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Difference between revisions of "cpp/types/conditional"

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< cpp‎ | types
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{{cpp/title|conditional}}
 
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Provides member typedef {{tt|type}}, which is defined as {{tt|T}} if {{tt|B}} is {{c|true}} at compile time, or as {{tt|F}} if {{tt|B}} is {{c|false}}.
 
Provides member typedef {{tt|type}}, which is defined as {{tt|T}} if {{tt|B}} is {{c|true}} at compile time, or as {{tt|F}} if {{tt|B}} is {{c|false}}.
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===Member types===
 
===Member types===
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  | 1=
 
  | 1=
 
template<bool B, class T, class F>
 
template<bool B, class T, class F>
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struct conditional { typedef T type; };
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struct conditional { using type = T; };
  
 
template<class T, class F>
 
template<class T, class F>
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struct conditional<false, T, F> { typedef F type; };
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struct conditional<false, T, F> { using type = F; };
 
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int main()  
 
int main()  
 
{
 
{
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     typedef std::conditional<true, int, double>::type Type1;
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     using Type1 = std::conditional<true, int, double>::type;
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     typedef std::conditional<false, int, double>::type Type2;
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     using Type2 = std::conditional<false, int, double>::type;
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     typedef std::conditional<sizeof(int) >= sizeof(double), int, double>::type Type3;
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     using Type3 = std::conditional<sizeof(int) >= sizeof(double), int, double>::type;
 
    
 
    
 
     std::cout << typeid(Type1).name() << '\n';
 
     std::cout << typeid(Type1).name() << '\n';
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Latest revision as of 01:44, 5 June 2023

 
C++
 
Metaprogramming library
Type traits
Type categories
(C++11)
(C++14)  
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)  
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++11)
(C++11)
(C++14)
(C++11)
(C++17)
(C++23)
(C++11)
(C++11)
(C++11)
(C++11)(until C++20*)
(C++11)(deprecated in C++20)
(C++11)
(C++11)
(C++20)
(C++20)
(C++23)
(C++17)
Type trait constants
(C++11)(C++17)(C++11)(C++11)
Metafunctions
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(C++17)
(C++17)
Supported operations
Relationships and property queries
Type modifications
(C++11)(C++11)(C++11)
(C++11)(C++11)(C++11)
(C++11)
(C++11)
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)
(C++11)
(C++20)
(C++11)
(C++17)

conditional
(C++11)
(C++11)
(C++20)
(C++11)
(C++11)(until C++20*)(C++17)
(C++20)
Compile-time rational arithmetic
Compile-time integer sequences
(C++14)
 
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.

Contents

[edit] Member types

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

[edit] Helper types

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

[edit] 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; };

[edit] 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

[edit] See also

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