7 Expressions [expr]

7.5 Primary expressions [expr.prim]

7.5.4 Names [expr.prim.id]

id-expression:
	unqualified-id
	qualified-id
An id-expression is a restricted form of a primary-expression.
Note
: — end note
 ]
An id-expression that denotes a non-static data member or non-static member function of a class can only be used:
  • as part of a class member access in which the object expression refers to the member's class58 or a class derived from that class, or
  • to form a pointer to member ([expr.unary.op]), or
  • if that id-expression denotes a non-static data member and it appears in an unevaluated operand.
    Example
    :
    struct S {
      int m;
    };
    int i = sizeof(S::m);           // OK
    int j = sizeof(S::m + 42);      // OK
    
    — end example
     ]
An id-expression that denotes an immediate function ([dcl.constexpr]) shall appear as a subexpression of an immediate invocation or in an immediate function context ([expr.const]).
An id-expression that denotes the specialization of a concept ([temp.concept]) results in a prvalue of type bool.
The expression is true if the concept's normalized constraint-expression is satisfied ([temp.constr.constr]) by the specified template arguments and false otherwise.
Example
:
template<typename T> concept C = true;
static_assert(C<int>);          // OK
— end example
 ]
Note
:
A concept's constraints are also considered when using a template name ([temp.names]) and during overload resolution, and they are compared during the partial ordering of constraints ([temp.constr.order]).
— end note
 ]
A program that refers explicitly or implicitly to a function with a trailing requires-clause whose constraint-expression is not satisfied, other than to declare it, is ill-formed.
Example
:
void f(int) requires false;

void g() {
  f(0);                         // error: cannot call f
  void (*p1)(int) = f;          // error: cannot take the address of f
  decltype(f)* p2 = nullptr;    // error: the type decltype(f) is invalid
}
In each case, the constraints of f are not satisfied.
In the declaration of p2, those constraints are required to be satisfied even though f is an unevaluated operand.
— end example
 ]
This also applies when the object expression is an implicit (*this) ([class.mfct.non-static]).

7.5.4.1 Unqualified names [expr.prim.id.unqual]

unqualified-id:
	identifier
	operator-function-id
	conversion-function-id
	literal-operator-id
	~ type-name
	~ decltype-specifier
	template-id
An identifier is only an id-expression if it has been suitably declared ([dcl.dcl]) or if it appears as part of a declarator-id.
Note
:
A type-name or decltype-specifier prefixed by ~ denotes the destructor of the type so named; see [expr.prim.id.dtor].
Within the definition of a non-static member function, an identifier that names a non-static member is transformed to a class member access expression ([class.mfct.non-static]).
— end note
 ]
The result is the entity denoted by the identifier.
If the entity is a local entity and naming it from outside of an unevaluated operand within the declarative region where the unqualified-id appears would result in some intervening lambda-expression capturing it by copy ([expr.prim.lambda.capture]), the type of the expression is the type of a class member access expression ([expr.ref]) naming the non-static data member that would be declared for such a capture in the closure object of the innermost such intervening lambda-expression.
Note
:
If that lambda-expression is not declared mutable, the type of such an identifier will typically be const qualified.
— end note
 ]
If the entity is a template parameter object for a template parameter of type T ([temp.param]), the type of the expression is const T.
Otherwise, the type of the expression is the type of the result.
Note
:
The type will be adjusted as described in [expr.type] if it is cv-qualified or is a reference type.
— end note
 ]
The expression is an lvalue if the entity is a function, variable, structured binding ([dcl.struct.bind]), data member, or template parameter object and a prvalue otherwise ([basic.lval]); it is a bit-field if the identifier designates a bit-field.
Example
:
void f() {
  float x, &r = x;
  [=] {
    decltype(x) y1;             // y1 has type float
    decltype((x)) y2 = y1;      // y2 has type float const& because this lambda
                                // is not mutable and x is an lvalue
    decltype(r) r1 = y1;        // r1 has type float&
    decltype((r)) r2 = y2;      // r2 has type float const&
  };
}
— end example
 ]

7.5.4.2 Qualified names [expr.prim.id.qual]

qualified-id:
	nested-name-specifier template unqualified-id
nested-name-specifier:
	::
	type-name ::
	namespace-name ::
	decltype-specifier ::
	nested-name-specifier identifier ::
	nested-name-specifier template simple-template-id ::
The type denoted by a decltype-specifier in a nested-name-specifier shall be a class or enumeration type.
A nested-name-specifier that denotes a class, optionally followed by the keyword template ([temp.names]), and then followed by the name of a member of either that class ([class.mem]) or one of its base classes, is a qualified-id; [class.qual] describes name lookup for class members that appear in qualified-ids.
The result is the member.
The type of the result is the type of the member.
The result is an lvalue if the member is a static member function or a data member and a prvalue otherwise.
Note
:
A class member can be referred to using a qualified-id at any point in its potential scope ([basic.scope.class]).
— end note
 ]
Where type-name ​::​~ type-name is used, the two type-names shall refer to the same type (ignoring cv-qualifications); this notation denotes the destructor of the type so named ([expr.prim.id.dtor]).
The unqualified-id in a qualified-id shall not be of the form ~decltype-specifier.
The nested-name-specifier ​::​ names the global namespace.
A nested-name-specifier that names a namespace, optionally followed by the keyword template ([temp.names]), and then followed by the name of a member of that namespace (or the name of a member of a namespace made visible by a using-directive), is a qualified-id; [namespace.qual] describes name lookup for namespace members that appear in qualified-ids.
The result is the member.
The type of the result is the type of the member.
The result is an lvalue if the member is a function, a variable, or a structured binding ([dcl.struct.bind]) and a prvalue otherwise.
A nested-name-specifier that denotes an enumeration, followed by the name of an enumerator of that enumeration, is a qualified-id that refers to the enumerator.
The result is the enumerator.
The type of the result is the type of the enumeration.
The result is a prvalue.
In a qualified-id, if the unqualified-id is a conversion-function-id, its conversion-type-id is first looked up in the class denoted by the nested-name-specifier of the qualified-id and the name, if found, is used.
Otherwise, it is looked up in the context in which the entire qualified-id occurs.
In each of these lookups, only names that denote types or templates whose specializations are types are considered.

7.5.4.3 Destruction [expr.prim.id.dtor]

An id-expression that denotes the destructor of a type T names the destructor of T if T is a class type ([class.dtor]), otherwise the id-expression is said to name a pseudo-destructor.
If the id-expression names a pseudo-destructor, T shall be a scalar type and the id-expression shall appear as the right operand of a class member access ([expr.ref]) that forms the postfix-expression of a function call ([expr.call]).
Note
:
Such a call has no effect.
— end note
 ]
Example
:
struct C { };
void f() {
  C * pc = new C;
  using C2 = C;
  pc->C::~C2();     // OK, destroys *pc
  C().C::~C();      // undefined behavior: temporary of type C destroyed twice
  using T = int;
  0 .T::~T();       // OK, no effect
  0.T::~T();        // error: 0.T is a user-defined-floating-literal ([lex.ext])
}
— end example
 ]