etl/docs/types/type-traits.md

title
type_traits

Reverse engineered types traits classes from C++11 plus several ETL extensions.
This file is generated from type_traits_generator.h. See Generators
Not all traits have been defined as some rely on compiler intrinsics that are not available on all compiler platforms.

integral_constant
remove_reference
add_reference
remove_pointer
add_pointer
is_const
remove_const
add_const
is_volatile
remove_volatile
add_volatile
remove_cv
add_cv
add_volatile
remove_cvref 20.17.0
is_integral
is_signed
is_unsigned
is_floating_point
is_same
is_void
is_arithmetic
is_fundamental
is_compound
is_array
is_pointer
is_reference
is_base_of
make_signed
make_unsigned
enable_if
conditional
extent
remove_extent
remove_all_extents
rank
decay
alignment_of
conjunction 20.14.0
disjunction 20.14.0
negation
is_lvalue_assignable
void_t 20.28.0
declvar 20.28.0
common_type
is_enum 20.30.0
underlying_type 20.42.0
 Unless the ETL is set to use builtins, the user must specialise the template for their enumerations.

The definitions will wrap those defined in C++11's <type_traits> if available.

---

The following will be defined according to the C++ standard and user defined macros.

is_assignable
is_constructible
is_copy_constructible
is_move_constructible
is_trivially_constructible
is_trivially_copy_constructible
is_trivially_destructible
is_trivially_copy_assignable
is_trivially_copyable

Scenario 1

Using C++11 or above and the STL.

If ETL_CPP11_SUPPORTED and ETL_USING_STL == 1 and !defined(ETL_USE_TYPE_TRAITS_BUILTINS) and !defined(ETL_USER_DEFINED_TYPE_TRAITS) and ((!defined(ARDUINO) && ETL_NOT_USING_STLPORT) or defined(ETL_GCC_V5_TYPE_TRAITS_SUPPORTED))

The ETL's definitions wrap the STL definitions, except in the following case.
if ETL_CPP11_TYPE_TRAITS_IS_TRIVIAL_SUPPORTED is not defined then they will be defined as in the ETL_USER_DEFINED_TYPE_TRAITS option.

Scenario 2

Not using the STL but the compiler has type trait built-ins.

If the user has defined ETL_USE_TYPE_TRAITS_BUILTINS then the ETL will use the generally available compiler built-ins.
This option is useful for when you are not using the STL, but are using a compatible compiler.

Scenario 3

Not using the STL and have the compiler has no type trait built-ins.

If the user has defined ETL_USER_DEFINED_TYPE_TRAITS then the ETL will define these type traits for arithmetic and pointer types only. For all other types the traits will be undefined, unless the user explicitly specialises them.

  struct Copyable
  {
    Copyable() {}
    Copyable(const Copyable& other) {}
    Copyable& operator =(const Copyable& rhs) { return *this; }

    Copyable(Copyable&& other) = delete;
    Copyable& operator =(Copyable& rhs) = delete;
  };

  using etl::is_assignable;
  using etl::is_constructible;
  using etl::is_copy_constructible;
  using etl::is_move_constructible;

  template <>
  struct etl::is_assignable<Copyable, Copyable> : public etl::true_type
  {
  };

  template <>
  struct etl::is_constructible<Copyable> : public etl::true_type
  {
  };

  template <>
  struct etl::is_copy_constructible<Copyable> : public etl::true_type
  {
  };

  template <>
  struct etl::is_move_constructible<Copyable> : public etl::false_type
  {
  };

Scenario 4

Not using the STL, the compiler has no type trait built-ins, and no user define specialisations are defined.

The ETL will define these type traits as true for arithmetic and pointer types only. For all other types the traits will have a value of false.

is_one_of

ETL extension

C++03

template <typename T,
          typename T1,         typename T2  = void, typename T3  = void, typename T4  = void, 
          typename T5 = void,  typename T6  = void, typename T7  = void, typename T8  = void, 
          typename T9 = void,  typename T10 = void, typename T11 = void, typename T12 = void, 
          typename T13 = void, typename T14 = void, typename T15 = void, typename T16 = void>
struct is_one_of

By default the ETL allows up the 16 types. This may be changed by running the type traits generator.

C++11 or above

template <typename T, typename... TTypelist>
struct is_one_of

Members

value

Set to true if the first template type is one of the subsequent types, otherwise false.

If C++17 is supported then this definition available.

template <typename T>
constexpr bool is_one_of_v = etl::is_one_of<T, TRest...>::value;

Example

bool isOK;

isOK = etl::is_one_of<int, char, short, int, long>::value;    // Sets 'isOK' to true.
isOK = etl::is_one_of<double, char, short, int, long>::value; // Sets 'isOK' to false.

are_all_same

ETL extension
C++11 or above

template <typename T, typename... TTypelist>
struct are_all_same

Members

value

Set to true if all of the template types are the same, otherwise false.

