common-type-class.md

description	title	ms.date	f1_keywords	helpviewer_keywords	ms.assetid
Learn more about: common_type Class	common_type Class	11/04/2016	type_traits/std::common_type	common_type class common_type	02bc4e7b-c63d-49de-9f8a-511d3a5c1e7f

common_type Class

Determines the common type of one or more types.

Syntax

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;
};

Parameters

List of types that are either complete types or void.

Remarks

The type member is the common type to which all types in the parameter list can be converted.

Example

The following program demonstrates some correct usage scenarios and tests for results.

// 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

Requirements

Header: <type_traits>

Namespace: std

See also

<type_traits>