21 Metaprogramming library [meta]

21.3 Metaprogramming and type traits [type.traits]

21.3.5 Unary type traits [meta.unary]

21.3.5.4 Type properties [meta.unary.prop]

Table 47: Type property predicates [tab:meta.unary.prop]
Template
Condition
Preconditions
template<class T>
struct is_const;
T is const-qualified ([basic.type.qualifier])
template<class T>
struct is_volatile;
T is volatile-qualified ([basic.type.qualifier])
template<class T>
struct is_trivial;
T is a trivial type ([basic.types.general])
remove_all_extents_t<T> shall be a complete type or cv void.
template<class T>
struct is_trivially_copyable;
T is a trivially copyable type ([basic.types.general])
remove_all_extents_t<T> shall be a complete type or cv void.
template<class T>
struct is_standard_layout;
T is a standard-layout type ([basic.types.general])
remove_all_extents_t<T> shall be a complete type or cv void.
template<class T>
struct is_empty;
T is a class type, but not a union type, with no non-static data members other than subobjects of zero size, no virtual member functions, no virtual base classes, and no base class B for which is_empty_v<B> is false.
If T is a non-union class type, T shall be a complete type.
template<class T>
struct is_polymorphic;
T is a polymorphic class ([class.virtual])
If T is a non-union class type, T shall be a complete type.
template<class T>
struct is_abstract;
T is an abstract class ([class.abstract])
If T is a non-union class type, T shall be a complete type.
template<class T>
struct is_final;
T is a class type marked with the class-virt-specifier final ([class.pre]).
[Note 2: 
A union is a class type that can be marked with final.
β€” end note]
If T is a class type, T shall be a complete type.
template<class T>
struct is_aggregate;
T is an aggregate type ([dcl.init.aggr])
T shall be an array type, a complete type, or cv void.
template<class T>
struct is_signed;
If is_arithmetic_v<T> is true, the same result as T(-1) < T(0); otherwise, false
template<class T>
struct is_unsigned;
If is_arithmetic_v<T> is true, the same result as T(0) < T(-1); otherwise, false
template<class T>
struct is_bounded_array;
T is an array type of known bound ([dcl.array])
template<class T>
struct is_unbounded_array;
T is an array type of unknown bound ([dcl.array])
template<class T>
struct is_scoped_enum;
T is a scoped enumeration ([dcl.enum])
template<class T, class... Args>
struct is_constructible;
For a function type T or for a cv void type T, is_constructible_v<T, Args...> is false, otherwise see below
T and all types in the template parameter pack Args shall be complete types, cv void, or arrays of unknown bound.
template<class T>
struct is_default_constructible;
is_constructible_v<T> is true.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_copy_constructible;
For a referenceable type T ([defns.referenceable]), the same result as is_constructible_v<T, const T&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_move_constructible;
For a referenceable type T, the same result as is_constructible_v<T, T&&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U>
struct is_assignable;
The expression declval<T>() = declval<U>() is well-formed when treated as an unevaluated operand ([expr.context]).
Access checking is performed as if in a context unrelated to T and U.
Only the validity of the immediate context of the assignment expression is considered.
[Note 3: 
The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.
Such side effects are not in the β€œimmediate context” and can result in the program being ill-formed.
β€” end note]
T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T>
struct is_copy_assignable;
For a referenceable type T, the same result as is_assignable_v<T&, const T&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_move_assignable;
For a referenceable type T, the same result as is_assignable_v<T&, T&&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U>
struct is_swappable_with;
The expressions swap(declval<T>(), declval<U>()) and swap(declval<U>(), declval<T>()) are each well-formed when treated as an unevaluated operand ([expr.context]) in an overload-resolution context for swappable values ([swappable.requirements]).
Access checking is performed as if in a context unrelated to T and U.
Only the validity of the immediate context of the swap expressions is considered.
[Note 4: 
The compilation of the expressions can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.
Such side effects are not in the β€œimmediate context” and can result in the program being ill-formed.
β€” end note]
T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T>
struct is_swappable;
For a referenceable type T, the same result as is_swappable_with_v<T&, T&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_destructible;
Either T is a reference type, or T is a complete object type for which the expression declval<U&>().~U() is well-formed when treated as an unevaluated operand ([expr.context]), where U is remove_all_extents_t<T>.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class... Args>
struct
is_trivially_constructible;
is_constructible_v<T,
Args...> is true and the variable definition for is_constructible, as defined below, is known to call no operation that is not trivial ([basic.types.general], [special]).
