< cpp‎ | types
Revision as of 17:54, 2 November 2012 by P12bot (Talk | contribs)

Type support
Basic types
Fundamental types
Fixed width integer types (C++11)
Numeric limits
C numeric limits interface
Runtime type information
Type traits
Primary type categories
Type properties
Supported operations
Relationships and property queries
Type modifications
Type transformations
Type trait constants

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Defined in header <type_traits>

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<td >
template< class T, T v >
struct integral_constant;

<td class="t-dcl-nopad"> </td> <td > (since C++11) </td> </tr> Template:ddcl list end

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>


Member types

Type Definition
value_type T
type std::integral_constant<T,v>

Member constants

Name Value
value static constant expression of type T with value v

Member functions

operator value_type
converts the object to value_type, returns value
(public member function)

Possible implementation

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; }


#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 {
    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");