clang-head/include/c++/v1/__functional_base:374:28: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'std::reference_wrapper<int>' to 'std::reference_wrapper<const int> &&' for 1st argument
/opt/wandbox/clang-head/include/c++/v1/__functional_base:386:5: note: candidate constructor not viable: no known conversion from 'std::reference_wrapper<int>' to 'std::reference_wrapper<const int>::type &' (aka 'const int &') for 1st argument
reference_wrapper(type& __f) _NOEXCEPT
^
/opt/wandbox/clang-head/include/c++/v1/__functional_base:389:14: note: candidate constructor not viable: no known conversion from 'std::reference_wrapper<int>' to 'std::reference_wrapper<const int>::type &&' (aka 'const int &&') for 1st argument
private: reference_wrapper(type&&); public: // = delete; // do not bind to temps
^
/opt/wandbox/clang-head/include/c++/v1/__functional_base:394:5: note: candidate function
operator type&() const _NOEXCEPT {return *__f_;}
^
prog.cc:16:54: error: no viable conversion from 'optional<reference_wrapper<int>>' to 'optional<reference_wrapper<const int>>'
std::optional<std::reference_wrapper<const int>> optConstRef = optRef;
^ ~~~~~~
/opt/wandbox/clang-head/include/c++/v1/optional:689:41: note: candidate constructor not viable: no known conversion from 'std::optional<std::reference_wrapper<int>>' to 'const std::optional<std::reference_wrapper<const int>> &' for 1st argument
_LIBCPP_INLINE_VISIBILITY constexpr optional(const optional&) = default;
^
/opt/wandbox/clang-head/include/c++/v1/optional:690:41: note: candidate constructor not viable: no known conversion from 'std::optional<std::reference_wrapper<int>>' to 'std::optional<std::reference_wrapper<const int>> &&' for 1st argument
_LIBCPP_INLINE_VISIBILITY constexpr optional(optional&&) = default;
^
/opt/wandbox/clang-head/include/c++/v1/optional:691:41: note: candidate constructor not viable: no known conversion from 'std::optional<std::reference_wrapper<int>>' to 'std::nullopt_t' for 1st argument
_LIBCPP_INLINE_VISIBILITY constexpr optional(nullopt_t) noexcept {}
^
/opt/wandbox/clang-head/include/c++/v1/optional:715:15: note: candidate template ignored: substitution failure [with _Up = std::optional<std::reference_wrapper<int>> &]: no member named '_EnableIfImpl' in 'std::_MetaBase<false>'
constexpr optional(_Up&& __v)
^
/opt/wandbox/clang-head/include/c++/v1/optional:730:5: note: candidate template ignored: substitution failure [with _Up = std::reference_wrapper<int>]: no member named '_EnableIfImpl' in 'std::_MetaBase<false>'
optional(const optional<_Up>& __v)
^
/opt/wandbox/clang-head/include/c++/v1/optional:748:5: note: candidate template ignored: substitution failure [with _Up = std::reference_wrapper<int>]: no member named '_EnableIfImpl' in 'std::_MetaBase<false>'
optional(optional<_Up>&& __v)
^
/opt/wandbox/clang-head/include/c++/v1/optional:701:24: note: explicit constructor is not a candidate
constexpr explicit optional(_InPlaceT,_Args&&... __args)
^
/opt/wandbox/clang-head/include/c++/v1/optional:722:24: note: explicit constructor is not a candidate
constexpr explicit optional(_Up&& __v)
^
/opt/wandbox/clang-head/include/c++/v1/optional:738:14: note: explicit constructor is not a candidate
explicit optional(const optional<_Up>& __v)
^
/opt/wandbox/clang-head/include/c++/v1/optional:756:14: note: explicit constructor is not a candidate
explicit optional(optional<_Up>&& __v)
^
2 errors generated.
解决方法
您在 wandbox 上使用的标准库有一个错误。它只找到了 2 个构造函数和一个转换运算符。
编译器和标准库并不总是同步。
printf("Hello World");
int a = 10;
std::reference_wrapper<int> ref = a;
std::reference_wrapper<const int> constRef( ref );
std::optional<std::reference_wrapper<int>> optRef = a;
std::optional<std::reference_wrapper<const int>> optConstRef( optRef );
return 0;
通过使转换显式,它可以工作。我不知道为什么;在我阅读 reference_wrapper
构造和转换运算符时,没有任何显式转换差异。
但是缺少隐式引用包装转换可以解释为什么可选项会想要它。
无论如何,这显然是一个错误。 reference_wrapper<const int>
的通用转换构造函数应适用于:
void FUN(int const&) {}
FUN(ref)
重载解析有效,它适用于 reference_wrapper<int>
。
版权声明:本文内容由互联网用户自发贡献,该文观点与技术仅代表作者本人。本站仅提供信息存储空间服务,不拥有所有权,不承担相关法律责任。如发现本站有涉嫌侵权/违法违规的内容, 请发送邮件至 dio@foxmail.com 举报,一经查实,本站将立刻删除。