2904. Make variant move-assignment more exception safe

Section: 22.6.3.4 [variant.assign] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 119 and CH 7

The copy-assignment operator is very careful to not destroy the contained element until after a temporary has been constructed, which can be safely moved from.

This makes the valueless_by_exception state extremely rare, by design.

However, the same care and attention is not paid to the move-assignment operator, nor the assignment-from-deduced-value assignment template. This concern should be similarly important in these cases, especially the latter.

Proposed change: —

[2017-03-02, Kona, Casey comments and suggests wording]

The wording below has been developed with much input from Tomasz.

[Kona 2017-03-02]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

  1. Modify 22.6.3.4 [variant.assign] as indicated:

    [Drafting note: Presentation of para 9 immediately below has been split into individual bullets.] 

    variant& operator=(const variant& rhs);

    Let j be rhs.index().

    -1- Effects:

    1. (1.1) — If neither *this nor rhs holds a value, there is no effect. Otherwise,

    2. (1.2) — if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value. Otherwise,

    3. (1.3) — if index() == jrhs.index(), assigns the value contained in rhs to the value contained in *this. Otherwise,

    4. (1.?) — if is_nothrow_copy_constructible_v<Tj> || !is_nothrow_move_constructible_v<Tj> is true, equivalent to emplace<j>(get<j>(rhs)). Otherwise,

    5. (1.4) — equivalent to operator=(variant(rhs))copies the value contained in rhs to a temporary, then destroys any value contained in *this. Sets *this to hold the same alternative index as rhs and initializes the value contained in *this as if direct-non-list-initializing an object of type Tj with std::forward<Tj>(TMP), with TMP being the temporary and j being rhs.index().

    -2- Returns: *this.

    -3- Postconditions: index() == rhs.index().

    -4- Remarks: This function shall not participate in overload resolution unless is_copy_constructible_v<Ti> && is_move_constructible_v<Ti> && is_copy_assignable_v<Ti> is true for all i.

    1. (4.1) — If an exception is thrown during the call […]

    2. (4.2) — If an exception is thrown during the call […]

    3. (4.3) — If an exception is thrown during the call […]

    variant& operator=(variant&& rhs) noexcept(see below);

    Let j be rhs.index().

    -5- Effects:

    1. (5.1) — If neither *this nor rhs holds a value, there is no effect. Otherwise,

    2. (5.2) — if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value. Otherwise,

    3. (5.3) — if index() == jrhs.index(), assigns get<j>(std::move(rhs)) to the value contained in *this, with j being index(). Otherwise,

    4. (5.4) — equivalent to emplace<j>(get<j>(std::move(rhs)))destroys any value contained in *this. Sets *this to hold the same alternative index as rhs and initializes the value contained in *this as if direct-non-list-initializing an object of type Tj with get<j>(std::move(rhs)) with j being rhs.index().

    […]

    […] 

    template <class T> variant& operator=(T&& t) noexcept(see below);

    -8- […]

    -9- Effects:

    1. (9.1) — If *this holds a Tj, assigns std::forward<T>(t) to the value contained in *this. Otherwise,

    2. (9.?) — if is_nothrow_constructible_v<Tj, T> || !is_nothrow_move_constructible_v<Tj> is true, equivalent to emplace<j>(std::forward<T>(t)). Otherwise,

    3. (9.3) — equivalent to operator=(variant(std::forward<T>(t)))destroys any value contained in *this, sets *this to hold the alternative type Tj as selected by the imaginary function overload resolution described above, and direct-initializes the contained value as if direct-non-list-initializing it with std::forward<T>(t).