common_type 类

确定一个或多个类型的常规类型。

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;
};

参数

类型列表是已完成类型或无效。

备注

type 成员是所有参数列的所有类型都可以转换的常规类型。

示例

以下程序演示一些正确使用方案并测试结果。

// 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;
}

Output

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

要求

标头： <type_traits>

命名空间: std

请参见

参考

<type_traits>

其他资源

type_traits 成员