std::integral_constant - cppreference.com (original) (raw)
| | | | | ---------------------------------------------------- | | ------------- | | 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.
Contents
- 1 Helper alias templates
- 2 Specializations
- 3 Member types
- 4 Member constants
- 5 Member functions
- 6 std::integral_constant::operator value_type
- 7 std::integral_constant::operator()
[edit] 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) | | ------------------------------------------------------------------------- | | ------------- |
[edit] Specializations
Two typedefs for the common case where T is bool are provided:
| Name | Definition |
| true_type | std::integral_constant<bool, true> |
| false_type | std::integral_constant<bool, false> |
[edit] Member types
| Name | Definition |
|---|---|
| value_type | T |
| type | std::integral_constant<T, v> |
[edit] Member constants
| Name | Value |
|---|---|
| constexpr T value[static] | v (public static member constant) |
[edit] Member functions
| | returns the wrapped value (public member function) [edit] | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | returns the wrapped value (public member function) [edit] |
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.
[edit] 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 };
[edit] 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 |
[edit] Example
#include 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, "22 != 4"); static_assert(two_t() << 1 == four_t() >> 0, "22 != 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() {}