type traits

Overview

trait template classes were first introduced in basic_string class so that the common operations such as finding the length of the string can be customized based on the character type and is injected separately during compile time. 

For example, the implementation of length() uses strlen() for char and wcslen() for wchar.

typedef basic_string<char, char_traits<char>, allocator<char> >
	string;
typedef basic_string<wchar_t, char_traits<wchar_t>, allocator<wchar_t> >
	wstring;

Details

type_traits were introduced in C++11, basically defines templates for compile-time constants that give information about the properties of their type arguments, or produce transformed types. The classes and templates in type_traits are used to support type inference, classification, and transformation at compile time. 

type_traits  can be divided into four categories. 

Name	Description
Unary type traits	Describe a property of a type
Binary type traits	Describe a relationship between types.
Transformation traits	Modify a property of a type.
Memory Allocation	These traits  allocate uninitialized memory of POD type.

Utility classes

type traits classes internally uses a number of utility classes  such as integral_constant in its implementation. 

The integral_constant is a template based class wraps a static constant of specified type in a member called value. It is the base class for the C++ type traits. 

For example, helper class true_type and false_type are basically typedefs of the classes based on  integral_constant of type bool, whose value member return true or false respectively.

Name	Description
integral_constant	Makes an integral constant from a type and a value.
true_type	Holds integral constant with true value.
false_type	Holds integral constant with false value.

The following code depctis implementation of integral_constant class and typedefs true_type or false_type. It also defines a trait class is_void that checks if a type is void and returns true or false.

template <typename T, T val>
struct integral_constant
{
	static constexpr T value = val;
	typedef T value_type;
	typedef integral_constant<T, val> type;
	constexpr operator value_type() const
	{
		return value;
	}
};
typedef integral_constant<bool, true> true_type;
typedef integral_constant<bool, false> false_type;

//Example
//The following defines a trait class is_void 
//it checks if a type is void and returns true or false.

//default
template<class T>
struct is_void	: std::false_type {};

//specialization
template<>
struct is_void<void> : std::true_type {};

//Usage
is_void<int*>::value; //returns false
is_void<void>::value;  //returns true

It's depicted in this example.

Unary type traits

Unary Type Traits describe a property of a single type. Most Unary Type Traits return true or false and are derived from true_type or false_type.  For example, is_void discussed above.

A few of the Unary Type Traits are not based on bool. An example is the type trait extent, which gives the number of elements in an array type is based on size_t.

Unary type trats can be further classified as below. 

1. Primary Type Categories
2. Composite Type Traits
3. Type Properties
4. Type property queries

Primary type categories

These traits basically return true_type  or false_type based on evaluation.

Name	Description
is_void	Tests whether the type is void.
is_null_pointer	Tests whether the type is nullptr_t.
is_integral	Tests whether the type is integral.
is_floating_point	Tests whether the type is floating-point.
is_array	Tests whether the type is an array.
is_pointer	Tests whether the type is a pointer.
is_lvalue_reference	Tests if type is an lvalue reference.
is_rvalue_reference	Tests if type is an rvalue reference.
is_member_object_pointer	Tests whether the type is a pointer to a member object.
is_member_function_pointer	Tests whether the type is a pointer to a member function.
is_enum	Tests whether the type is an enumeration.
is_union	Tests whether the type is a union.
is_class	Tests whether the type is a class.
is_function	Tests whether the type is a function type.

The example 2 depicts the usage.

Composite type categories

These traits basically return Traits return  true_type  or false_type based on evaluation.

Name	Description
is_reference	Tests whether the type is a reference.
is_arithmetic	Tests whether the type is arithmetic.
is_fundamental	Tests whether the type is void or arithmetic.
is_object	Tests whether the type is an object type.
is_scalar	Tests whether the type is scalar.
is_compound	Tests whether the type is not scalar.
is_member_pointer	Tests whether the type is a pointer to a member.

Type properties

These traits basically return Traits return  true_type  or false_type based on evaluation.

