20 General utilities library [utilities]

20.7 Variants [variant]

20.7.3 Class template variant [variant.variant]

20.7.3.5 Modifiers [variant.mod]

template<class T, class... Args> constexpr T& emplace(Args&&... args);
Constraints: is_­constructible_­v<T, Args...> is true, and T occurs exactly once in Types.
Effects: Equivalent to: return emplace<I>(std::forward<Args>(args)...); where I is the zero-based index of T in Types.
template<class T, class U, class... Args> constexpr T& emplace(initializer_list<U> il, Args&&... args);
Constraints: is_­constructible_­v<T, initializer_­list<U>&, Args...> is true, and T occurs exactly once in Types.
Effects: Equivalent to: return emplace<I>(il, std::forward<Args>(args)...); where I is the zero-based index of T in Types.
template<size_t I, class... Args> constexpr variant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);
Mandates: .
Constraints: is_­constructible_­v<, Args...> is true.
Effects: Destroys the currently contained value if valueless_­by_­exception() is false.
Then initializes the contained value as if direct-non-list-initializing a value of type with the arguments std​::​forward<Args>(args)....
Postconditions: index() is I.
Returns: A reference to the new contained value.
Throws: Any exception thrown during the initialization of the contained value.
Remarks: If an exception is thrown during the initialization of the contained value, the variant is permitted to not hold a value.
template<size_t I, class U, class... Args> constexpr variant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U> il, Args&&... args);
Mandates: .
Constraints: is_­constructible_­v<, initializer_­list<U>&, Args...> is true.
Effects: Destroys the currently contained value if valueless_­by_­exception() is false.
Then initializes the contained value as if direct-non-list-initializing a value of type with the arguments il, std​::​forward<Args>(args)....
Postconditions: index() is I.
Returns: A reference to the new contained value.
Throws: Any exception thrown during the initialization of the contained value.
Remarks: If an exception is thrown during the initialization of the contained value, the variant is permitted to not hold a value.