P3612R0
Harmonize proxy-reference operations (LWG 3638 and 4187)

1. Changelog

• R0 (pre-Hagenberg 2025): Initial revision.

2. Background

From scratch, the simplest proxy-reference type for bool would look like this:

struct Reference {
  bool *p_;
  Reference(bool& r) : p_(&r) {}
  Reference(const Reference&) = default;
  Reference operator=(Reference rhs) const {
    *p_ = *rhs.p_;
    return *this;
  }
  operator bool() const {
    return *p_;
  }
  Reference operator=(bool b) const {
    *p_ = b;
    return *this;
  }
  friend void swap(Reference lhs, Reference rhs) {
    std::swap(*lhs.p_, *rhs.p_);
  }
};

1. Let r be a variable of type Reference. We need operator=(bool) so that r = true will compile. But we also need operator=(Reference) so that the compiler will not generate us a defaulted copy-assignment operator. Both operator=s are const-qualified following [P2321]’s guidance (§5.3); see also "Field-testing -Wassign-to-class-rvalue".

2. Now, C++20’s vector<bool>::reference already had a non-const-qualified operator=(bool). It would have broken ABI if [P2321] had simply added const to that member function. Therefore P2321 added a whole new const-qualified overload of operator=(bool) alongside the pre-existing non-const-qualified operator=(bool). We wouldn’t do that from scratch, but we do it for backward compatibility.

3. P2321 also established the precedent that operator=(bool) and operator=(bool) const return lvalue references to Reference, rather than prvalues of type Reference. We wouldn’t do that from scratch either, but we do it now because of P2321.

4. We don’t need an operator=(const Reference&) const because Reference is already implicitly convertible to bool. If cr is of type const Reference, then cr = cr2 will end up calling cr.operator=(bool(cr2)), which is fine. (Again, [P2321] has already established this pattern.)

5. We must provide ADL swap. The generic std::swap is inappropriate for two reasons:

• We need swap(v[1], v[2]) to work even when the expression v[1] is a prvalue Reference. The generic std::swap(T&, T&) doesn’t accept prvalue arguments.

• The generic algorithm Reference t = r1; r1 = r2; r2 = t; fails to swap the values of the referents of r1 and r2.

If this Reference is the iter_reference_t of some iterator type, then that iterator type might do well also to provide an iter_swap; but that doesn’t affect the rationale above; we must invariably provide an ADL swap.

6. Consider swap(r, b), where b is an lvalue of type bool. If bool& is implicitly convertible to Reference, this will happily use the single overload of ADL swap above. But in the STL, vector<bool>::reference and bitset<N>::reference are not implicitly convertible from bool&. (This is as it should be: you shouldn’t be able to create one referring to an arbitrary bool of your own.) So, in order to make swap(r, b) and swap(b, r) compile, we’ll need two additional overloads of swap. This matches the proposed resolution of [LWG3638].

The final product (omitting constexpr and noexcept) looks like this:

struct Reference {
  bool *p_;
  explicit Reference(bool* p) : p_(p) {} // exposition only
  Reference(const Reference&) = default;
  Reference& operator=(const Reference& rhs) { // C++98
    *p_ = *rhs.p_;
    return *this;
  }
  operator bool() const {
    return *p_;
  }
  Reference& operator=(bool b) { // C++98
    *p_ = b;
    return *this;
  }
  const Reference& operator=(bool b) const { // P2321
    *p_ = b;
    return *this;
  }
  friend void swap(Reference lhs, Reference rhs) {
    std::swap(*lhs.p_, *rhs.p_);
  }
  friend void swap(Reference lhs, bool& rhs) {
    std::swap(*lhs.p_, rhs);
  }
  friend void swap(bool& lhs, Reference rhs) {
    std::swap(lhs, *rhs.p_);
  }
};

We propose to apply this pattern consistently in both vector<bool>::reference (resolving [LWG3638]) and bitset<N>::reference (resolving [LWG4187]).

3. Don’t mandate triviality

Today the copy constructors of vector<bool>::reference and bitset::reference are specified with =default; but because we don’t specify their data members, this says nothing normative about the copy constructor’s noexceptness and triviality. These explicitly defaulted declarations were added to C++20 by P0619, merely to avoid relying on [depr.impldec]. Prior to C++20 these types did not specify a copy constructor at all, which was interpreted as a request for implicitly defaulted copy and move constructors, which still said nothing normative about noexceptness and triviality.

Both copy constructors are trivial in practice, on all three vendors.

However, there is implementation divergence on the destructors. All vendors give vector<bool>::reference a trivial destructor; but only libc++ gives bitset::reference a trivial destructor. libstdc++ and Microsoft give bitset::reference a non-trivial user-provided destructor. This means that the two types have visibly different calling conventions (Godbolt). We cannot mandate a change here, because ABI.

We conceivably could require triviality of any operation with "Yes"es all across its row above. But, since we can’t get it for bitset::reference’s destructor; I don’t want to introduce gratuitous differences in specification between bitset::reference and vector<bool>::reference; and it doesn’t seem very important whether a proxy reference’s copy constructor is normatively specified to be trivial —​I don’t think it’s worth the bother to specify.

4. Proposed wording

4.1. [template.bitset.general]

DRAFTING NOTE: This resolves [LWG4187]. We don’t add a const-qualified overload of reference::flip because I don’t think anyone cares about flip. We don’t rearrange the members to match [vector.bool]'s order because that can be done later, editorially.

