# std::variant<Types...>::emplace

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< cpp‎ | utility‎ | variant

template <class T, class... Args> T& emplace(Args&&... args); | (1) | (since C++17)
template <class T, class U, class... Args> T& emplace(std::initializer_list<U> il, Args&&... args); | (2) | (since C++17)
template <size_t I, class... Args> std::variant_alternative_t<I, variant>& emplace(Args&&... args); | (3) | (since C++17)
template <size_t I, class U, class... Args> std::variant_alternative_t<I, variant>& emplace(std::initializer_list<U> il, Args&&... args); | (4) | (since C++17)

Creates a new value in-place, in an existing `variant` object

1) Equivalent to emplace<I>(std::forward<Args>(args)...), where `I` is the zero-based index of `T` in `Types...`. This overload only participates in overload resolution if `std::is_constructible_v<T, Args...>` is `true`, and `T` occurs exactly once in `Types...`
2) Equivalent to emplace<I>(il, std::forward<Args>(args)...), where `I` is the zero-based index of `T` in `Types...`. This overload only participates in overload resolution if `std::is_constructible_v<T, std::initializer_list<U>&, Args...>` is `true`, and `T` occurs exactly once in `Types...`
3) First, destroys the currently contained value (if any). Then direct-initializes the contained value as if constructing a value of type `T_I` with the arguments std::forward<Args>(args).... If an exception is thrown, `*this` may become valueless_by_exception. This overload only participates in overload resolution if `std::is_constructible_v<T_I, Args...>` is `true`. The behavior is undefined if `I` is not less than sizeof...(Types).
4) First, destroys the currently contained value (if any). Then direct-initializes the contained value as if constructing a value of type `T_I` with the arguments il, std::forward<Args>(args).... If an exception is thrown, `*this` may become valueless_by_exception. This overload only participates in overload resolution if `std::is_constructible_v<T_I, initializer_list<U>&, Args...>` is `true`. The behavior is undefined if `I` is not less than sizeof...(Types).

### Parameters

args | - | constructor arguments to use when constructing the new value
il | - | initializer_list argument to use when constructing the new value

### Return value

A reference to the new contained value.

### Exceptions

1-4) Any exception thrown during the initialization of the contained value.

### Example

Run this code

    #include <iostream>
    #include <string>
    #include <variant>

    int main()
    {
      std::variant<std::string> v1;
      v1.emplace<0>("abc"); // OK
      std::cout << std::get<0>(v1) << '\n';
      v1.emplace<std::string>("def"); // OK
      std::cout << std::get<0>(v1) << '\n';

      std::variant<std::string, std::string> v2;
      v2.emplace<1>("ghi"); // OK
      std::cout << std::get<1>(v2) << '\n';
      // v2.emplace<std::string>("abc"); -> Error
    }

Output:

    abc
    def
    ghi

### See also

operator= | assigns a variant (public member function)

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