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4015. LWG 3973 broke const overloads of std::optional monadic operations

Section: 22.5.3.7 [optional.monadic] Status: New Submitter: Jonathan Wakely Opened: 2023-11-24 Last modified: 2023-11-29

Priority: Not Prioritized

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Discussion:

The resolution of LWG 3973 (adopted in Kona) changed all occurrences of value() to *val. The intention was not to change the meaning, just avoid the non-freestanding value() function, and avoid ADL that would be caused by using **this. However, in the const overloads such as and_then(F&&) const the type of value() was const T&, but the type of *val is always T&. This implies that the const overloads invoke the callable with a non-const argument, which is incorrect (and would be undefined behaviour for a const std::optional<T>).

On the LWG reflector it was suggested that we should rewrite the specification of std::optional to stop using an exposition-only data member of type T*. No such member ever exists in real implemetations, so it is misleading and leads to specification bugs of this sort.

Change the class definition in 22.5.3.1 [optional.optional.general] to use a union, and update every use of val accordingly throughout 22.5.3 [optional.optional]. For consistency with 22.8.6.1 [expected.object.general] we might also want to introduce a bool has_val member and refer to that in the specification.


  private:
    T *val;         // exposition only
    bool has_val;   // exposition only
    union {
      T val;        // exposition only
    };
  };

For example, in 22.5.3.8 [optional.mod]:

-1- Effects: If *this contains a value, calls val->.T::~T() to destroy the contained value and sets has_val to false; otherwise no effect.

[2023-11-26; Daniel provides wording]

The proposed wording is considerably influenced by that of the specification of expected, but attempts to reduce the amount of changes to not perfectly mimic it. Although "the contained value" is a magic word of power it seemed feasible and simpler to use the new exposition-only member val directly in some (but not all) places, usually involved with initializations.

Furthermore, I have only added "and sets has_val to true/false" where either the Effects wording says "otherwise no effect" or in other cases if the postconditions did not already say that indirectly. I also added extra mentioning of has_val changes in tables where different cells had very different effects on that member (unless these cells specify postconditions), to prevent misunderstanding.

Proposed resolution:

This wording is relative to N4964 after application of the wording of LWG 3973.

  1. Modify 22.5.3.1 [optional.optional.general], class template optional synopsis, as indicated:

    namespace std {
      template<class T>
      class optional {
      public:
        using value_type = T;
        […]
      private:
        bool has_val; // exposition only
        union {
          T val*val; // exposition only
        };
      };
      
      […]
    }
    
  2. Modify 22.5.3.1 [optional.optional.general] as indicated:

    -2- Member has_val indicates whether an optional<T> object contains a valueWhen an optional<T> object contains a value, member val points to the contained value.

  3. Modify 22.5.3.2 [optional.ctor] as indicated:

    [Drafting note: Normatively, this subclause doesn't require any changes, but I'm suggesting to replace phrases of the form "[…]initializes the contained value with"] by "[…]initializes val with" as we do in 22.8.6.2 [expected.object.cons]. I intentionally did not add extra "and sets has_val to true/false" since those effects are already guaranteed by the postconditions]

    constexpr optional(const optional& rhs);
    

    -4- Effects: If rhs contains a value, direct-non-list-initializes valthe contained value with *rhs.val.

    -5- Postconditions: rhs.has_value() == this->has_value().

    […]

    constexpr optional(optional&& rhs) noexcept(see below);
    

    -8- Constraints: […]

    -9- Effects: If rhs contains a value, direct-non-list-initializes valthe contained value with std::move(*rhs.val). rhs.has_value() is unchanged.

    -10- Postconditions: rhs.has_value() == this->has_value().

    […]

    template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);
    

    -13- Constraints: […]

    -14- Effects: Direct-non-list-initializes valthe contained value with std::forward<Args>(args)....

    -15- Postconditions: *this contains a value.

