c++ - Is it okay to define a totally general swap() function?

Question

The following snippet:

#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();
}

does not compile for me:

$ 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)

Is this my fault for declaring a swap() function so general that it conflicts with std::swap?

If so, is there a way to define foo::swap() so that it doesn't get hauled in by Koenig lookup?

question from:https://stackoverflow.com/questions/36850522/is-it-okay-to-define-a-totally-general-swap-function

Answer 1

• unique_ptr<T> requires T* to be a NullablePointer [unique.ptr]p3
• NullablePointer requires lvalues of T* to be Swappable [nullablepointer.requirements]p1
• Swappable essentially requires using std::swap; swap(x, y); to select an overload for x, y being lvalues of type T* [swappable.requirements]p3

In the last step, your type foo::bar produces an ambiguity and therefore violates the requirements of unique_ptr. libstdc++'s implementation is conforming, although I'd say this is rather surprising.

---

The wording is of course a bit more convoluted, because it is generic.

[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.

(emphasis mine)

[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.

Note that for a pointer type T*, for purposes of ADL, the associated namespaces and classes are derived from the type T. Hence, foo::bar* has foo as an associated namespace. ADL for swap(x, y) where either x or y is a foo::bar* will therefore find foo::swap.