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.
The behavior of a program that adds specializations for integral_constant 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) | |
| Feature-test macro: | __cpp_lib_bool_constant |
Specializations
Two typedefs for the common case where T is bool are provided:
| Defined in header
<type_traits> | |
| Type | Definition |
true_type
|
std::integral_constant<bool, true> |
false_type
|
std::integral_constant<bool, false> |
Member types
| Type | Definition |
value_type
|
T
|
type
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std::integral_constant<T,v> |
Member constants
| Name | Value |
| constexpr T value [static] |
static constant of type T with value 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.
| Feature-test macro: | __cpp_lib_integral_constant_callable |
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 }; |
Example
Run this code
#include <type_traits> int main() { typedef std::integral_constant<int, 2> two_t; typedef std::integral_constant<int, 4> four_t; // static_assert(std::is_same<two_t, four_t>::value, // "two_t and four_t are not equal!"); // error: static assertion failed: "two_t and four_t are not equal!" static_assert(two_t::value*2 == four_t::value, "2*2 != 4" ); enum class my_e { e1, e2 }; typedef std::integral_constant<my_e, my_e::e1> my_e_e1; typedef std::integral_constant<my_e, my_e::e2> my_e_e2; static_assert(my_e_e1() == my_e::e1); // static_assert(my_e_e1::value == my_e::e2, // "my_e_e1::value != my_e::e2"); // error: static assertion failed: "my_e_e1::value != my_e::e2" static_assert(std::is_same<my_e_e2, my_e_e2>::value, "my_e_e2 != my_e_e2"); }