    […]

    template<class U, class... Args>
      constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
    

    -18- Constraints: […]

    -19- Effects: Direct-non-list-initializes valthe contained value with il, std::forward<Args>(args)....

    -20- Postconditions: *this contains a value.

    […]

    template<class U = T> constexpr explicit(see below) optional(U&& v);
    

    -23- Constraints: […]

    -24- Effects: Direct-non-list-initializes valthe contained value with std::forward<U>(v).

    -25- Postconditions: *this contains a value.

    […]

    template<class U> constexpr explicit(see below) optional(const optional<U>& rhs);
    

    -28- Constraints: […]

    -29- Effects: If rhs contains a value, direct-non-list-initializes valthe contained value with *rhs.val.

    -30- Postconditions: rhs.has_value() == this->has_value().

    […]

    template<class U> constexpr explicit(see below) optional(optional<U>&& rhs);
    

    -33- Constraints: […]

    -34- Effects: If rhs contains a value, direct-non-list-initializes valthe contained value with std::move(*rhs.val). rhs.has_value() is unchanged.

    -35- Postconditions: rhs.has_value() == this->has_value().

    […]

  4. Modify 22.5.3.3 [optional.dtor] as indicated:

    constexpr ~optional();
    

    -1- Effects: If is_trivially_destructible_v<T> != true and *this contains a value, calls val->val.T::~T().

  5. Modify 22.5.3.4 [optional.assign] as indicated:

    constexpr optional<T>& operator=(nullopt_t) noexcept;
    

    -1- Effects: If *this contains a value, calls val->val.T::~T() to destroy the contained value and sets has_val to false; otherwise no effect.

    -2- Postconditions: *this does not contain a value.

    constexpr optional<T>& operator=(const optional& rhs);
    

    -4- Effects: See Table 58.

    Table 58 — optional::operator=(const optional&) effects [tab:optional.assign.copy]
    *this contains a value *this does not contain a value
    rhs contains a value assigns *rhs.val to valthe contained value direct-non-list-initializes valthe contained value with *rhs.val
    and sets has_val to true
    rhs does not contain a value destroys the contained value by calling val->val.T::~T()
    and sets has_val to false
    no effect

    -5- Postconditions: rhs.has_value() == this->has_value().

    […]

    constexpr optional<T>& operator=(optional&& rhs) noexcept(see below);
    

    -8- Constraints: […]

    -9- Effects: See Table 59. The result of the expression rhs.has_value() remains unchanged.

    -10- Postconditions: rhs.has_value() == this->has_value().

    -11- Returns: *this.

    Table 59 — optional::operator=(optional&&) effects [tab:optional.assign.move]
    *this contains a value *this does not contain a value
    rhs contains a value assigns std::move(*rhs.val) to valthe contained value direct-non-list-initializes valthe contained value with std::move(*rhs.val) and sets has_val to true
    rhs does not contain a value destroys the contained value by calling
    val->val.T::~T()and sets has_val to false
    no effect

    -12- Remarks: […]

    -13- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's move constructor, the state of *rhs.valval is determined by the exception safety guarantee of T's move constructor. If an exception is thrown during the call to T's move assignment, the state of *valval and *rhs.valval is determined by the exception safety guarantee of T's move assignment.

    template<class U = T> constexpr optional<T>& operator=(U&& v);
    

    -14- Constraints: […]

    -15- Effects: If *this contains a value, assigns std::forward<U>(v) to valthe contained value; otherwise direct-non-list-initializes valthe contained value with std::forward<U>(v).

    -16- Postconditions: *this contains a value.

    -17- Returns: *this.

    -18- Remarks: If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of v is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the state of val*val and v is determined by the exception safety guarantee of T's assignment.

    template<class U> constexpr optional<T>& operator=(const optional<U>& rhs);
    

    -19- Constraints: […]

    -20- Effects: See Table 60.

