Std::variant::emplace (3) Linux Manual Page

std::variant<Types…>::emplace – std::variant<Types…>::emplace

Synopsis

template <class T, class... Args>                                                             (1) (since C++17)

T& emplace(Args&&... args);

template <class T, class U, class... Args>                                                    (2) (since C++17)

T& emplace(std::initializer_list<U> il, Args&&... args);

template <size_t I, class... Args>                                                            (3) (since C++17)

std::variant_alternative_t<I, variant>& emplace(Args&&... args);

template <size_t I, class U, class... Args>                                                   (4) (since C++17)

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

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

std::variant::emplace (3) Linux Manual Page