std:: variant_alternative, std:: variant_alternative_t
|
Defined in header
<variant>
|
||
|
template
<
std::
size_t
I,
class
T
>
struct variant_alternative ; /* undefined */ |
(1) | (since C++17) |
|
template
<
std::
size_t
I,
class
...
Types
>
struct variant_alternative < I, variant < Types... >> ; |
(2) | (since C++17) |
|
template
<
std::
size_t
I,
class
T
>
class
variant_alternative
<
I,
const
T
>
;
|
(3) | (since C++17) |
|
template
<
std::
size_t
I,
class
T
>
class
variant_alternative
<
I,
volatile
T
>
;
|
(3) |
(since C++17)
(deprecated in C++20) |
Provides compile-time indexed access to the types of the alternatives of the possibly cv-qualified variant, combining cv-qualifications of the variant (if any) with the cv-qualifications of the alternative.
Formally,
type
equal to the type of the alternative with index
I
type
that names, respectively,
std::
add_const_t
<
std
::
variant_alternative_t
<
I,T
>>
,
std::
add_volatile_t
<
std
::
variant_alternative_t
<
I,T
>>
, and
std::
add_cv_t
<
std
::
variant_alternative_t
<
I,T
>>
Member types
| Member type | Definition |
| type |
the type of
I
th alternative of the variant, where
I
must be in
[0, sizeof...(Types))
, otherwise the program is ill-formed.
|
Helper template alias
|
template
<
size_t I,
class
T
>
using variant_alternative_t = typename variant_alternative < I, T > :: type ; |
(since C++17) | |
Example
#include <variant> #include <iostream> using my_variant = std::variant<int, float>; static_assert(std::is_same_v <int, std::variant_alternative_t<0, my_variant>>); static_assert(std::is_same_v <float, std::variant_alternative_t<1, my_variant>>); // cv-qualification on the variant type propagates to the extracted alternative type. static_assert(std::is_same_v <const int, std::variant_alternative_t<0, const my_variant>>); int main() { std::cout << "All static assertions passed.\n"; }
Output:
All static assertions passed.
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2974 | C++17 | out-of-bounds index resulted in undefined behavior | made ill-formed |
See also
|
(C++17)
|
obtains the size of the
variant
's list of alternatives at compile time
(class template) (variable template) |
|
(C++11)
|
obtains the type of the specified element
(class template specialization) |