Modify [template.bitset.general] as follows:

namespace std {
  template<size_t N> class bitset {
  public:
    // bit reference
    class reference {
    public:
      constexpr reference(const reference& x) noexcept = default;
      constexpr ~reference();
      constexpr reference& operator=(bool x) noexcept;              // for b[i] = x;
      constexpr reference& operator=(const reference& x) noexcept;    // for b[i] = b[j];
      constexpr const reference& operator=(bool x) const noexcept;
      constexpr bool operator~() const noexcept;                    // flips the bit
      constexpr operator bool() const noexcept;                     // for x = b[i];
      constexpr reference& flip() noexcept;                         // for b[i].flip();
      friend constexpr void swap(reference x, reference y) noexcept;
      friend constexpr void swap(reference x, bool& y) noexcept;
      friend constexpr void swap(bool& x, reference y) noexcept;
    };

[...]

  };

  // [bitset.hash], hash support
  template<class T> struct hash;
  template<size_t N> struct hash<bitset<N>>;
}

1․ The class template bitset<N> describes an object that can store a sequence consisting of a fixed number of bits, N.

2․ Each bit represents either the value zero (reset) or one (set). To toggle a bit is to change the value zero to one, or the value one to zero. Each bit has a non-negative position pos. When converting between an object of class type bitset<N> and a value of some integral type, bit position pos corresponds to the bit value 1 << pos. The integral value corresponding to two or more bits is the sum of their bit values.

x․ reference is a class that simulates a reference to a single bit in the sequence.

  constexpr reference::reference(const reference& x) noexcept;

x․ Effects: Initializes *this to refer to the same bit as x.

  constexpr reference::~reference();

x․ Effects: None.

  constexpr reference& reference::operator=(bool x) noexcept;
  constexpr reference& reference::operator=(const reference& x) noexcept;
  constexpr const reference& reference::operator=(bool x) const noexcept;

x․ Effects: Sets the bit referred to by *this if bool(x) is true, and clears it otherwise.

x․ Returns: *this.

 constexpr void swap(reference x, reference y) noexcept;
 constexpr void swap(reference x, bool& y) noexcept;
 constexpr void swap(bool& x, reference y) noexcept;

x․ Effects: Exchanges the values denoted by x and y as if by:

bool b = x;
x = y;
y = b;

  constexpr reference& reference::flip() noexcept;

x․ Effects: *this = !*this;

3․ The functions described in [template.bitset] can report three kinds of errors, each associated with a distinct exception [...]

4.2. [vector.bool]

DRAFTING NOTE: This resolves [LWG3638].

Modify [vector.bool] as follows:

namespace std {
  template<class Allocator>
  class vector<bool, Allocator> {
  public:
    // types
    using value_type             = bool;
    using allocator_type         = Allocator;
    using pointer                = implementation-defined;
    using const_pointer          = implementation-defined;
    using const_reference        = bool;
    using size_type              = implementation-defined; // see [container.requirements]
    using difference_type        = implementation-defined; // see [container.requirements]
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

    // bit reference
    class reference {
    public:
      constexpr reference(const reference&) noexcept = default;
      constexpr ~reference();
      constexpr operator bool() const noexcept;
      constexpr reference& operator=(bool x) noexcept;
      constexpr reference& operator=(const reference& x) noexcept;
      constexpr const reference& operator=(bool x) const noexcept;
      constexpr void flip() noexcept;   // flips the bit
      friend constexpr void swap(reference x, reference y) noexcept;
      friend constexpr void swap(reference x, bool& y) noexcept;
      friend constexpr void swap(bool& x, reference y) noexcept;
    };

[...]

    constexpr void swap(vector&)
      noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value ||
               allocator_traits<Allocator>::is_always_equal::value);
    static constexpr void swap(reference x, reference y) noexcept;
    constexpr void flip() noexcept;     // flips all bits
    constexpr void clear() noexcept;
  };
}

[...]

4․ reference is a class that simulates a reference to a single bit in the sequence. the behavior of references of a single bit in vector<bool>. The conversion function returns true when the bit is set, and false otherwise. The assignment operators set the bit when the argument is (convertible to) true and clear it otherwise. flip reverses the state of the bit.

  constexpr reference::reference(const reference& x) noexcept;

x․ Effects: Initializes *this to refer to the same bit as x.

  constexpr reference::~reference();

x․ Effects: None.

  constexpr reference& reference::operator=(bool x) noexcept;
  constexpr reference& reference::operator=(const reference& x) noexcept;
  constexpr const reference& reference::operator=(bool x) const noexcept;

x․ Effects: Sets the bit referred to by *this when bool(x) is true, and clears it otherwise.

x․ Returns: *this.

  constexpr void reference::flip() noexcept;

x․ Effects: *this = !*this;

 constexpr void swap(reference x, reference y) noexcept;
 constexpr void swap(reference x, bool& y) noexcept;
 constexpr void swap(bool& x, reference y) noexcept;

x․ Effects: Exchanges the values denoted by x and y as if by:

bool b = x;
x = y;
y = b;

  constexpr reference& reference::flip() noexcept;

x․ Effects: *this = !*this;

  constexpr void flip() noexcept;

5․ Effects: Replaces each element in the container with its complement.

  static constexpr void swap(reference x, reference y) noexcept;

6․ Effects: Exchanges the contents of x and y as if by:

bool b = x;
x = y;
y = b;

4.3. [depr.vector.bool.swap]

Create a new subclause [depr.vector.bool.swap] under [depr]:

D.? Deprecated vector<bool, Allocator> swap [depr.vector.bool.swap]

x․ The following member is declared in addition to those members specified in [vector.bool]:

namespace std {
  template<class Allocator> class vector<bool, Allocator> {
  public:
    static constexpr void swap(reference x, reference y) noexcept;
  };
}

  static constexpr void swap(reference x, reference y) noexcept;

x․ Effects: Exchanges the values denoted by x and y as if by:

bool b = x;
x = y;
y = b;