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

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std::variant

Member functions
variant::variant
variant::~variant
variant::operator=
Observers
variant::index
variant::valueless_by_exception
Modifiers
variant::emplace
variant::swap
Visitation
variant::visit (C++26)
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visit(std::variant)
holds_alternative
get(std::variant)
get_if
operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)
swap(std::variant)
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monostate
bad_variant_access
variant_size
variant_alternative
hash<std::variant>
Helper objects
variant_npos

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template<class T,class...Args> T& emplace( Args&&...args);	(1)	(since C++17) (constexpr since C++20)
template<class T,class U,class...Args> T& emplace(std::initializer_list<U> il, Args&&...args);	(2)	(since C++17) (constexpr since C++20)
template<std::size_t I,class...Args> std::variant_alternative_t<I, variant>& emplace( Args&&...args);	(3)	(since C++17) (constexpr since C++20)
template<std::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) (constexpr since C++20)

Creates a new value in-place, in an existingvariant object

1) Equivalent toemplace<I>(std::forward<Args>(args)...), whereI is the zero-based index ofT inTypes....

This overload participates in overload resolution only ifstd::is_constructible_v<T, Args...> istrue, andT occurs exactly once inTypes....

2) Equivalent toemplace<I>(il,std::forward<Args>(args)...), whereI is the zero-based index ofT inTypes....

This overload participates in overload resolution only ifstd::is_constructible_v<T,std::initializer_list<U>&, Args...> istrue, andT occurs exactly once inTypes....

3) First, destroys the currently contained value (if any). Thendirect-initializes the contained value as if constructing a value of typeT_I with the argumentsstd::forward<Args>(args).... If an exception is thrown,*this may becomevalueless_by_exception.

This overload participates in overload resolution only ifstd::is_constructible_v<T_I, Args...> istrue.
It is a compile-time error ifI is not less thansizeof...(Types).

4) First, destroys the currently contained value (if any). Thendirect-initializes the contained value as if constructing a value of typeT_I with the argumentsil,std::forward<Args>(args).... If an exception is thrown,*this may becomevalueless_by_exception.

This overload participates in overload resolution only ifstd::is_constructible_v<T_I,std::initializer_list<U>&, Args...> istrue.
It is a compile-time error ifI is not less thansizeof...(Types).

[edit]Parameters

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

[edit]Return value

A reference to the new contained value.

[edit]Exceptions

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

[edit]Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_variant`	`202106L`	(C++20) (DR)	Fullyconstexpr`std::variant`(1-4)

[edit]Example

Run this code

#include <iostream>#include <string>#include <variant> int main(){std::variant<std::string> v1;    v1.emplace<0>("abc");// OKstd::cout<< std::get<0>(v1)<<'\n';    v1.emplace<std::string>("def");// OKstd::cout<< std::get<0>(v1)<<'\n'; std::variant<std::string,std::string> v2;    v2.emplace<1>("ghi");// OKstd::cout<< std::get<1>(v2)<<'\n';// v2.emplace<std::string>("abc"); -> Error}