22 General utilities library [utilities]

22.6 Variants [variant]

22.6.7 Visitation [variant.visit]

template<class Visitor, class... Variants>
  constexpr see below visit(Visitor&& vis, Variants&&... vars);
template<class R, class Visitor, class... Variants>
  constexpr R visit(Visitor&& vis, Variants&&... vars);

Let as-variant denote the following exposition-only function templates: template<class... Ts> auto&& as-variant(variant<Ts...>& var) { return var; } template<class... Ts> auto&& as-variant(const variant<Ts...>& var) { return var; } template<class... Ts> auto&& as-variant(variant<Ts...>&& var) { return std::move(var); } template<class... Ts> auto&& as-variant(const variant<Ts...>&& var) { return std::move(var); }

Let n be sizeof...(Variants).

For each

0 \leq i < n

, let

V_{i}

denote the type
decltype(as-variant(std::forward<

{Variants}_{i}

{vars}_{i}

))).

Constraints:

V_{i}

is a valid type for all

0 \leq i < n

Let V denote the pack of types

V_{i}

Let m be a pack of n values of type size_t.

Such a pack is valid if

0 \leq m_{i} < {variant_size_v<remove_reference_t<V}_{i} >>

for all

0 \leq i < n

For each valid pack m, let e(m) denote the expression: INVOKE(std::forward<Visitor>(vis), get<m>(std::forward<V>(vars))...) // see [func.require] for the first form and INVOKE<R>(std::forward<Visitor>(vis), get<m>(std::forward<V>(vars))...) // see [func.require] for the second form.

Mandates: For each valid pack m, e(m) is a valid expression.

All such expressions are of the same type and value category.

Returns: e(m), where m is the pack for which

m_{i}

is as-variant(vars

_{i}

).index() for all

0 \leq i < n

The return type is decltype(e(m)) for the first form.

Throws: bad_variant_access if (as-variant(vars).valueless_by_exception() || ...) is true.

Complexity: For n ≤ 1, the invocation of the callable object is implemented in constant time, i.e., for

n = 1

, it does not depend on the number of alternative types of

V_{0}

For

n > 1

, the invocation of the callable object has no complexity requirements.