20 General utilities library [utilities]

20.10 Metaprogramming and type traits [meta]

20.10.7 Transformations between types [meta.trans] Other transformations [meta.trans.other]

Table 57 — Other transformations
template <std::size_t Len,
std::size_t Align
= default-alignment>
struct aligned_storage;
Len shall not be zero. Align shall be equal to alignof(T) for some type T or to default-alignment. The value of default-alignment shall be the most stringent alignment requirement for any C++ object type whose size is no greater than Len ([basic.types]). The member typedef type shall be a POD type suitable for use as uninitialized storage for any object whose size is at most Len and whose alignment is a divisor of Align.
template <std::size_t Len,
class... Types>
struct aligned_union;
At least one type is provided. The member typedef type shall be a POD type suitable for use as uninitialized storage for any object whose type is listed in Types; its size shall be at least Len. The static member alignment_value shall be an integral constant of type std::size_t whose value is the strictest alignment of all types listed in Types.
template <class T> struct decay; Let U be remove_reference_t<T>. If is_array<U>::value is true, the member typedef type shall equal remove_extent_t<U>*. If is_function<U>::value is true, the member typedef type shall equal add_pointer_t<U>. Otherwise the member typedef type equals remove_cv_t<U>. [ Note: This behavior is similar to the lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) conversions applied when an lvalue expression is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing.  — end note ]
template <bool B, class T = void> struct enable_if; If B is true, the member typedef type shall equal T; otherwise, there shall be no member type.
template <bool B, class T, class F> struct conditional; If B is true, the member typedef type shall equal T. If B is false, the member typedef type shall equal F.
template <class... T> struct common_type; The member typedef type shall be defined as set out below. All types in the parameter pack T shall be complete or (possibly cv) void. A program may specialize this trait if at least one template parameter in the specialization is a user-defined type. [ Note: Such specializations are needed when only explicit conversions are desired among the template arguments.  — end note ]
template <class T>
struct underlying_type;
T shall be an enumeration type ([dcl.enum]) The member typedef type shall name the underlying type of T.
template <class Fn,
class... ArgTypes> struct result_of<Fn(ArgTypes...)>;
Fn and all types in the parameter pack ArgTypes shall be complete types, (possibly cv-qualified) void, or arrays of unknown bound. If the expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) is well formed when treated as an unevaluated operand (Clause [expr]), the member typedef type shall name the type decltype(INVOKE(declval<Fn>(), declval<ArgTypes>()...)); otherwise, there shall be no member type. Access checking is performed as if in a context unrelated to Fn and ArgTypes. Only the validity of the immediate context of the expression is considered. [ Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the “immediate context” and can result in the program being ill-formed.  — end note ]

Note: A typical implementation would define aligned_storage as:

template <std::size_t Len, std::size_t Alignment>
struct aligned_storage {
  typedef struct {
    alignas(Alignment) unsigned char __data[Len];
  } type;

 — end note ]

It is implementation-defined whether any extended alignment is supported ([basic.align]).

The nested typedef common_type::type shall be defined as follows:

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;

Example: Given these definitions:

typedef bool (&PF1)();
typedef short (*PF2)(long);

struct S {
  operator PF2() const;
  double operator()(char, int&);
  void fn(long) const;
  char data;

typedef void (S::*PMF)(long) const;
typedef char S::*PMD;

the following assertions will hold:

static_assert(is_same<result_of_t<S(int)>, short>::value, "Error!");
static_assert(is_same<result_of_t<S&(unsigned char, int&)>, double>::value, "Error!");
static_assert(is_same<result_of_t<PF1()>, bool>::value, "Error!");
static_assert(is_same<result_of_t<PMF(unique_ptr<S>, int)>, void>::value, "Error!");
static_assert(is_same<result_of_t<PMD(S)>, char&&>::value, "Error!");
static_assert(is_same<result_of_t<PMD(const S*)>, const char&>::value, "Error!");

 — end example ]