If C++17 is supported then this definition available.

template <typename T>
constexpr bool are_all_same_v = etl::is_one_of<T, TRest...>::value;

Example

bool isOK;

isOK = etl::are_all_same<int, int, int, int>::value;    // Sets 'isOK' to true.
isOK = etl::is_one_of<int, int, char, int>::value; // Sets 'isOK' to false.

conditional_integral_constant

ETL extension

Members

value

Set to the constant determined by the condition.

Example
int value;

value = etl::conditional_integral_constant<true, 1, 2>::value; // Sets value to 1 value = etl::conditional_integral_constant<false, 1, 2>::value; // Sets value to 2

types

ETL extension
Extracts the basic types from a template type.

type
The underlying type.

---

reference
A reference to the underlying type.

---

const_reference
A const reference to the underlying type.

---

pointer
A pointer to the underlying type.

---

const_pointer

A const pointer to the underlying type.

---

Example

typedef const int* const MyType;

etl::types<MyType>::type                int
etl::types<MyType>::reference           int&
etl::types<MyType>::const_reference     const int&
etl::types<MyType>::pointer             int*
etl::types<MyType>::const_pointer       const int*
etl::types<MyType>::const_pointer_const const int* const

unsigned_type

ETL extension
From: 20.29.0

template <typename T>
struct unsigned_type

Defines one of five unsigned types that has the same size as T.
Defines one of unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long.

---

template <typename T>
using unsigned_type_t = typename unsigned_type<T>::type;

C++11

signed_type

ETL extension
From: 20.29.0

template <typename T>
struct signed_type

Defines one of five signed types that has the same size as T.
Defines one of char, short, int, long, long long.

---

template <typename T>
using signed_type_t = typename signed_type<T>::type;

C++11

---

has_duplicates

ETL extension

template <typename... TTypes>
struct has_duplicates

From: 20.39.1 C++11 or above

Members

value
Set to true if the typelist TTypes contains any duplicate types, otherwise false.

---

If C++17 is supported then this definition is available

template <typname... TTypes>
constexpr bool has_duplicates_v = etl::has_duplicates<TTypes...>::value;

has_duplicates_of

ETL extension

template <typename T, typename... TTypes>
struct has_duplicates_of

From: 20.39.1
C++11 or above

Members

value Set to true if the typelist TTypes contains duplicates of types T, otherwise false.

---

If C++17 is supported then this definition is available.

template <typename T, typname... TTypes>
constexpr bool has_duplicates_of_v = etl::has_duplicates_of<T, TTypes...>::value;

count_of

ETL extension

template <typename T, typename... TTypes>
struct count_of

From: 20.39.1
C++11 or above

Members

value
The count of the occurrences of type T in the typelist TTypes.

---

If C++17 is supported then this definition is available.

template <typename T, typname... TTypes>
constexpr bool count_of_v = etl::count_of<T, TTypes...>::value;

conjunction

From: 20.40.1

template <typename... Types>
struct conjunction;

Forms the logical conjunction (AND) of Types...
The result is found in the member value.

template <typename... Types>
inline constexpr bool conjunction_v;

Forms the logical conjunction (AND) of Types...
C++17

disjunction

From: 20.40.1

template <typename... Types>
struct disjunction;

Forms the logical disjunction (OR) of Types...
The result is found in the member value.

template <typename... Types>
inline constexpr bool disjunction_v;

Forms the logical disjunction (OR) of Types...
C++17

exclusive_disjunction

From: 20.40.1

template <typename... Types>
struct exclusive_disjunction;

Forms the logical exclusive disjunction (XOR) of Types...
The result is found in the member value.

template <typename... Types>
inline constexpr bool exclusive_disjunction_v;

Forms the logical exclusive disjunction (XOR) of Types...
C++17

unwrap_reference, unwrap_ref_decay

From: 20.40.1

template <class T>
struct unwrap_reference;

Unwraps any etl::reference_wrapper, changing etl::reference_wrapper<U> to U&.
If T is a specialisation of etl::reference_wrapper, unwraps it, otherwise T remains the same.

template <typename T>
using unwrap_reference_t;

C++11

---

template <typename T>
struct unwrap_ref_decay;

Unwraps any etl::reference_wrapper, changing etl::reference_wrapper<U> to U&.
If decayed T is a specialisation of etl::reference_wrapper, unwraps it, otherwise T is decayed.

template <typename T>
using unwrap_ref_decay_t;

C++11

is_specialization

From: 20.41.0

template <typename T, template <typename...> class Template>
struct is_specialization

Checks if T is a specialization of template type Template.
The result is found in the member value.
C++11

template <typename T, template <typename...> class Template>
inline constexpr bool is_specialization_v = etl::is_specialization<T, Template>::value;

C++17

is_object_pointer

From: 20.49.0

template <typename T>
struct is_object_pointer

Checks if T is a pointer to an object.
Pointers to functions and member functions return false.
The result is found in the member value.

template <typename T>
inline constexpr bool is_object_pointer_v = etl::is_object_pointer<T>::value;

C++17