7 Declarations [dcl.dcl]

The simple type specifiers are

simple-type-specifier:
    nested-name-specifieropt type-name
    nested-name-specifier template simple-template-id
    nested-name-specifieropt template-name
    char
    char16_t
    char32_t
    wchar_t
    bool
    short
    int
    long
    signed
    unsigned
    float
    double
    void
    auto
    decltype-specifier

type-name:
    class-name
    enum-name
    typedef-name
    simple-template-id

decltype-specifier:
  decltype ( expression )
  decltype ( auto )

The simple-type-specifier auto is a placeholder for a type to be deduced ([dcl.spec.auto]). A type-specifier of the form typename_opt nested-name-specifier_opt template-name is a placeholder for a deduced class type and shall appear only as a decl-specifier in the decl-specifier-seq of a simple-declaration ([dcl.type.class.deduct]) or as the simple-type-specifier in an explicit type conversion (functional notation) ([expr.type.conv]). The template-name shall name a class template that is not an injected-class-name. The other simple-type-specifiers specify either a previously-declared type, a type determined from an expression, or one of the fundamental types ([basic.fundamental]). Table [tab:simple.type.specifiers] summarizes the valid combinations of simple-type-specifiers and the types they specify.

When multiple simple-type-specifiers are allowed, they can be freely intermixed with other decl-specifiers in any order. [ Note: It is implementation-defined whether objects of char type are represented as signed or unsigned quantities. The signed specifier forces char objects to be signed; it is redundant in other contexts.  — end note ]

For an expression e, the type denoted by decltype(e) is defined as follows:

• (4.1)

  if e is an unparenthesized id-expression naming an lvalue or reference introduced from the identifier-list of a decomposition declaration, decltype(e) is the referenced type as given in the specification of the decomposition declaration ([dcl.decomp]);

• (4.2)

  otherwise, if e is an unparenthesized id-expression or an unparenthesized class member access ([expr.ref]), decltype(e) is the type of the entity named by e. If there is no such entity, or if e names a set of overloaded functions, the program is ill-formed;

• (4.3)

  otherwise, if e is an xvalue, decltype(e) is T&&, where T is the type of e;

• (4.4)

  otherwise, if e is an lvalue, decltype(e) is T&, where T is the type of e;

• (4.5)

  otherwise, decltype(e) is the type of e.

The operand of the decltype specifier is an unevaluated operand (Clause [expr]).

[ Example:

const int&& foo();
int i;
struct A { double x; };
const A* a = new A();
decltype(foo()) x1 = 17;        // type is const int&&
decltype(i) x2;                 // type is int
decltype(a->x) x3;              // type is double
decltype((a->x)) x4 = x3;       // type is const double&

 — end example ] [ Note: The rules for determining types involving decltype(auto) are specified in [dcl.spec.auto].  — end note ]

If the operand of a decltype-specifier is a prvalue, the temporary materialization conversion is not applied ([conv.rval]) and no result object is provided for the prvalue. The type of the prvalue may be incomplete. [ Note: As a result, storage is not allocated for the prvalue and it is not destroyed. Thus, a class type is not instantiated as a result of being the type of a function call in this context. In this context, the common purpose of writing the expression is merely to refer to its type. In that sense, a decltype-specifier is analogous to a use of a typedef-name, so the usual reasons for requiring a complete type do not apply. In particular, it is not necessary to allocate storage for a temporary object or to enforce the semantic constraints associated with invoking the type's destructor.  — end note ] [ Note: Unlike the preceding rule, parentheses have no special meaning in this context.  — end note ] [ Example:

template<class T> struct A { ~A() = delete; };
template<class T> auto h()
  -> A<T>;
template<class T> auto i(T)     // identity
  -> T;
template<class T> auto f(T)     // #1
  -> decltype(i(h<T>()));       // forces completion of A<T> and implicitly uses
                                // A<T>::~A() for the temporary introduced by the
                                // use of h(). (A temporary is not introduced
                                // as a result of the use of i().)
template<class T> auto f(T)     // #2
  -> void;
auto g() -> void {
  f(42);                        // OK: calls #2. (#1 is not a viable candidate: type
                                // deduction fails ([temp.deduct]) because A<int>::~A()
                                // is implicitly used in its decltype-specifier)
}
template<class T> auto q(T)
  -> decltype((h<T>()));        // does not force completion of A<T>; A<T>::~A() is
                                // not implicitly used within the context of this decltype-specifier
void r() {
  q(42);                        // Error: deduction against q succeeds, so overload resolution
                                // selects the specialization “q(T) -> decltype((h<T>())) [with T=int]”.
                                // The return type is A<int>, so a temporary is introduced and its
                                // destructor is used, so the program is ill-formed.
}

 — end example ]