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
.
sizeof...(T)
is zero, there is no member type
. 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
. sizeof...(T)
is two (i.e., T...
contains exactly two types T1
and T2
), 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
. std::common_type<T1, T2>
, the member type
denotes the type std::decay<decltype(false ? std::declval<T1>() : std::declval<T2>())>::type
, if that conditional expression is wellformed; if not, there is no member type
. 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<std::common_type<T1, T2>::type, R...>::type
if such a type exists. In all other cases, there is no member type
. The types in the parameter pack T
shall each be a complete type, (possibly cvqualified) void
, or an array of unknown bound. Otherwise, the behavior is undefined.
Name  Definition 

type  the common type for all T... 
template< class... T > using common_type_t = typename common_type<T...>::type;  (since C++14) 
Users may specialize common_type
for types T1
and T2
if.
T1
and T2
depends on a userdefined 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 type that names a cvunqualified nonreference 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 from <type_traits>
is undefined.
The following specializations are already provided by the standard library:
specializes the std::common_type trait (class template specialization) 

specializes the std::common_type trait (class template specialization) 
// primary template (used for zero types) template <class ...T> struct common_type { }; //////// one type template <class T> struct common_type<T> { using type = std::decay_t<T>; }; //////// two types // default implementation for two types template<class T1, class T2> using cond_t = decltype(false ? std::declval<T1>() : std::declval<T2>()); template<class T1, class T2, class=void> struct common_type_2_default { }; template<class T1, class T2> struct common_type_2_default<T1, T2, std::void_t<cond_t<T1, T2>>> { using type = std::decay_t<cond_t<T1, T2>>; }; // dispatcher to decay the type before applying specializations template<class T1, class T2, class D1 = std::decay_t<T1>, class D2=std::decay_t<T2>> struct common_type_2_impl : common_type<D1, D2> {}; template<class D1, class D2> struct common_type_2_impl<D1, D2, D1, D2> : common_type_2_default<D1, D2> {}; template <class T1, class T2> struct common_type<T1, T2> : common_type_2_impl<T1, T2> { }; //////// 3+ types 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<std::void_t<common_type_t<T1, T2>>, T1, T2, R...> : common_type<common_type_t<T1, T2>, R...> { }; template <class T1, class T2, class... R> struct common_type<T1, T2, R...> : common_type_multi_impl<void, T1, T2, R...> { }; 
For arithmetic types not subject to promotion, the common type may be viewed as the type of the (possibly mixedmode) arithmetic expression such as T0() + T1() + ... + Tn()
.
The following behaviorchanging defect reports were applied retroactively to previously published C++ standards.
DR  Applied to  Behavior as published  Correct behavior 

LWG 2141  C++11  common_type<int, int>::type is int&&  decayed result type 
LWG 2408  C++11  common_type is not SFINAEfriendly  made SFINAEfriendly 
LWG 2460  C++11  common_type specializations are nearly impossible to write  reduced number of specializations needed 
Demonstrates mixedmode arithmetic on a userdefined 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
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