< cpp‎ | types
Metaprogramming library
Type traits
Type categories
Type properties
(C++11)(until C++20*)
(C++11)(deprecated in C++20)
Type trait constants
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)

(C++11)(until C++20*)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
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. If such a type exists (as determined according to the rules below), the member type names that type. Otherwise, there is no member type.

  • If sizeof...(T) is zero, there is no member type.
  • If sizeof...(T) is one (i.e., T... contains only one type T0), the member type names the same type as std::common_type<T0, T0>::type if it exists; otherwise there is no member type.
  • If sizeof...(T) is two (i.e., T... contains exactly two types T1 and T2),
  • If applying std::decay to at least one of T1 and T2 produces a different type, the member type names the same type as std::common_type<std::decay<T1>::type, std::decay<T2>::type>::type, if it exists; if not, there is no member type;
  • Otherwise, if there is a user specialization for std::common_type<T1, T2>, that specialization is used;
  • Otherwise, if std::decay<decltype(false ? std::declval<T1>() : std::declval<T2>())>::type is a valid type, the member type denotes that type, see the conditional operator;
(since C++20)
  • Otherwise, there is no member type.
  • If sizeof...(T) is greater than two (i.e., T... consists of the types T1, T2, R...), then if std::common_type<T1, T2>::type exists, the member type denotes std::common_type<typename std::common_type<T1, T2>::type, R...>::type if such a type exists. In all other cases, there is no member type.

If any type in the parameter pack T is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.


[edit] Nested 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

Users may specialize common_type for types T1 and T2 if

  • At least one of T1 and T2 depends on a user-defined type, and
  • std::decay is an identity transformation for both T1 and T2.

If such a specialization has a member named type, it must be a public and unambiguous member that names a cv-unqualified non-reference type to which both T1 and T2 are explicitly convertible. Additionally, std::common_type<T1, T2>::type and std::common_type<T2, T1>::type must denote the same type.

A program that adds common_type specializations in violation of these rules has undefined behavior.

Note that the behavior of a program that adds a specialization to any other template (except for std::basic_common_reference)(since C++20) from <type_traits> is undefined.

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]
determines the common type of two pairs
(class template specialization) [edit]
determines the common type of a tuple and a tuple-like type
(class template specialization) [edit]
determines the common type of an iterator and an adapted basic_const_iterator type
(class template specialization) [edit]

[edit] Possible implementation

// primary template (used for zero types)
struct common_type {};
// one type
template<class T>
struct common_type<T> : common_type<T, T> {};
namespace detail
    using void_t = void;
    template<class T1, class T2>
    using conditional_result_t = decltype(false ? std::declval<T1>() : std::declval<T2>());
    template<class, class, class = void>
    struct decay_conditional_result {};
    template<class T1, class T2>
    struct decay_conditional_result<T1, T2, void_t<conditional_result_t<T1, T2>>>
        : std::decay<conditional_result_t<T1, T2>> {};
    template<class T1, class T2, class = void>
    struct common_type_2_impl : decay_conditional_result<const T1&, const T2&> {};
    // C++11 implementation:
    // template<class, class, class = void>
    // struct common_type_2_impl {};
    template<class T1, class T2>
    struct common_type_2_impl<T1, T2, void_t<conditional_result_t<T1, T2>>>
        : decay_conditional_result<T1, T2> {};
// two types
template<class T1, class T2>
struct common_type<T1, T2> 
    : std::conditional<std::is_same<T1, typename std::decay<T1>::type>::value &&
                       std::is_same<T2, typename std::decay<T2>::type>::value,
                       detail::common_type_2_impl<T1, T2>,
                       common_type<typename std::decay<T1>::type,
                                   typename std::decay<T2>::type>>::type {};
// 3+ types
namespace detail
    template<class AlwaysVoid, class T1, class T2, class... R>
    struct common_type_multi_impl {};
    template<class T1, class T2, class...R>
    struct common_type_multi_impl<void_t<typename common_type<T1, T2>::type>, T1, T2, R...>
        : common_type<typename common_type<T1, T2>::type, R...> {};
template<class T1, class T2, class... R>
struct common_type<T1, T2, R...>
    : detail::common_type_multi_impl<void, T1, T2, R...> {};

[edit] Notes

For arithmetic types not subject to promotion, the common type may be viewed as the type of the (possibly mixed-mode) arithmetic expression such as T0() + T1() + ... + Tn().

[edit] Examples

Demonstrates mixed-mode arithmetic on a program-defined class:

#include <iostream>
#include <type_traits>
template<class T>
struct Number { T n; };
template<class T, class U>
constexpr Number<std::common_type_t<T, U>>
    operator+(const Number<T>& lhs, const Number<U>& rhs)
    return {lhs.n + rhs.n};
void describe(const char* expr, const Number<int>& x)
    std::cout << expr << "  is  Number<int>{" << x.n << "}\n";
void describe(const char* expr, const Number<double>& x)
    std::cout << expr << "  is  Number<double>{" << x.n << "}\n";
int main()
    Number<int> i1 = {1}, i2 = {2};
    Number<double> d1 = {2.3}, d2 = {3.5};
    describe("i1 + i2", i1 + i2);
    describe("i1 + d2", i1 + d2);
    describe("d1 + i2", d1 + i2);
    describe("d1 + d2", d1 + d2);


i1 + i2  is  Number<int>{3}
i1 + d2  is  Number<double>{4.5}
d1 + i2  is  Number<double>{4.3}
d1 + d2  is  Number<double>{5.8}

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2141 C++11 the result type of the conditional operator was not decayed decayed the result type
LWG 2408 C++11 common_type was not SFINAE-friendly made SFINAE-friendly
LWG 2460 C++11 common_type specializations were nearly impossible to write reduced the number of
specializations needed

[edit] See also

specifies that two types share a common type
(concept) [edit]