Does C++11 change the behavior of explicitly calli

问题:

Background

Consider for this question the following code:

#include <utility>

namespace ns
{
    struct foo
    {
        foo() : i(0) {}
        int i;

    private:
        foo(const foo&); // not defined,
        foo& operator=(const foo&); // non-copyable
    };

    void swap(foo& lhs, foo& rhs)
    {
        std::swap(lhs.i, rhs.i);
    }
}

template <typename T>
void do_swap(T& lhs, T& rhs); // implementation to be determined

int main()
{
    ns::foo a, b;
    do_swap(a, b);
}

In C++03, this implementation of do_swap would be considered "broken":

template <typename T>
void do_swap(T& lhs, T& rhs)
{
    std::swap(lhs, rhs);
}

By explicitly specifying std::, it prohibits ns::swap from being found via argument-dependent lookup. (It then fails to compile because std::swap tries to copy a foo, which is not allowed.) Instead, we do this:

template <typename T>
void do_swap(T& lhs, T& rhs)
{
    using std::swap; // allow std::swap as a backup if ADL fails to find a swap
    swap(lhs, rhs); // unqualified call to swap, allow ADL to operate
}

Now ns::swap is found and std::swap, being less specialized, is not used. It's uglier, but it works and is understandable in hind-sight. boost::swap wraps this up nicely for us (and provides array overloads):

#include <boost/swap.hpp>

template <typename T>
void do_swap(T& lhs, T& rhs)
{
    boost::swap(lhs, rhs); // internally does what do_swap did above
}

Question

My question is thus: does std::swap take on the behavior of boost::swap in C++11? If not, why?

To me it seems obvious that it ought to. Any code broken by the change was probably quite flimsy in the first place (algorithms and containers, like std::sort and std::vector, were underspecified; implementations were allowed to call ADL swap's or not indeterminately), so the change would be for the better. Additionally, std::swap is now defined for arrays, so change at all certainly isn't out of the question.

However, while §17.6.3.2 specifies that all calls to swap within the standard library must be done without std:: qualification (fixing the problem with algorithms and containers noted above), it fails to touch on std::swap itself. It even gives examples of swapping values that include using std::swap;. Likewise §20.2.2 (where std::swap is specified) doesn't say a word on ADL.

Lastly, GCC does not enable ADL in their std::swap implementation (nor does MSVC, but that's not saying much). So I must be wrong that std::swap takes on the behavior of boost::swap, but I don't understand why the change wasn't made. :( And I'm not alone!

回答1:

I would have had to vote against your proof-of-concept implementation had it been proposed. I fear it would break the following code, which I'm pretty sure I've seen in the wild at least once or twice over the past dozen years.

namespace oops
{

    struct foo
    {
        foo() : i(0) {}
        int i;

        void swap(foo& x) {std::swap(*this, x);}
    };

    void swap(foo& lhs, foo& rhs)
    {
        lhs.swap(rhs);
    }

}

Whether you think the above is good code or bad, it works as the author intends in C++98/03 and so the bar for silently breaking it is pretty high. Telling users that in C++11 they would no longer have to write using std::swap; isn't a sufficiently high benefit to outweigh the disadvantage of silently turning the above code into infinite recursion.

Another way to get out of writing using std::swap; is to use std::iter_swap instead:

template <typename T>
void do_swap(T& lhs, T& rhs)
{
    std::iter_swap(&lhs, &rhs); // internally does what do_swap did above
}

回答2:

Here's a proof-of-concept implementation:

#include <utility>

// exposition implementation
namespace std_
{
    namespace detail
    {
        // actual fallback implementation
        template <typename T>
        void swap(T& lhs, T& rhs)
        {
            T temp = std::move(lhs);
            lhs = std::move(rhs);
            rhs = std::move(temp);
        }
    }

    template <typename T>
    void swap(T& lhs, T& rhs)
    {
        using detail::swap; // shadows std_::swap, stops recursion
        swap(lhs, rhs); // unqualified call, allows ADL
    }
}

namespace ns
{
    struct foo
    {
        foo() : i(0) {}
        int i;

    private:
        foo(const foo&); // not defined,
        foo& operator=(const foo&); // non-copyable
    };

    void swap(foo& lhs, foo& rhs)
    {
        std::swap(lhs.i, rhs.i);
    }
}


int main()
{
    int i = 0, j = 0;
    std_::swap(i, j);

    ns::foo a, b;
    std_::swap(a, b);
}

回答3:

Well, boost::swap() dispatches to std::swap(). To have std::swap() do something similar to boost::swap() it would need to delegate somewhere else. What is this somewhere else? The standard doesn't mandate another version of swap() which provides the actual implementation. This can be done but the standard doesn't mandate it.

Why it doesn't do it? I didn't see any proposal proposing this implementation. If someone had wanted this to do done I'm sure it would have been proposed.