std::conditional

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conditional
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Compile-time rational arithmetic
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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

If the program adds specializations for std::conditional, the behavior is undefined.

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 { using type = T; };
 
template<class T, class F>
struct conditional<false, T, F> { using type = F; };

Example

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

Possible output: 

int
double
double

See also

(C++11)
 conditionally removes a function overload or template specialization from overload resolution
(class template)
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