https://en.cppreference.com/w/cpp/language/elaborated_type_specifier.html
https://vsdmars.blogspot.com/2014/02/c11-extended-friend-declaration.html
class T
{
public:
class U;
private:
int U;
};
int main()
{
int T;
T t; // error: the local variable T is found
class T t; // OK: finds ::T, the local variable T is ignored
T::U* u; // error: lookup of T::U finds the private data member
class T::U* u; // OK: the data member is ignored
}
template<typename T>
struct Node
{
struct Node* Next; // OK: lookup of Node finds the injected-class-name
struct Data* Data; // OK: declares type Data at global scope
// and also declares the data member Data
friend class ::List; // error: cannot introduce a qualified name
enum Kind* kind; // error: cannot introduce an enum
};
Data* p; // OK: struct Data has been declared
template<typename T>
class Node
{
friend class T; // error: type parameter cannot appear in an elaborated type specifier;
// note that similar declaration `friend T;` is OK.
};
class A {};
enum b { f, t };
int main()
{
class A a; // OK: equivalent to 'A a;'
enum b flag; // OK: equivalent to 'b flag;'
}
enum class E { a, b };
enum E x = E::a; // OK
enum class E y = E::b; // error: 'enum class' cannot introduce an elaborated type specifier
struct A {};
class A a; // OK
