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.

__underlying_type (type)

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