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4493. Specification for some functions of bit reference types seems missing

Section: 22.9.2.1 [template.bitset.general], 23.3.14.1 [vector.bool.pspc] Status: New Submitter: Jiang An Opened: 2025-12-16 Last modified: 2025-12-20

Priority: Not Prioritized

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Discussion:

We haven't explicitly specified the return values of bitset<N>::reference::operator bool() and vector<bool, A>::reference::operator bool(), although the intended return values can be inferred from "a class that simulates a reference to a single bit in the sequence". Moreover, specification for bitset<N>::reference::operator~ seems completely missing, and the comment "flip the bit" seems misleading. Implementations consistently make the operator~ return !operator bool().

Proposed resolution:

This wording is relative to N5032.

1. Modify 22.9.2.1 [template.bitset.general] as indicated:

   […]
   // bit reference
   class reference {
   public:
     constexpr reference(const reference& x) noexcept;
     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 operator bool() const noexcept; // for x = b[i];
     constexpr bool operator~() const noexcept; // flips the bit
     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;
   };
   […]
   

   […]

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

   -6- Effects: Sets the bit referred to by *this if bool(x) is true, and clears it otherwise.

   -7- Returns: *this.

   constexpr reference::operator bool() const noexcept;
   

   -?- Returns: true if the value of the referred to bit is one, false otherwise.

   constexpr bool reference::operator~() const noexcept;
   

   -?- Returns: !bool(*this).

2. Modify 23.3.14.1 [vector.bool.pspc] as indicated:

   […]

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

   -7- Effects: Sets the bit referred to by *this when bool(x) is true, and clears it otherwise.

   -8- Returns: *this.

   constexpr reference::operator bool() const noexcept;
   

   -?- Returns: true if the value of the referred to bit is one, false otherwise.