20 General utilities library [utilities]

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 Clause, 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.

[ Example:

is_const<const volatile int>::value     // true
is_const<const int*>::value             // false
is_const<const int&>::value             // false
is_const<int[3]>::value                 // false
is_const<const int[3]>::value           // true

 — end example ]

[ Example:

remove_const_t<const volatile int>  // volatile int
remove_const_t<const int* const>    // const int*
remove_const_t<const int&>          // const int&
remove_const_t<const int[3]>        // int[3]

 — end example ]

[ Example:

// Given:
struct P final { };
union U1 { };
union U2 final { };

// the following assertions hold:
static_assert(!is_final<int>::value, "Error!");
static_assert( is_final<P>::value, "Error!");
static_assert(!is_final<U1>::value, "Error!");
static_assert( is_final<U2>::value, "Error!");

 — end example ]

Given the following function prototype:

template <class T>
  add_rvalue_reference_t<T> create() noexcept;

the predicate condition for a template specialization is_constructible<T, Args...> shall be satisfied if and only if the following variable definition would be well-formed for some invented variable t:

T t(create<Args>()...);

[ Note: These tokens are never interpreted as a function declaration.  — end note ] Access checking is performed as if in a context unrelated to T and any of the Args. Only the validity of the immediate context of the variable initialization is considered. [ Note: The evaluation of the initialization 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 ]