T and all types in the template parameter pack Args shall be complete types, cv void, or arrays of unknown bound.
template<class T>
struct is_trivially_default_constructible;
is_trivially_constructible_v<T> is true.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_trivially_copy_constructible;
For a referenceable type T, the same result as is_trivially_constructible_v<T, const T&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_trivially_move_constructible;
For a referenceable type T, the same result as is_trivially_constructible_v<T, T&&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U>
struct is_trivially_assignable;
is_assignable_v<T, U> is true and the assignment, as defined by is_assignable, is known to call no operation that is not trivial ([basic.types.general], [special]).
T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T>
struct is_trivially_copy_assignable;
For a referenceable type T, the same result as is_trivially_assignable_v<T&, const T&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_trivially_move_assignable;
For a referenceable type T, the same result as is_trivially_assignable_v<T&, T&&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_trivially_destructible;
is_destructible_v<T> is true and remove_all_extents_t<T> is either a non-class type or a class type with a trivial destructor.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class... Args>
struct is_nothrow_constructible;
is_constructible_v<T, Args...> is true and the variable definition for is_constructible, as defined below, is known not to throw any exceptions ([expr.unary.noexcept]).
T and all types in the template parameter pack Args shall be complete types, cv void, or arrays of unknown bound.
template<class T>
struct is_nothrow_default_constructible;
is_nothrow_constructible_v<T> is true.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_nothrow_copy_constructible;
For a referenceable type T, the same result as is_nothrow_constructible_v<T, const T&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_nothrow_move_constructible;
For a referenceable type T, the same result as is_nothrow_constructible_v<T, T&&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U>
struct is_nothrow_assignable;
is_assignable_v<T, U> is true and the assignment is known not to throw any exceptions ([expr.unary.noexcept]).
T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T>
struct is_nothrow_copy_assignable;
For a referenceable type T, the same result as is_nothrow_assignable_v<T&, const T&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_nothrow_move_assignable;
For a referenceable type T, the same result as is_nothrow_assignable_v<T&, T&&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U>
struct is_nothrow_swappable_with;
is_swappable_with_v<T, U> is true and each swap expression of the definition of is_swappable_with<T, U> is known not to throw any exceptions ([expr.unary.noexcept]).
T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T>
struct is_nothrow_swappable;
For a referenceable type T, the same result as is_nothrow_swappable_with_v<T&, T&>, otherwise false.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_nothrow_destructible;
is_destructible_v<T> is true and the indicated destructor is known not to throw any exceptions ([expr.unary.noexcept]).
T shall be a complete type, cv void, or an array of unknown bound.
template<class T>
struct is_implicit_lifetime;
T is an implicit-lifetime type ([basic.types.general]).
T shall be an array type, a complete type, or cv void.
template<class T>
struct has_virtual_destructor;
T has a virtual destructor ([class.dtor])
If T is a non-union class type, T shall be a complete type.
template<class T>
struct has_unique_object_representations;
For an array type T, the same result as has_unique_object_representations_v<remove_all_extents_t<T>>, otherwise see below.
T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U>
struct reference_constructs_from_temporary;
T is a reference type, and the initialization T t(VAL<U>); is well-formed and binds t to a temporary object whose lifetime is extended ([class.temporary]).
Access checking is performed as if in a context unrelated to T and U.
Only the validity of the immediate context of the variable initialization is considered.
[Note 5: 
The initialization can result in effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.
Such effects are not in the β€œimmediate context” and can result in the program being ill-formed.
β€” end note]
T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T, class U>
struct reference_converts_from_temporary;
T is a reference type, and the initialization T t = VAL<U>; is well-formed and binds t to a temporary object whose lifetime is extended ([class.temporary]).
Access checking is performed as if in a context unrelated to T and U.
Only the validity of the immediate context of the variable initialization is considered.
[Note 6: 
The initialization can result in effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.
Such effects are not in the β€œimmediate context” and can result in the program being ill-formed.
β€” end note]
T and U shall be complete types, cv void, or arrays of unknown bound.