These templates provide access to some of the more important properties of types.
It is unspecified whether the library defines any full or partial specializations of any of these templates.
For all of the class templates X declared in this subclause, instantiating that template with a template argument that is a class template specialization may result in the implicit instantiation of the template argument if and only if the semantics of X require that the argument must be a complete type.
For the purpose of defining the templates in this subclause, a function call expression declval<T>() for any type T is considered to be a trivial ( ISO/IEC 14882:2014 §[basic.types], ISO/IEC 14882:2014 §[special]) function call that is not an odr-use ( ISO/IEC 14882:2014 §[basic.def.odr]) of declval in the context of the corresponding definition notwithstanding the restrictions of ( ISO/IEC 14882:2014 §[declval]).
| Template | Condition | Precondition |
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template <class T, class U> struct is_swappable_with; | The expressions ranges::swap(declval<T>(), declval<U>()) and ranges::swap(declval<U>(), declval<T>()) are each well-formed when treated as an unevaluated operand (Clause ISO/IEC 14882:2014 §[expr]). 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: 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 “immediate context” and can result in the program being ill-formed. — end note ] | T and U shall be complete types, (possibly cv-qualified) 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, (possibly cv-qualified) 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 ( ISO/IEC 14882:2014 §[expr.unary.noexcept]). | T and U shall be complete types, (possibly cv-qualified) 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, (possibly cv-qualified) void, or an array of unknown bound. |