Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | utility‎ | variant
 
 
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
(C++26)
Three-way comparison
(C++20)
(C++20)(C++20)(C++20)
(C++20)(C++20)(C++20)
General utilities
Date and time
Function objects
Formatting library (C++20)
(C++11)
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)(C++20)(C++20)   
(C++20)
Swap and type operations
(C++14)
(C++11)
(C++11)
(C++11)
(C++17)
Common vocabulary types
(C++11)
(C++17)
(C++17)
(C++17)
(C++11)
(C++17)
(C++23)
Elementary string conversions
(C++17)
(C++17)

 
 
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 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 participates in overload resolution only 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....
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 participates in overload resolution only if std::is_constructible_v<T_I, Args...> is true.
  • It is a compile-time error 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.

Contents

[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)
Fully constexpr std::variant (1-4)

[edit] Example

#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

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P2231R1 C++20 emplace was not constexpr while the required operations can be constexpr in C++20 made constexpr

[edit] See also

assigns a variant
(public member function) [edit]