[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);

Effects: Destroys the currently contained value if valueless_by_exception() is false.

Then direct-non-list-initializes the contained value of type

T {I with the arguments std ​::​ forward < Args > (args) . .}_{I with the arguments std ​::​ forward < Args > (args) . .}

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);

Effects: Destroys the currently contained value if valueless_by_exception() is false.

Then direct-non-list-initializes the contained value of type

T {I with il, std ​::​ forward < Args > (args) . .}_{I with il, std ​::​ forward < Args > (args) . .}

Remarks: If an exception is thrown during the initialization of the contained value, the variant is permitted to not hold a value.

22 General utilities library [utilities]