Type Traits - Using the GNU Compiler Collection (GCC)
Description: Using the GNU Compiler Collection (GCC)

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### 7.9 Type Traits

The C++ front-end implements syntactic extensions that allow to determine at compile time 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.