common_type, classe
Détermine le type commun d'un ou plusieurs types.
Syntaxe
template <class... T>
struct common_type;
template <class T>
struct common_type<T> {
typedef typename decay<T>::type type;
};
template <class T, class U>
struct common_type<T, U> {
typedef typename decay<decltype(true declval<T>() :
declval<U>())>::type type;
};
template <class T, class U, class... V>
struct common_type<T, U, V...> {
typedef typename common_type<typename common_type<T, U>::type, V...>::type type;
};
Paramètres
Liste des types qui sont des types complets ou void.
Notes
Le membre type est le type commun vers lequel tous les types dans la liste de paramètres peuvent être convertis.
Exemple
Le programme suivant illustre certains scénarios d'utilisation correcte et teste les résultats.
// Compile using cl.exe /EHsc
// common_type sample
#include <iostream>
#include <type_traits>
struct Base {};
struct Derived : Base {};
int main()
{
typedef std::common_type<unsigned char, short, int>::type NumericType;
typedef std::common_type<float, double>::type FloatType;
typedef std::common_type<const int, volatile int>::type ModifiedIntType;
typedef std::common_type<Base, Derived>::type ClassType;
std::cout << std::boolalpha;
std::cout << "Test for typedefs of common_type int" << std::endl;
std::cout << "NumericType: " << std::is_same<int, NumericType>::value << std::endl;
std::cout << "FloatType: " << std::is_same<int, FloatType>::value << std::endl;
std::cout << "ModifiedIntType: " << std::is_same<int, ModifiedIntType>::value << std::endl;
std::cout << "ClassType: " << std::is_same<int, ClassType>::value << std::endl;
std::cout << "---------------------------" << std::endl;
std::cout << "Test for typedefs of common_type double" << std::endl;
std::cout << "NumericType: " << std::is_same<double, NumericType>::value << std::endl;
std::cout << "FloatType: " << std::is_same<double, FloatType>::value << std::endl;
std::cout << "ModifiedIntType: " << std::is_same<double, ModifiedIntType>::value << std::endl;
std::cout << "ClassType: " << std::is_same<double, ClassType>::value << std::endl;
std::cout << "---------------------------" << std::endl;
std::cout << "Test for typedefs of common_type Base" << std::endl;
std::cout << "NumericType: " << std::is_same<Base, NumericType>::value << std::endl;
std::cout << "FloatType: " << std::is_same<Base, FloatType>::value << std::endl;
std::cout << "ModifiedIntType: " << std::is_same<Base, ModifiedIntType>::value << std::endl;
std::cout << "ClassType: " << std::is_same<Base, ClassType>::value << std::endl;
return 0;
}
Sortie
Test for typedefs of common_type int
NumericType: true
FloatType: false
ModifiedIntType: true
ClassType: false
---------------------------
Test for typedefs of common_type double
NumericType: false
FloatType: true
ModifiedIntType: false
ClassType: false
---------------------------
Test for typedefs of common_type Base
NumericType: false
FloatType: false
ModifiedIntType: false
ClassType: true
Spécifications
Header :<type_traits>
Espace de noms : std