std::variant::emplace

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Utilities library

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Library feature-test macros
Date and time
Function objects
initializer_list (C++11)
bitset
hash (C++11)
integer_sequence (C++14)
Relational operators (deprecated)
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Comparisons (C++20)
strong_order weak_order partial_order strong_equal weak_equal	strong_ordering weak_ordering partial_ordering strong_equality weak_equality	is_eqis_neqis_ltis_lteqis_gtis_gteq
common_comparison_category
Common vocabulary types
pair tuple (C++11) apply (C++17) make_from_tuple (C++17)	optional (C++17) any (C++17) variant (C++17)
Swap, forward and move
swap
exchange (C++14)
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
Elementary string conversions
to_chars (C++17)
from_chars (C++17)
chars_format (C++17)
Type operations
declval (C++11)
as_const (C++17)

std::variant

Member functions

variant::variant

variant::~variant

variant::operator=

Observers

variant::index

variant::valueless_by_exception

Modifiers

variant::emplace

variant::swap

Non-member functions

visit

holds_alternative

get

get_if

operator==operator!=operator<operator<=operator>operator>=

swap

Helper classes

monostate

bad_variant_access

variant_size

variant_alternative

hash

Helper objects

variant_npos

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)