std::variant<Types...>::emplace
Declarations
- C++20
- C++17
// 1)
template <class T, class... Args>
constexpr T& emplace(Args&&... args);
// 2)
template <class T, class U, class... Args>
constexpr T& emplace( std::initializer_list<U> il, Args&&... args );
// 3)
template <std::size_t I, class... Args>
constexpr std::variant_alternative_t<I, variant>& emplace( Args&&... args );
// 4)
template <std::size_t I, class U, class... Args>
constexpr std::variant_alternative_t<I, variant>&
emplace( std::initializer_list<U> il, Args&&... args );
// 1)
template <class T, class... Args>
T& emplace(Args&&... args);
// 2)
template <class T, class U, class... Args>
T& emplace( std::initializer_list<U> il, Args&&... args );
// 3)
template <std::size_t I, class... Args>
std::variant_alternative_t<I, variant>& emplace( Args&&... args );
// 4)
template <std::size_t I, class U, class... Args>
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
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, andT
occurs exactly once inTypes...
Equivalent to emplace<I>(il, 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, std::initializer_list<U>&, Args...>
is true, andT
occurs exactly once inTypes...
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 thansizeof...(Types)
.
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 participates in overload resolution only if
std::is_constructible_v<T_I, std::initializer_list<U>&, Args...>
is true. - It is a compile-time error if
I
is not less thansizeof...(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
#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
}
abc
def
ghi
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 |