Name	Description
is_const	Tests whether the type is const.
is_volatile	Tests whether the type is volatile.
is_trivial	Tests whether the type is trivial.
is_trivially_copyable	Tests whether the type is trivially copyable.
is_standard_layout	Tests if type is a standard layout type.
is_pod	Tests whether the type is a POD.
is_literal_type	Tests whether the type can be a constexpr variable or used in a constexpr function.
is_empty	Tests whether the type is an empty class.
is_polymorphic	Tests whether the type is a polymorphic class.
is_abstract	Tests whether the type is an abstract class.
is_final	Tests whether the type is a class type marked final.
is_aggregate	Tests whether the type is a class type marked aggregate.
is_signed	Tests whether the type is a signed integer.
is_unsigned	Tests whether the type is an unsigned integer.
is_constructible	Tests whether the type is constructible using the specified argument types.
is_default_constructible	Tests whether the type has a default constructor.
is_copy_constructible	Tests whether the type has a copy constructor.
is_move_constructible	Tests whether the type has a move constructor.
is_assignable	Tests whether the first type can be assigned a value of the second type.
is_copy_assignable	Tests whether a type can be assigned a const reference value of the type.
is_move_assignable	Tests whether a type can be assigned an rvalue reference of the type.
is_destructible	Tests whether the type is destructible.
is_trivially_constructible	Tests whether the type uses no non-trivial operations when constructed using the specified types.
is_trivially_default_constructible	Tests whether the type uses no non-trivial operations when default constructed.
is_trivially_copy_constructible	Tests whether the type uses no non-trivial operations when copy constructed.
is_trivially_move_constructible	Tests whether the type uses no non-trivial operations when move constructed.
is_trivially_assignable	Tests whether the types are assignable and the assignment uses no non-trivial operations.
is_trivially_copy_assignable	Tests whether the type is copy assignable and the assignment uses no non-trivial operations.
is_trivially_move_assignable	Tests whether the type is move assignable and the assignment uses no non-trivial operations.
is_trivially_destructible	Tests whether the type is destructible and the destructor uses no non-trivial operations.
is_nothrow_constructible	Tests whether the type is constructible and is known not to throw when constructed using the specified types.
is_nothrow_default_constructible	Tests whether the type is default constructible and is known not to throw when default constructed.
is_nothrow_copy_constructible	Tests whether the type is copy constructible and the copy constructor is known not to throw.
is_nothrow_move_constructible	Tests whether the type is move constructible and the move constructor is known not to throw.
is_nothrow_assignable	Tests whether the type is assignable using the specified type and the assignment is known not to throw.
is_nothrow_copy_assignable	Tests whether the type is copy assignable and the assignment is known not to throw.
is_nothrow_move_assignable	Tests whether the type is move assignable and the assignment is known not to throw.
is_nothrow_destructible	Tests whether the type is destructible and the destructor is known not to throw.
has_virtual_destructor	Tests whether the type has a virtual destructor.

Type property queries

These traits basically return a value based on evaluation.

Name	Description
alignment_of	Gets the alignment of a type.
rank	Gets the number of array dimensions.
extent	Gets the number of elements in the specified array dimension.

The example 3 depicts the usage.

Binary Type Traits 

Binary Type Traits provide information about a relationship between two types. Every Binary Type Trait return  true_type  or false_type. based on the presence or absence of a specific relationship between the two argument types.

Type relations

These traits basically return Traits return  true_type  or false_type based on evaluation.

Name	Description
is_same	Tests whether two types are the same.
is_base_of	Tests whether one type is a base of another.
is_convertible	Tests whether one type is convertible to another.

Transformation Type Traits

Transformation Type Traits modify a type. Every Transformation Type Trait possesses a nested typedef named type that represents the result of the modification. 

Transformation type trats can be further classified as below. 

1. Const-volatile modifications
2. Reference modifications
3. Sign modifications
4. Array modifications
5. Pointer  modifications
6. Other modifications

Const-volatile modifications

These traits basically return a type based on modification.

Name	Description
add_const	Produces a const type from type.
add_volatile	Produces a volatile type from type.
add_cv	Produces a const volatile type from type.
remove_const	Produces a non-const type from type.
remove_volatile	Produces a non-volatile type from type.
remove_cv	Produces a non-const non-volatile type from type.

Reference modifications

These traits basically return a type based on modification.

Name	Description
add_lvalue_reference	Produces a reference to type from type.
add_rvalue_reference	Produces an rvalue reference to type from type
remove_reference	Produces a non-reference type from type.

Sign modifications

These traits basically return a type based on modification.

Name	Description
make_signed	Produces the type if signed or the smallest signed type greater than or equal in size to type.
make_unsigned	Produces the type if unsigned or the smallest unsigned type greater than or equal in size to type.

Array modifications

These traits basically return a type based on modification.

Name	Description
remove_all_extents	Produces a non-array type from an array type.
remove_extent	Produces the element type from an array type.

Pointer  modifications

These traits basically return a type based on modification.

Name	Description
add_pointer	Produces a pointer to type from type.
remove_pointer	Produces a type from a pointer to type.

The example 4  depicts the usage.

Other modifications

These traits basically return a type based on modification.

Name	Description
conditional	If the condition is true, produces the first specified type, otherwise the second specified type.
decay	Produces the type as passed by value. Makes non-reference, non-const, or non-volatile type, or makes a pointer to type.
enable_if	If the condition is true, produces the specified type, otherwise no type.
underlying_type	Produces the underlying integral type for an enumeration type.

The example 5  depicts the usage.

Memory Allocation

These traits  allocate uninitialized memory of POD type.

Name	Description
aligned_storage	Allocates uninitialized memory for an aligned type.
aligned_union	Allocates uninitialized memory for an aligned union with a non-trivial constructor or destructor.

The example 6  depicts the usage.

Summary of Examples

Name
Description
github
wandbox
  Example       
Example Implementation
   source    output
   source + output    
  Example 2
Primary Types
   source    output 
   source + output
  Example 3
Type Property Queries
   source    output  
   source + output
  Example 4
Pointer Modifications
   source    output  
   source + output
  Example 5
Other Modifications
   source    output  
   source + output
  Example 6
Memory Allocation
   source    output  
   source + output