.. _type-traits:

Type Traits
***********

The C++ front end implements syntactic extensions that allow
compile-time determination of 
various characteristics of a type (or of a
pair of types).

``__has_nothrow_assign (type)``
  If ``type`` is const qualified or is a reference type then the trait is
  false.  Otherwise if ``__has_trivial_assign (type)`` is true then the trait
  is true, else if ``type`` is a cv class or union type with copy assignment
  operators that are known not to throw an exception then the trait is true,
  else it is false.  Requires: ``type`` shall be a complete type,
  (possibly cv-qualified) ``void``, or an array of unknown bound.

``__has_nothrow_copy (type)``
  If ``__has_trivial_copy (type)`` is true then the trait is true, else if
  ``type`` is a cv class or union type with copy constructors that
  are known not to throw an exception then the trait is true, else it is false.
  Requires: ``type`` shall be a complete type, (possibly cv-qualified)
  ``void``, or an array of unknown bound.

``__has_nothrow_constructor (type)``
  If ``__has_trivial_constructor (type)`` is true then the trait is
  true, else if ``type`` is a cv class or union type (or array
  thereof) with a default constructor that is known not to throw an
  exception then the trait is true, else it is false.  Requires:
  ``type`` shall be a complete type, (possibly cv-qualified)
  ``void``, or an array of unknown bound.

``__has_trivial_assign (type)``
  If ``type`` is const qualified or is a reference type then the trait is
  false.  Otherwise if ``__is_pod (type)`` is true then the trait is
  true, else if ``type`` is a cv class or union type with a trivial
  copy assignment ([class.copy]) then the trait is true, else it is
  false.  Requires: ``type`` shall be a complete type, (possibly
  cv-qualified) ``void``, or an array of unknown bound.

``__has_trivial_copy (type)``
  If ``__is_pod (type)`` is true or ``type`` is a reference type
  then the trait is true, else if ``type`` is a cv class or union type
  with a trivial copy constructor ([class.copy]) then the trait
  is true, else it is false.  Requires: ``type`` shall be a complete
  type, (possibly cv-qualified) ``void``, or an array of unknown bound.

``__has_trivial_constructor (type)``
  If ``__is_pod (type)`` is true then the trait is true, else if
  ``type`` is a cv class or union type (or array thereof) with a
  trivial default constructor ([class.ctor]) then the trait is true,
  else it is false.  Requires: ``type`` shall be a complete
  type, (possibly cv-qualified) ``void``, or an array of unknown bound.

``__has_trivial_destructor (type)``
  If ``__is_pod (type)`` is true or ``type`` is a reference type then
  the trait is true, else if ``type`` is a cv class or union type (or
  array thereof) with a trivial destructor ([class.dtor]) then the trait
  is true, else it is false.  Requires: ``type`` shall be a complete
  type, (possibly cv-qualified) ``void``, or an array of unknown bound.

``__has_virtual_destructor (type)``
  If ``type`` is a class type with a virtual destructor
  ([class.dtor]) then the trait is true, else it is false.  Requires:
  ``type`` shall be a complete type, (possibly cv-qualified)
  ``void``, or an array of unknown bound.

``__is_abstract (type)``
  If ``type`` is an abstract class ([class.abstract]) then the trait
  is true, else it is false.  Requires: ``type`` shall be a complete
  type, (possibly cv-qualified) ``void``, or an array of unknown bound.

``__is_base_of (base_type, derived_type)``
  If ``base_type`` is a base class of ``derived_type``
  ([class.derived]) then the trait is true, otherwise it is false.
  Top-level cv qualifications of ``base_type`` and
  ``derived_type`` are ignored.  For the purposes of this trait, a
  class type is considered is own base.  Requires: if ``__is_class
  (base_type)`` and ``__is_class (derived_type)`` are true and
  ``base_type`` and ``derived_type`` are not the same type
  (disregarding cv-qualifiers), ``derived_type`` shall be a complete
  type.  Diagnostic is produced if this requirement is not met.

``__is_class (type)``
  If ``type`` is a cv class type, and not a union type
  ([basic.compound]) the trait is true, else it is false.

``__is_empty (type)``
  If ``__is_class (type)`` is false then the trait is false.
  Otherwise ``type`` is considered empty if and only if: ``type``
  has no non-static data members, or all non-static data members, if
  any, are bit-fields of length 0, and ``type`` has no virtual
  members, and ``type`` has no virtual base classes, and ``type``
  has no base classes ``base_type`` for which
  ``__is_empty (base_type)`` is false.  Requires: ``type`` shall
  be a complete type, (possibly cv-qualified) ``void``, or an array
  of unknown bound.

``__is_enum (type)``
  If ``type`` is a cv enumeration type ([basic.compound]) the trait is
  true, else it is false.

``__is_literal_type (type)``
  If ``type`` is a literal type ([basic.types]) the trait is
  true, else it is false.  Requires: ``type`` shall be a complete type,
  (possibly cv-qualified) ``void``, or an array of unknown bound.

``__is_pod (type)``
  If ``type`` is a cv POD type ([basic.types]) then the trait is true,
  else it is false.  Requires: ``type`` shall be a complete type,
  (possibly cv-qualified) ``void``, or an array of unknown bound.

``__is_polymorphic (type)``
  If ``type`` is a polymorphic class ([class.virtual]) then the trait
  is true, else it is false.  Requires: ``type`` shall be a complete
  type, (possibly cv-qualified) ``void``, or an array of unknown bound.

``__is_standard_layout (type)``
  If ``type`` is a standard-layout type ([basic.types]) the trait is
  true, else it is false.  Requires: ``type`` shall be a complete
  type, (possibly cv-qualified) ``void``, or an array of unknown bound.

``__is_trivial (type)``
  If ``type`` is a trivial type ([basic.types]) the trait is
  true, else it is false.  Requires: ``type`` shall be a complete
  type, (possibly cv-qualified) ``void``, or an array of unknown bound.

``__is_union (type)``
  If ``type`` is a cv union type ([basic.compound]) the trait is
  true, else it is false.

``__underlying_type (type)``
  The underlying type of ``type``.  Requires: ``type`` shall be
  an enumeration type ([dcl.enum]).