Using the GNU Compiler Collection (GCC): Type Traits

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### 7.9 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-qualified 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-qualified 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_trivial (type)` is `true` then the trait is `true`, else if `type` is a cv-qualified 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_trivial (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_trivial (type)` is `true` then the trait is `true`, else if `type` is a cv-qualified 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_trivial (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. A diagnostic is produced if this requirement is not met.

`__is_class (type)`

  If `type` is a cv-qualified 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]).

`__integer_pack (length)`

  When used as the pattern of a pack expansion within a template definition, expands to a template argument pack containing integers from `0` to `length-1`. This is provided for efficient implementation of `std::make_integer_sequence`.

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