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std::common_type

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common_type
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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;
(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.

The types in the parameter pack T shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, 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.

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

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 type 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]

[edit] Possible implementation

// primary template (used for zero types)
template<class...>
struct common_type {};
 
//////// one type
template <class T>
struct common_type<T> : common_type<T, T> {};
 
namespace detail {
template<class...>
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<T2>::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 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

[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 common_type<int, int>::type is int&& decayed result type
LWG 2408 C++11 common_type is not SFINAE-friendly made SFINAE-friendly
LWG 2460 C++11 common_type specializations are nearly impossible to write reduced number of specializations needed