This sub-clause contains templates that may be used to transform one type to another following some predefined rule.
Each of the templates in this subclause shall be a TransformationTrait ([meta.rqmts]).
| Template | Comments |
|
template <class T> struct remove_const; | The member typedef type shall name the same type as T except that any top-level const-qualifier has been removed. [ Example: remove_const_t<const volatile int> evaluates to volatile int, whereas remove_const_t<const int*> evaluates to const int*. — end example ] |
|
template <class T> struct remove_volatile; | The member typedef type shall name the same type as T except that any top-level volatile-qualifier has been removed. [ Example: remove_volatile_t<const volatile int> evaluates to const int, whereas remove_volatile_t<volatile int*> evaluates to volatile int*. — end example ] |
|
template <class T> struct remove_cv; | The member typedef type shall be the same as T except that any top-level cv-qualifier has been removed. [ Example: remove_cv_t<const volatile int> evaluates to int, whereas remove_cv_t<const volatile int*> evaluates to const volatile int*. — end example ] |
|
template <class T> struct add_const; | If T is a reference, function, or top-level const-qualified type, then type shall name the same type as T, otherwise T const. |
|
template <class T> struct add_volatile; | If T is a reference, function, or top-level volatile-qualified type, then type shall name the same type as T, otherwise T volatile. |
|
template <class T> struct add_cv; | The member typedef type shall name the same type as add_const_t<add_volatile_t<T>>. |
| Template | Comments |
|
template <class T> struct remove_reference; | If T has type “reference to T1” then the member typedef type shall name T1; otherwise, type shall name T. |
| template <class T> struct add_lvalue_reference; | If T names an object or function type then the member typedef type shall name T&; otherwise, if T names a type “rvalue reference to T1” then the member typedef type shall name T1&; otherwise, type shall name T. |
| template <class T> struct add_rvalue_reference; | If T names an object or function type then the member typedef type shall name T&&; otherwise, type shall name T. [ Note: This rule reflects the semantics of reference collapsing ([dcl.ref]). For example, when a type T names a type T1&, the type add_rvalue_reference_t<T> is not an rvalue reference. — end note ] |
| Template | Comments |
|
template <class T> struct make_signed; |
If T names a (possibly cv-qualified) signed integer
type ([basic.fundamental]) then the member typedef
type shall name the type T; otherwise,
if T names a (possibly cv-qualified) unsigned integer
type then type shall name the corresponding
signed integer type, with the same cv-qualifiers as T;
otherwise, type shall name the signed integer type with smallest
rank ([conv.rank]) for which
sizeof(T) == sizeof(type), with the same
cv-qualifiers as T. Requires: T shall be a (possibly cv-qualified) integral type or enumeration but not a bool type. |
|
template <class T> struct make_unsigned; |
If T names a (possibly cv-qualified) unsigned integer
type ([basic.fundamental]) then the member typedef
type shall name the type T; otherwise,
if T names a (possibly cv-qualified) signed integer
type then type shall name the corresponding
unsigned integer type, with the same cv-qualifiers as T;
otherwise, type shall name the unsigned integer type with smallest
rank ([conv.rank]) for which
sizeof(T) == sizeof(type), with the same
cv-qualifiers as T. Requires: T shall be a (possibly cv-qualified) integral type or enumeration but not a bool type. |
| Template | Comments |
|
template <class T> struct remove_extent; | If T names a type “array of U”, the member typedef type shall be U, otherwise T. [ Note: For multidimensional arrays, only the first array dimension is removed. For a type “array of const U”, the resulting type is const U. — end note ] |
|
template <class T> struct remove_all_extents; | If T is “multi-dimensional array of U”, the resulting member typedef type is U, otherwise T. |
[Example
// the following assertions hold:
assert((is_same<remove_extent_t<int>, int>::value));
assert((is_same<remove_extent_t<int[2]>, int>::value));
assert((is_same<remove_extent_t<int[2][3]>, int[3]>::value));
assert((is_same<remove_extent_t<int[][3]>, int[3]>::value));
— end example ]
[Example
// the following assertions hold:
assert((is_same<remove_all_extents_t<int>, int>::value));
assert((is_same<remove_all_extents_t<int[2]>, int>::value));
assert((is_same<remove_all_extents_t<int[2][3]>, int>::value));
assert((is_same<remove_all_extents_t<int[][3]>, int>::value));
— end example ]
| Template | Comments |
|
template <class T> struct remove_pointer; | If T has type “(possibly cv-qualified) pointer to T1” then the member typedef type shall name T1; otherwise, it shall name T. |
|
template <class T> struct add_pointer; | The member typedef type shall name the same type as remove_reference_t<T>*. |
| Template | Condition | Comments |
|
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 ]