std::integral_constant
From Cppreference
| Defined in header <type_traits>
|
||
| template< class T, T v >
struct integral_constant; |
(C++11 feature) | |
std::integral_constant wraps a static constant of specified type. It is the base class for the C++ type traits.
Two specializations for the type bool are provided:
| Defined in header
<type_traits> | |
| Type | Definition |
| true_type | std::integral_constant<bool, true> |
| false_type | std::integral_constant<bool, false> |
Contents |
[edit] Member types
| Type | Definition |
| value_type | T |
| type | std::integral_constant<T,v> |
[edit] Member constants
| Name | Value |
| value | static constant expression of type T with value v |
[edit] Member functions
| operator value_type | converts the object to value_type, returns value (public member function) |
[edit] Equivalent definition
template<class T, T v> struct integral_constant { static constexpr T value = v; typedef T value_type; typedef integral_constant type; constexpr operator value_type() const { return value; } }; |
[edit] Example
#include <iostream> #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 a same types" 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::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"); }