std::any 的 std::unique_ptr 不起作用？

问题描述

请考虑以下 C++20 程序：

#include <any>
#include <memory>

int main() {
  std::any a;
  std::unique_ptr<int> x = std::make_unique<int>(42);
  a.emplace<std::unique_ptr<int>>(std::move(x));
}

我得到：

test.cc: In function ‘int main()’:
test.cc:7:47: error: no matching function for call to ‘std::any::emplace<std::unique_ptr<int,std::default_delete<int> > >(std::remove_reference<std::unique_ptr<int>&>::type)’
    7 |   a.emplace<std::unique_ptr<int>>(std::move(x));
      |                                               ^
In file included from test.cc:1:
/usr/include/c++/9/any:273:7: note: candidate: ‘template<class _ValueType,class ... _Args> typename std::any::__any_constructible<std::any::_Decay<_ValueType>&,std::any::_Decay<_ValueType>,_Args&& ...>::type std::any::emplace(_Args&& ...)’
  273 |       emplace(_Args&&... __args)
      |       ^~~~~~~
/usr/include/c++/9/any:273:7: note:   template argument deduction/substitution Failed:
/usr/include/c++/9/any: In substitution of ‘template<class _ValueType,_Args&& ...>::type std::any::emplace(_Args&& ...) [with _ValueType = std::unique_ptr<int>; _Args = {std::unique_ptr<int,std::default_delete<int> >}]’:
test.cc:7:47:   required from here
/usr/include/c++/9/any:273:7: error: no type named ‘type’ in ‘struct std::enable_if<false,std::unique_ptr<int>&>’
/usr/include/c++/9/any:288:7: note: candidate: ‘template<class _ValueType,class _Up,std::initializer_list<_Up>,_Args&& ...>::type std::any::emplace(std::initializer_list<_Up>,_Args&& ...)’
  288 |       emplace(initializer_list<_Up> __il,_Args&&... __args)
      |       ^~~~~~~
/usr/include/c++/9/any:288:7: note:   template argument deduction/substitution Failed:
test.cc:7:47: note:   ‘std::unique_ptr<int>’ is not derived from ‘std::initializer_list<_Up>’
    7 |   a.emplace<std::unique_ptr<int>>(std::move(x));

有什么想法吗？

解决方法

类 std::any 是可复制的。尽管它的名字，它不能包装“任何”和所有类型。具体来说，它不能包装不可复制的类型。 std::unique_ptr 不可复制。 std::unique_ptr 不能存储在 std::any 中。