1 Revision History

1.1 R1

• Added noexcept specifier.
• Changed constraints wording to say “T and U are similar types” per LWG feedback
• Added converting constructor to specializations for integral types
• Added converting constructor to specializations for floating-point types
• Added converting constructor to specializations for pointer types

1.2 R0

• Initial revision

2 Abstract

This paper proposes a converting constructor for atomic_ref as the resolution for UK NB Comment GB13-309.

3 Introduction

The UK NB Comment GB13-309 pointed out that atomic_ref<T> is not convertible to atomic_ref<const T>. [P3323R1] added cv qualifiers to atomic and atomic_ref. However, the conversion constructor of atomic_ref between different cv qualifiers is overlooked.

4 Wording

Change 32.5.7.1 General [atomics.ref.generic.general] as follows:

    constexpr explicit atomic_ref(T&);
    constexpr atomic_ref(const atomic_ref&) noexcept;
    template <class U>
    constexpr atomic_ref(const atomic_ref<U>&) noexcept;

Add a new entry to 32.5.7.2 Operations [atomics.ref.ops] after paragraph 8:

template <class U>
constexpr atomic_ref(const atomic_ref<U>& ref) noexcept;

Change 32.5.7.3 Specializations for integral types [atomics.ref.int] as follows:

    constexpr explicit atomic_ref(integral-type&);
    constexpr atomic_ref(const atomic_ref&) noexcept;
    template <class U>
    constexpr atomic_ref(const atomic_ref<U>&) noexcept;

Change 32.5.7.4 Specializations for floating-point types [atomics.ref.float] as follows:

    constexpr explicit atomic_ref(floating-point-type&);
    constexpr atomic_ref(const atomic_ref&) noexcept;
    template <class U>
    constexpr atomic_ref(const atomic_ref<U>&) noexcept;

Change 32.5.7.5 Specialization for pointers [atomics.ref.pointer] as follows:

    constexpr explicit atomic_ref(pointer-type&);
    constexpr atomic_ref(const atomic_ref&) noexcept;
    template <class U>
    constexpr atomic_ref(const atomic_ref<U>&) noexcept;

5 Implementation Experience

[P3323R1] is currently being implemented in libc++ and adding the proposed constructor is under way.

6 Feature Test Macro

[P3323R1] does not seem to mention Feature Test Macro

7 References