使用 std::p air 作为运算符 = 的参数时出现编译问题

Question

我花了几个小时试图弄清楚为什么下面的代码无法编译，如果有人能指出我缺少什么，我将不胜感激。

代码只是我在实际项目代码中遇到的编译问题的简化示例。

#include <map>
#include <string>
#include <utility>
class A
{
public:
A() = default;
A(const A& v) = default;
A(A&& v) = default;
A& operator=(const A& v) = default;

A& operator=(const std::pair<A, A>& v)
{
return *this;
}
};
void func(const std::pair<int, A>& obj);
int main(int argc, char *argv[])
{
std::pair<int, A> obj;
func(obj);
return 0;
}
void func(const std::pair<int, A>& obj)
{
A a, b;
a = b;
}

这个问题似乎与"A& operator=(const std：:p air<A，>&v)">有关。让我感到困惑的是，如果我移动函数"void func(...">之前main()一切正确编译。

使用 gcc (C++17) 编译。我得到的错误如下所示：

In file included from /usr/include/c++/7/bits/move.h:54:0,
from /usr/include/c++/7/bits/stl_pair.h:59,
from /usr/include/c++/7/bits/stl_algobase.h:64,
from /usr/include/c++/7/bits/stl_tree.h:63,
from /usr/include/c++/7/map:60,
from main.cpp:1:
/usr/include/c++/7/type_traits: In instantiation of ‘struct std::__and_<std::is_copy_assignable<A>, std::is_copy_assignable<A> >’:
/usr/include/c++/7/bits/stl_pair.h:378:7:   required from ‘struct std::pair<A, A>’
/usr/include/c++/7/type_traits:1259:45:   required by substitution of ‘template<class _Tp1, class _Up1, class> static std::true_type std::__is_assignable_helper<A&, const A&>::__test<_Tp1, _Up1, <template-parameter-1-3> >(int) [with _Tp1 = A&; _Up1 = const A&; <template-parameter-1-3> = <missing>]’
/usr/include/c++/7/type_traits:1268:40:   required from ‘class std::__is_assignable_helper<A&, const A&>’
/usr/include/c++/7/type_traits:1273:12:   required from ‘struct std::is_assignable<A&, const A&>’
/usr/include/c++/7/type_traits:1285:12:   required from ‘struct std::__is_copy_assignable_impl<A, true>’
/usr/include/c++/7/type_traits:1291:12:   required from ‘struct std::is_copy_assignable<A>’
/usr/include/c++/7/type_traits:143:12:   required from ‘struct std::__and_<std::is_copy_assignable<int>, std::is_copy_assignable<A> >’
/usr/include/c++/7/bits/stl_pair.h:378:7:   required from ‘struct std::pair<int, A>’
<span class="error_line" onclick="ide.gotoLine('main.cpp',23)">main.cpp:23:20</span>:   required from here
/usr/include/c++/7/type_traits:143:12: error: incomplete type ‘std::is_copy_assignable’ used in nested name specifier
struct __and_<_B1, _B2>
^~~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/bits/stl_algobase.h:64:0,
from /usr/include/c++/7/bits/stl_tree.h:63,
from /usr/include/c++/7/map:60,
from main.cpp:1:
/usr/include/c++/7/bits/stl_pair.h: In instantiation of ‘struct std::pair<A, A>’:
/usr/include/c++/7/type_traits:1259:45:   required by substitution of ‘template<class _Tp1, class _Up1, class> static std::true_type std::__is_assignable_helper<A&, const A&>::__test<_Tp1, _Up1, <template-parameter-1-3> >(int) [with _Tp1 = A&; _Up1 = const A&; <template-parameter-1-3> = <missing>]’
/usr/include/c++/7/type_traits:1268:40:   required from ‘class std::__is_assignable_helper<A&, const A&>’
/usr/include/c++/7/type_traits:1273:12:   required from ‘struct std::is_assignable<A&, const A&>’
/usr/include/c++/7/type_traits:1285:12:   required from ‘struct std::__is_copy_assignable_impl<A, true>’
/usr/include/c++/7/type_traits:1291:12:   required from ‘struct std::is_copy_assignable<A>’
/usr/include/c++/7/type_traits:143:12:   required from ‘struct std::__and_<std::is_copy_assignable<int>, std::is_copy_assignable<A> >’
/usr/include/c++/7/bits/stl_pair.h:378:7:   required from ‘struct std::pair<int, A>’
<span class="error_line" onclick="ide.gotoLine('main.cpp',23)">main.cpp:23:20</span>:   required from here
/usr/include/c++/7/bits/stl_pair.h:378:7: error: ‘value’ is not a member of ‘std::__and_, std::is_copy_assignable >’
operator=(typename conditional<
^~~~~~~~

Answer 1

您现在遇到的是未定义行为的一个主要示例，因为std::pair实现依赖于未为不完整类型定义的type_traits。

在A范围内但在任何成员函数之外A被视为不完整类型。您正在尝试定义一个接受std::pair<A, A>的函数，从而用不完整的类型实例化std::pair。

如果A是模板化类型，则情况并非如此，因为类模板的函数在实际调用之前不会实例化。