    Table 60 — optional::operator=(const optional<U>&) effects [tab:optional.assign.copy.templ]
    *this contains a value *this does not contain a value
    rhs contains a value assigns *rhs.val to valthe contained value direct-non-list-initializes valthe contained value with *rhs.val and sets has_val to true
    rhs does not contain a value destroys the contained value by calling
    val->val.T::~T() and sets has_val to false
    no effect

    -21- Postconditions: rhs.has_value() == this->has_value().

    -22- Returns: *this.

    -23- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.valval is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the state of val*val and *rhs.valval is determined by the exception safety guarantee of T's assignment.

    template<class U> constexpr optional<T>& operator=(optional<U>&& rhs);
    

    -24- Constraints: […]

    -25- Effects: See Table 61. The result of the expression rhs.has_value() remains unchanged.

    Table 61 — optional::operator=(optional<U>&&) effects [tab:optional.assign.move.templ]
    *this contains a value *this does not contain a value
    rhs contains a value assigns std::move(*rhs.val) to valthe contained value direct-non-list-initializes valthe contained value with
    std::move(*rhs.val) and sets has_val to true
    rhs does not contain a value destroys the contained value by calling
    val->val.T::~T() and sets has_val to false
    no effect

    -26- Postconditions: rhs.has_value() == this->has_value().

    -27- Returns: *this.

    -28- If any exception is thrown, the result of the expression this->has_value() remains unchanged. If an exception is thrown during the call to T's constructor, the state of *rhs.valval is determined by the exception safety guarantee of T's constructor. If an exception is thrown during the call to T's assignment, the state of val*val and *rhs.valval is determined by the exception safety guarantee of T's assignment.

    template<class... Args> constexpr T& emplace(Args&&... args);
    

    -29- Mandates: […]

    -30- Effects: Calls *this = nullopt. Then direct-non-list-initializes valthe contained value with std::forward<Args>(args)....

    -31- Postconditions: *this contains a value.

    -32- Returns: valA reference to the new contained value.

    […]

    -34- Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous val*val (if any) has been destroyed.

    template<class U, class... Args> constexpr T& emplace(initializer_list<U> il, Args&&... args);
    

    -35- Constraints: […]

    -36- Effects: Calls *this = nullopt. Then direct-non-list-initializes valthe contained value with il, std::forward<Args>(args)....

    -37- Postconditions: *this contains a value.

    -38- Returns: valA reference to the new contained value.

    […]

    -40- Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous val*val (if any) has been destroyed.

  6. Modify 22.5.3.5 [optional.swap] as indicated:

    constexpr void swap(optional& rhs) noexcept(see below);
    

    -1- Mandates: […]

    -2- Preconditions: […]

    -3- Effects: See Table 62.

    Table 62 — optional::swap(optional&) effects [tab:optional.swap]
    *this contains a value *this does not contain a value
    rhs contains a value calls swap(val*(*this), *rhs.val) direct-non-list-initializes valthe contained value of *this
    with std::move(*rhs.val), followed by rhs.val.val->T::~T();
    postcondition is that *this contains a value and rhs does
    not contain a value
    rhs does not contain a value direct-non-list-initializes the contained value of rhs.val
    with std::move(val*(*this)), followed by val.val->T::~T();
    postcondition is that *this does not contain a value and rhs
    contains a value
    no effect

    -4- Throws: […]

    -5- Remarks: […]

    -6- If any exception is thrown, the results of the expressions this->has_value() and rhs.has_value() remain unchanged. If an exception is thrown during the call to function swap, the state of val*val and *rhs.valval is determined by the exception safety guarantee of swap for lvalues of T. If an exception is thrown during the call to T's move constructor, the state of val*val and *rhs.valval is determined by the exception safety guarantee of T's move constructor.

  7. Modify 22.5.3.6 [optional.observe] as indicated:

    constexpr const T* operator->() const noexcept;
    constexpr T* operator->() noexcept;
    

    -1- Preconditions: *this contains a value.

    -2- Returns: addressof(val)val.

