class 中的 C++17 变体<any>

Question

以下代码编译良好：

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

但是当我尝试将 variant<any> 设为 class 成员时

struct MyClass{
    variant<any> var;
};

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

它不编译。我是做错了什么还是有什么错误？

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

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

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

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

只有当表达式 FUN(std::forward<T>(t)) 的格式正确时，它才会参与重载决策，其中 FUN 是一组根据 [variant.ctor]/12 生成的重载函数。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，其签名为FUN(std::any)。现在，编译器需要决定 FUN 是否可以用 const std::variant<std::any>&¹ 调用。在这个过程中，编译器需要知道std::any是否可以用const std::variant<std::any>&¹.

来构造

这将触发 std::any 的构造函数模板 template<class T> any(T&& value); 的实例化，它仅在 std::is_copy_constructible_v<VT> 为 true 时才参与重载决议（VT是 std::decay_t<T>，T 是 const std::variant<std::any>&)。

现在为了查看 VT（即 std::variant<std::any>）是否可复制构造，编译器需要尝试实例化所有 std::variant 的构造函数模板...并且这是我们开始的地方，我们陷入了一个循环。

这就可以解释为什么我们在错误信息中看到template<class _Tp, class> constexpr std::variant<std::any>::variant(_Tp&&)和__overload_set<std::any>::_S_fun（对应上面提到的函数FUN），以及为什么我们会看到同样的错误出现好几个次。

GCC 如何从上述循环中跳出，以及为什么调整程序可以阻止 GCC 报告错误，这仍然是一个问题。也许这些是一些错误的迹象。

^{1.严格来说应该是"an lvalue of type const std::variant<std::any>"而不是"a const std::variant<std::any>&".}

^{2。该标准还要求此构造函数仅在 is_same_v<decay_t<T>, variant> 为 false 时才参与重载决策。 GCC (libstdc++) 选择稍后检查。不知道合不合}

class 中的 C++17 变体<any>

C++17 variant<any> inside the class

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variant