c++ - 可以定义一个完全通用的 swap() 函数吗？

Question

以下片段:

#include <memory>
#include <utility>

namespace foo
{
    template <typename T>
    void swap(T& a, T& b)
    {
        T tmp = std::move(a);
        a = std::move(b);
        b = std::move(tmp);
    }

    struct bar { };
}

void baz()
{
    std::unique_ptr<foo::bar> ptr;
    ptr.reset();
}

不为我编译:

$ g++ -std=c++11 -c foo.cpp
In file included from /usr/include/c++/5.3.0/memory:81:0,
                 from foo.cpp:1:
/usr/include/c++/5.3.0/bits/unique_ptr.h: In instantiation of ‘void std::unique_ptr<_Tp, _Dp>::reset(std::unique_ptr<_Tp, _Dp>::pointer) [with _Tp = foo::bar; _Dp = std::default_delete<foo::bar>; std::unique_ptr<_Tp, _Dp>::pointer = foo::bar*]’:
foo.cpp:20:15:   required from here
/usr/include/c++/5.3.0/bits/unique_ptr.h:342:6: error: call of overloaded ‘swap(foo::bar*&, foo::bar*&)’ is ambiguous
  swap(std::get<0>(_M_t), __p);
      ^
In file included from /usr/include/c++/5.3.0/bits/stl_pair.h:59:0,
                 from /usr/include/c++/5.3.0/bits/stl_algobase.h:64,
                 from /usr/include/c++/5.3.0/memory:62,
                 from foo.cpp:1:
/usr/include/c++/5.3.0/bits/move.h:176:5: note: candidate: void std::swap(_Tp&, _Tp&) [with _Tp = foo::bar*]
     swap(_Tp& __a, _Tp& __b)
     ^
foo.cpp:7:10: note: candidate: void foo::swap(T&, T&) [with T = foo::bar*]
     void swap(T& a, T& b)

我声明一个 swap() 函数如此笼统以至于与 std::swap 冲突，这是我的错吗？

如果是这样，有没有办法定义 foo::swap() 以便它不会被 Koenig 查找拖入？

Answer 1

• unique_ptr<T>需要 T*成为 NullablePointer [unique.ptr]p3
• NullablePointer需要 T* 的左值成为 Swappable [nullablepointer.requirements]p1
• Swappable本质上需要 using std::swap; swap(x, y);为 x 选择过载, y是 T* 类型的左值[swappable.requirements]p3

在最后一步，您的类型 foo::bar产生歧义，因此违反 unique_ptr 的要求. libstdc++ 的实现是符合标准的，尽管我会说这相当令人惊讶。

---

措辞当然有点复杂，因为它是通用的。

[unique.ptr]p3

If the type remove_reference_t<D>::pointer exists, then unique_ptr<T, D>::pointer shall be a synonym for remove_reference_t<D>::pointer. Otherwise unique_ptr<T,
D>::pointer shall be a synonym for T*. The type unique_ptr<T,
D>::pointer shall satisfy the requirements of NullablePointer.

(强调我的)

[nullablepointer.requirements]p1

A NullablePointer type is a pointer-like type that supports null values. A type P meets the requirements of NullablePointer if:

• [...]
• lvalues of type P are swappable (17.6.3.2),
• [...]

[swappable.requirements]p2

An object t is swappable with an object u if and only if:

• the expressions swap(t, u) and swap(u, t) are valid when evaluated in the context described below, and
• [...]

[swappable.requirements]p3

The context in which swap(t, u) and swap(u, t) are evaluated shall ensure that a binary non-member function named “swap” is selected via overload resolution on a candidate set that includes:

• the two swap function templates defined in <utility> and
• the lookup set produced by argument-dependent lookup.

请注意，对于指针类型 T* ，对于 ADL，相关的命名空间和类派生自类型 T .因此，foo::bar*有 foo作为关联的命名空间。 swap(x, y) 的 ADL其中x或 y是 foo::bar*因此会找到 foo::swap .