std:: variant_size, std:: variant_size_v
|
Defined in header
<variant>
|
||
|
template
<
class
T
>
struct variant_size ; /* undefined */ |
(1) | (since C++17) |
|
template
<
class
...
Types
>
struct
variant_size
<
std::
variant
<
Types...
>>
|
(2) | (since C++17) |
|
template
<
class
T
>
class variant_size < const T > ; |
(3) | (since C++17) |
|
template
<
class
T
>
class variant_size < volatile T > ; |
(4) |
(since C++17)
(deprecated in C++20) |
|
template
<
class
T
>
class variant_size < const volatile T > ; |
(5) |
(since C++17)
(deprecated in C++20) |
Provides access to the number of alternatives in a possibly cv-qualified variant as a compile-time constant expression.
Formally,
Helper variable template
|
template
<
class
T
>
constexpr std:: size_t variant_size_v = std :: variant_size < T > :: value ; |
(since C++17) | |
Inherited from std:: integral_constant
Member constants
|
value
[static]
|
sizeof...(Types)
(public static member constant) |
Member functions
|
operator std::size_t
|
converts the object to
std::
size_t
, returns
value
(public member function) |
|
operator()
(C++14)
|
returns
value
(public member function) |
Member types
| Type | Definition |
value_type
|
std:: size_t |
type
|
std:: integral_constant < std:: size_t , value > |
Notes
All specializations of
std::variant_size
satisfy
UnaryTypeTrait
with
base characteristic
std::
integral_constant
<
std::
size_t
, N
>
for some
N
.
Example
#include <any> #include <variant> static_assert(std::variant_size_v<std::variant<>> == 0); static_assert(std::variant_size_v<std::variant<int>> == 1); static_assert(std::variant_size_v<std::variant<int, int>> == 2); static_assert(std::variant_size_v<std::variant<int, int, int>> == 3); static_assert(std::variant_size_v<std::variant<int, float, double>> == 3); static_assert(std::variant_size_v<std::variant<std::monostate, void>> == 2); static_assert(std::variant_size_v<std::variant<const int, const float>> == 2); static_assert(std::variant_size_v<std::variant<std::variant<std::any>>> == 1); int main() {}
See also
|
obtains the type of the alternative specified by its index, at compile time
(class template) (alias template) |
|
|
(C++11)
|
obtains the size of
a
|