c++ - 为私有类专门化一个函数？

Question

有没有办法std::swap为私有类专门化一个函数（比如，）？

例如，当我测试这个时：

#include <algorithm>

class Outer
{
    struct Inner
    {
        int a;
        void swap(Inner &other)
        {
            using std::swap;
            swap(this->a, other.a);
        }
    };
public:
    static void test();
};

namespace std
{
    template<> void swap<Outer::Inner>(Outer::Inner &a, Outer::Inner &b)
    { a.swap(b); }
}
void Outer::test()
{
    using std::swap;
    Inner a, b;
    swap(a, b);
}
int main()
{
    Outer::test();
    return 0;
}

我明白了：

Test.cpp:20:47: error: 'Inner' is a private member of 'Outer'
    template<> void swap<Outer::Inner>(Outer::Inner &a, Outer::Inner &b)
                                              ^
Test.cpp:5:12: note: implicitly declared private here
    struct Inner
           ^
Test.cpp:20:64: error: 'Inner' is a private member of 'Outer'
    template<> void swap<Outer::Inner>(Outer::Inner &a, Outer::Inner &b)
                                                               ^
Test.cpp:5:12: note: implicitly declared private here
    struct Inner
           ^
Test.cpp:20:33: error: 'Inner' is a private member of 'Outer'
    template<> void swap<Outer::Inner>(Outer::Inner &a, Outer::Inner &b)
                                ^
Test.cpp:5:12: note: implicitly declared private here
    struct Inner

（我确实意识到声明swap可以通过 ADL 找到的朋友可以避免这个问题swap，但这与我的问题无关。swap只是一个例子。）

Answer 1

您可以添加内部friend声明std::swap<Inner>(Inner&, Inner&)Outer

#include <algorithm>

class Outer
{
    struct Inner
    {
        int a;
        void swap(Inner &other)
        {
            using std::swap;
            swap(this->a, other.a);
        }
    };

    friend void std::swap<Inner>(Inner&, Inner&) noexcept;
public:
    static void test();
};

namespace std
{
    template<> void swap<Outer::Inner>(Outer::Inner &a, Outer::Inner &b) noexcept
    { a.swap(b); }
}

void Outer::test()
{
    using std::swap;
    Inner a, b;
    swap(a, b);
}

int main()
{
    Outer::test();
    return 0;
}

现场示例

Answer 2

不要扩展std命名空间。

如果要为 创建交换函数Inner，请将其设为私有函数Outer

#include <algorithm>

class Outer
{
    struct Inner
    {
        int a;
        void swap(Inner &other)
        {
            std::swap(this->a, other.a);
        }
    };

    static void swap(Inner& a, Inner& b);

public:
    static void test();
};

void Outer::test()
{
    Inner a, b;
    swap(a, b);
}

void Outer::swap(Inner& a, Inner& b)
{
    a.swap(b);
}

int main()
{
    Outer::test();
    return 0;
}