    -3- […]

    constexpr const T& operator*() const & noexcept;
    constexpr T& operator*() & noexcept;
    

    -4- Preconditions: *this contains a value.

    -5- Returns: val*val.

    -6- […]

    constexpr T&& operator*() && noexcept;
    constexpr const T&& operator*() const && noexcept;
    

    -7- Preconditions: *this contains a value.

    -8- Effects: Equivalent to: return std::move(val*val);

    constexpr explicit operator bool() const noexcept;
    

    -9- Returns: true if and only if *this contains a value.

    -10- Remarks: This function is a constexpr function.

    constexpr bool has_value() const noexcept;
    

    -11- Returns: has_valtrue if and only if *this contains a value.

    -12- Remarks: These functions areThis function is a constexpr functions.

    constexpr const T& value() const &;
    constexpr T& value() &;
    

    -13- Effects: Equivalent to:

    return has_value() ? val*val : throw bad_optional_access();
    
    constexpr T&& value() &&;
    constexpr const T&& value() const &&;
    

    -14- Effects: Equivalent to:

    return has_value() ? std::move(val*val) : throw bad_optional_access();
    
    template<class U> constexpr T value_or(U&& v) const &;
    

    -15- Mandates: […]

    -16- Effects: Equivalent to:

    return has_value() ? val**this : static_cast<T>(std::forward<U>(v));
    
    template<class U> constexpr T value_or(U&& v) &&;
    

    -17- Mandates: […]

    -18- Effects: Equivalent to:

    return has_value() ? std::move(val**this) : static_cast<T>(std::forward<U>(v));
    
  8. Modify 22.5.3.7 [optional.monadic] as indicated:

    template<class F> constexpr auto and_then(F&& f) &;
    template<class F> constexpr auto and_then(F&& f) const &;
    

    -1- Let U be invoke_result_t<F, decltype((val)*val)>.

    -2- Mandates: […]

    -3- Effects: Equivalent to:

    if (*this) {
      return invoke(std::forward<F>(f), val*val);
    } else {
      return remove_cvref_t<U>();
    }
    
    template<class F> constexpr auto and_then(F&& f) &&;
    template<class F> constexpr auto and_then(F&& f) const &&;
    

    -4- Let U be invoke_result_t<F, decltype(std::move(val*val))>.

    -5- Mandates: […]

    -6- Effects: Equivalent to:

    if (*this) {
      return invoke(std::forward<F>(f), std::move(val*val));
    } else {
      return remove_cvref_t<U>();
    }
    
    template<class F> constexpr auto transform(F&& f) &;
    template<class F> constexpr auto transform(F&& f) const &;
    

    -7- Let U be remove_cv_t<invoke_result_t<F, decltype((val)*val)>>.

    -8- Mandates: U is a non-array object type other than in_place_t or nullopt_t. The declaration

    U u(invoke(std::forward<F>(f), val*val));
    

    is well-formed for some invented variable u.

    […]

    -9- Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized with invoke(std::forward<F>(f), val*val); otherwise, optional<U>().

    template<class F> constexpr auto transform(F&& f) &&;
    template<class F> constexpr auto transform(F&& f) const &&;
    

    -10- Let U be remove_cv_t<invoke_result_t<F, decltype(std::move(val*val))>>.

    -11- Mandates: U is a non-array object type other than in_place_t or nullopt_t. The declaration

    U u(invoke(std::forward<F>(f), std::move(val*val)));
    

    is well-formed for some invented variable u.

    […]

    -12- Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized with invoke(std::forward<F>(f), std::move(val*val)); otherwise, optional<U>().

  9. Modify 22.5.3.8 [optional.mod] as indicated:

    constexpr void reset() noexcept;
    

    -1- Effects: If *this contains a value, calls val->val.T::~T() to destroy the contained value and sets has_val to false; otherwise no effect.

    -2- Postconditions: *this does not contain a value.