std::variant<Types...>:: emplace
From cppreference.com
|
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
>
&
|
(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
Toccurs exactly once inTypes....
2)
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
, and
Toccurs exactly once inTypes....
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
Iis 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 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
Iis 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.
Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_variant
|
202106L |
(C++20)
(DR) |
Fully
constexpr
std::variant
(
1-4
)
|
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
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 |
See also
assigns a
variant
(public member function) |