std::integral_constant
From cppreference.com
| Defined in header <type_traits>
|
||
| template< class T, T v > struct integral_constant; |
(since C++11) | |
std::integral_constant wraps a static constant of specified type. It is the base class for the C++ type traits.
If the program adds specializations for std::integral_constant, the behavior is undefined.
Helper alias templates
A helper alias template std::bool_constant is defined for the common case where T is bool.
| template< bool B > using bool_constant = integral_constant<bool, B>; |
(since C++17) | |
Specializations
Two typedefs for the common case where T is bool are provided:
| Defined in header
<type_traits> | |
| Name | Definition |
true_type
|
std::integral_constant<bool, true> |
false_type
|
std::integral_constant<bool, false> |
Member types
| Name | Definition |
value_type
|
T |
type
|
std::integral_constant<T, v> |
Member constants
| Name | Value |
| constexpr T value [static] |
v (public static member constant) |
Member functions
| operator value_type |
returns the wrapped value (public member function) |
| operator() (C++14) |
returns the wrapped value (public member function) |
std::integral_constant::operator value_type
| constexpr operator value_type() const noexcept; |
||
Conversion function. Returns the wrapped value.
std::integral_constant::operator()
| constexpr value_type operator()() const noexcept; |
(since C++14) | |
Returns the wrapped value. This function enables std::integral_constant to serve as a source of compile-time function objects.
Possible implementation
template<class T, T v> struct integral_constant { static constexpr T value = v; using value_type = T; using type = integral_constant; // using injected-class-name constexpr operator value_type() const noexcept { return value; } constexpr value_type operator()() const noexcept { return value; } // since c++14 }; |
Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_integral_constant_callable |
201304L | (C++14) | std::integral_constant::operator()
|
__cpp_lib_bool_constant |
201505L | (C++17) | std::bool_constant
|
Example
Run this code
#include <type_traits> using two_t = std::integral_constant<int, 2>; using four_t = std::integral_constant<int, 4>; static_assert(not std::is_same_v<two_t, four_t>); static_assert(two_t::value * 2 == four_t::value, "2*2 != 4"); static_assert(two_t() << 1 == four_t() >> 0, "2*2 != 4"); enum class E{ e1, e2 }; using c1 = std::integral_constant<E, E::e1>; using c2 = std::integral_constant<E, E::e2>; static_assert(c1::value != E::e2); static_assert(c1() == E::e1); static_assert(std::is_same_v<c2, c2>); int main() {}
See also
| (C++14) |
implements compile-time sequence of integers (class template) |