C++ 17类中的变种<any>

Question

以下代码编译得很好:

int main()
{
    variant<any> var;
    var = 5;
    cout << any_cast<int>(get<any>(var)) << endl;
    return 0;
}

但是当我试图把它variant<any>作为一个班级成员

struct MyClass{
    variant<any> var;
};

int main()
{
    MyClass s;
    return 0;
}

它不编译.我做错了什么或者是一些错误？

我正在使用gcc 7.1.0

In file included from /home/zak/Projects/Anytest/main.cpp:3:0:
/usr/local/gcc-7.1/include/c++/7.1.0/variant: In instantiation of ‘struct std::__detail::__variant::__accepted_index<const std::variant<std::any>&, std::variant<std::any>, void>’:
/usr/local/gcc-7.1/include/c++/7.1.0/variant:911:26:   required from ‘constexpr const size_t std::variant<std::any>::__accepted_index<const std::variant<std::any>&>’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:940:6:   required by substitution of ‘template<class _Tp, class> constexpr std::variant<std::any>::variant(_Tp&&) [with _Tp = const std::variant<std::any>&; <template-parameter-1-2> = <missing>]’
/home/zak/Projects/Anytest/main.cpp:14:13:   required from here
/usr/local/gcc-7.1/include/c++/7.1.0/variant:559:49: error: no matching function for call to ‘std::__detail::__variant::__overload_set<std::any>::_S_fun(const std::variant<std::any>&)’
       decltype(__overload_set<_Types...>::_S_fun(std::declval<_Tp>()),
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note: candidate: static std::integral_constant<long unsigned int, sizeof... (_Rest)> std::__detail::__variant::__overload_set<_First, _Rest ...>::_S_fun(_First) [with _First = std::any; _Rest = {}]
       static integral_constant<size_t, sizeof...(_Rest)> _S_fun(_First);
                                                          ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note:   no known conversion for argument 1 from ‘const std::variant<std::any>’ to ‘std::any’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note: candidate: static void std::__detail::__variant::__overload_set<_Types>::_S_fun() [with _Types = {}]
     { static void _S_fun(); };
                   ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note:   candidate expects 0 arguments, 1 provided
/usr/local/gcc-7.1/include/c++/7.1.0/variant:559:49: error: no matching function for call to ‘std::__detail::__variant::__overload_set<std::any>::_S_fun(const std::variant<std::any>&)’
       decltype(__overload_set<_Types...>::_S_fun(std::declval<_Tp>()),
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note: candidate: static std::integral_constant<long unsigned int, sizeof... (_Rest)> std::__detail::__variant::__overload_set<_First, _Rest ...>::_S_fun(_First) [with _First = std::any; _Rest = {}]
       static integral_constant<size_t, sizeof...(_Rest)> _S_fun(_First);
                                                          ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note:   no known conversion for argument 1 from ‘const std::variant<std::any>’ to ‘std::any’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note: candidate: static void std::__detail::__variant::__overload_set<_Types>::_S_fun() [with _Types = {}]
     { static void _S_fun(); };
                   ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note:   candidate expects 0 arguments, 1 provided
/usr/local/gcc-7.1/include/c++/7.1.0/variant:559:49: error: no matching function for call to ‘std::__detail::__variant::__overload_set<std::any>::_S_fun(const std::variant<std::any>&)’
       decltype(__overload_set<_Types...>::_S_fun(std::declval<_Tp>()),
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note: candidate: static std::integral_constant<long unsigned int, sizeof... (_Rest)> std::__detail::__variant::__overload_set<_First, _Rest ...>::_S_fun(_First) [with _First = std::any; _Rest = {}]
       static integral_constant<size_t, sizeof...(_Rest)> _S_fun(_First);
                                                          ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note:   no known conversion for argument 1 from ‘const std::variant<std::any>’ to ‘std::any’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note: candidate: static void std::__detail::__variant::__overload_set<_Types>::_S_fun() [with _Types = {}]
     { static void _S_fun(); };
                   ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note:   candidate expects 0 arguments, 1 provided
/usr/local/gcc-7.1/include/c++/7.1.0/variant: In instantiation of ‘constexpr const size_t std::__detail::__variant::__accepted_index<const std::variant<std::any>&, std::variant<std::any>, void>::value’:
/usr/local/gcc-7.1/include/c++/7.1.0/variant:911:26:   required from ‘constexpr const size_t std::variant<std::any>::__accepted_index<const std::variant<std::any>&>’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:940:6:   required by substitution of ‘template<class _Tp, class> constexpr std::variant<std::any>::variant(_Tp&&) [with _Tp = const std::variant<std::any>&; <template-parameter-1-2> = <missing>]’
/home/zak/Projects/Anytest/main.cpp:14:13:   required from here
/usr/local/gcc-7.1/include/c++/7.1.0/variant:564:12: error: no matching function for call to ‘std::__detail::__variant::__overload_set<std::any>::_S_fun(const std::variant<std::any>&)’
  - decltype(__overload_set<_Types...>::
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~
      _S_fun(std::declval<_Tp>()))::value;
      ~~~~~~^~~~~~~~~~~~~~~~~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note: candidate: static std::integral_constant<long unsigned int, sizeof... (_Rest)> std::__detail::__variant::__overload_set<_First, _Rest ...>::_S_fun(_First) [with _First = std::any; _Rest = {}]
       static integral_constant<size_t, sizeof...(_Rest)> _S_fun(_First);
                                                          ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note:   no known conversion for argument 1 from ‘const std::variant<std::any>’ to ‘std::any’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note: candidate: static void std::__detail::__variant::__overload_set<_Types>::_S_fun() [with _Types = {}]
     { static void _S_fun(); };
                   ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note:   candidate expects 0 arguments, 1 provided

