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std::conditional

Defined in header <type_traits> 
template< bool B, class T, class F >
struct conditional;
 (since C++11)

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< bool B, class T, class F >
using conditional_t = typename conditional<B,T,F>::type;
 (since C++14)

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

#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

(C++11)
 hides a function overload or template specialization based on compile-time boolean
(class template)

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