std::conditional

 

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Template:ddcl list begin <tr class="t-dsc-header">

<td>

Defined in header <type_traits>

</td>

<td></td> <td></td> </tr> <tr class="t-dcl ">

<td >

template< bool B, class T, class F >
struct conditional;

</td>

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

Provides member typedef type, which is defined as T if B is true at compile time, or as F if B is false.

Member types

Member type	Definition
type	T if B == true, F if B == false

Helper types

Template:ddcl list begin <tr class="t-dcl ">

<td >

template< bool B, class T, class F >
using conditional_t = typename conditional<B,T,F>::type;

</td>

<td class="t-dcl-nopad"> </td> <td > (since C++14) </td> </tr> Template:ddcl list end

Possible implementation

template<bool B, class T, class F> struct conditional { typedef T type; };   template<class T, class F> struct conditional<false, T, F> { typedef F type; };

Example

Run this code

#include <iostream>
#include <type_traits>
#include <typeinfo>
 
int main() 
{
    typedef std::conditional<true, int, double>::type Type1;
    typedef std::conditional<false, int, double>::type Type2;
    typedef std::conditional<sizeof(int) >= sizeof(double), int, double>::type Type3;
 
    std::cout << typeid(Type1).name() << '\n';
    std::cout << typeid(Type2).name() << '\n';
    std::cout << typeid(Type3).name() << '\n';
}

Output:

i
d
d

See also

enable_if (C++11)

conditionally removes a function overload or template specialization from overload resolution (class template)