Answer 1

MyClass当它试图复制构造类型的成员时,问题在于隐式定义的复制构造函数std::variant<std::any>.

要执行重载解析,编译器首先需要尝试实例化所有std::variant的构造函数模板,函数参数为const std::variant<std::any>&¹.我们感兴趣的构造函数是这样的:

template <class T> constexpr variant(T&& t) noexcept(/*...*/);

它只参与重载解析,如果表达式FUN(?std?::?forward<T>(t))很好,其中FUN是根据[variant.ctor]/12生成的一组重载函数.²

在这种情况下,只有一个替代类型(std::any),因此只有一个虚函数FUN,其签名是FUN(std::any).现在,编译器需要决定是否FUN可以使用const std::variant<std::any>&¹调用.在此过程中,编译器需要知道是否std::any可以用const std::variant<std::any>&¹构造.

这将触发实例化std::any的构造函数模板template<class T> any(T&& value);,如果std::is_­copy_­constructible_­v<VT>是true(VT正在std::decay_t<T>和T正在const std::variant<std::any>&),它只参与重载解析.

现在为了看看VT(即std::variant<std::any>)是否是可复制构造的,编译器需要尝试实例化所有std::variant的构造函数模板......这就是我们开始的地方,我们陷入了循环.

这可以解释为什么我们在错误消息中看到template<class _Tp, class> constexpr std::variant<std::any>::variant(_Tp&&)和__overload_set<std::any>::_S_fun(对应于FUN上面提到的函数),以及为什么我们看到多次出现相同的错误.

GCC如何脱离上述循环仍然是个问题,为什么调整程序可以阻止GCC报告错误.也许这些都是一些错误的迹象.

^{严格来说,它应该是"一种类型的左右const std::variant<std::any>"而不是"一种const std::variant<std::any>&".}

^{2.该标准还要求,如果此构造方法只能参与重载决议is_­same_­v<decay_­t<T>, variant>是false.GCC(libstdc ++)选择稍后检查.我不知道这是否符合要求.}