20 General utilities library [utilities]

20.7 Variants [variant]

template<class T, class... Args> 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> 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>
  variant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);

Mandates:

I < sizeof...(Types)

Constraints: is_constructible_v<

T_{I}

, 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

T_{I}

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>
  variant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U> il, Args&&... args);