Namespaces
Variants
Views
Actions

std::common_type

From cppreference.com
< cpp‎ | types
 
 
 
Type support
Basic types
Fundamental types
Fixed width integer types (C++11)
Numeric limits
C numeric limits interface
Runtime type information
Type traits
Primary type categories
(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++14)
Supported operations
Relationships and property queries
(C++11)
(C++11)
(C++11)
(C++11)
Type modifications
(C++11)(C++11)(C++11)
Type transformations
(C++11)
(C++11)
common_type
(C++11)
(C++11)
Type trait constants
 
Defined in header <type_traits>
template< class... T >
struct common_type;
(since C++11)

Determines the common type among all types T..., that is the type all T... can be implicitly converted to.

  • std::common_type may be specialized for custom types, e.g. std::chrono::time_point.
  • For non-specialized std::common_type, the rules for determining the common type between every pair T1, T2 are exactly the rules for determining the return type of the ternary conditional operator where T1 and T2 are the types of its second and the third operands.
  • For arithmetic types, the common type may also be viewed as the type of the (possibly mixed-mode) arithmetic expression such as T0() + T1() + ... + Tn().

Contents

[edit] Member types

Name Definition
type the common type for all T...

[edit] Helper types

template< class... T >
using common_type_t = typename common_type<T...>::type;
(since C++14)

[edit] Specializations

Custom specializations of the type trait std::common_type are allowed. The following specializations are already provided by the standard library:

specializes the std::common_type trait
(class template specialization) [edit]
specializes the std::common_type trait
(class template specialization) [edit]

[edit] Possible implementation

template <class ...T> struct common_type;
 
template <class T>
struct common_type<T> {
    typedef decay_t<T> type;
};
 
template <class T, class U>
struct common_type<T, U> {
    typedef decay_t<decltype(true ? declval<T>() : declval<U>())> type;
};
 
template <class T, class U, class... V>
struct common_type<T, U, V...> {
    typedef common_type_t<common_type_t<T, U>, V...> type;
};

[edit] Examples

Demonstrates mixed-mode arithmetic on a user-defined class

#include <iostream>
#include <type_traits>
 
template <class T>
struct Number { T n; };
 
template <class T, class U>
Number<typename std::common_type<T, U>::type> operator+(const Number<T>& lhs,
                                                        const Number<U>& rhs) 
{
    return {lhs.n + rhs.n};
}
 
int main()
{
    Number<int> i1 = {1}, i2 = {2};
    Number<double> d1 = {2.3}, d2 = {3.5};
    std::cout << "i1i2: " << (i1 + i2).n << "\ni1d2: " << (i1 + d2).n << '\n'
              << "d1i2: " << (d1 + i2).n << "\nd1d2: " << (d1 + d2).n << '\n';
}

Output:

i1i2: 3
i1d2: 4.5
d1i2: 4.3
d1d2: 5.8

Demonstrates the use as the identity template

#include <type_traits>
#include <vector>
#include <iostream>
 
template <size_t N>
void fun(const int (&a)[N])
{
    std::cout << "Array overload: ";
    for (int n: a)
        std::cout << n << ' ';
    std::cout << '\n';
}
 
void fun(const std::vector<int>& v)
{
    std::cout << "Vector overload: ";
    for (int n: v)
        std::cout << n << ' ';
    std::cout << '\n';
}
 
int main()
{
    fun( {1, 2, 3} ); // calls the vector overload
    fun( std::common_type<int[]>::type {1, 2, 3} ); // calls the array overload
}

Output:

Vector overload: 1 2 3 
Array overload: 1 2 3