Nested Class Declarations
A class can be declared within the scope of another class. Such a class is called a "nested class." Nested classes are considered to be within the scope of the enclosing class and are available for use within that scope. To refer to a nested class from a scope other than its immediate enclosing scope, you must use a fully qualified name.
The following example shows how to declare nested classes:
// nested_class_declarations.cpp
class BufferedIO
{
public:
enum IOError { None, Access, General };
// Declare nested class BufferedInput.
class BufferedInput
{
public:
int read();
int good()
{
return _inputerror == None;
}
private:
IOError _inputerror;
};
// Declare nested class BufferedOutput.
class BufferedOutput
{
// Member list
};
};
int main()
{
}
BufferedIO::BufferedInput and BufferedIO::BufferedOutput are declared within BufferedIO. These class names are not visible outside the scope of class BufferedIO. However, an object of type BufferedIO does not contain any objects of types BufferedInput or BufferedOutput.
Nested classes can directly use names, type names, names of static members, and enumerators only from the enclosing class. To use names of other class members, you must use pointers, references, or object names.
In the preceding BufferedIO example, the enumeration IOError can be accessed directly by member functions in the nested classes, BufferedIO::BufferedInput or BufferedIO::BufferedOutput, as shown in function good.
注意
Nested classes declare only types within class scope. They do not cause contained objects of the nested class to be created. The preceding example declares two nested classes but does not declare any objects of these class types.
An exception to the scope visibility of a nested class declaration is when a type name is declared together with a forward declaration. In this case, the class name declared by the forward declaration is visible outside the enclosing class, with its scope defined to be the smallest enclosing non-class scope. For example:
// nested_class_declarations_2.cpp
class C
{
public:
typedef class U u_t; // class U visible outside class C scope
typedef class V {} v_t; // class V not visible outside class C
};
int main()
{
// okay, forward declaration used above so file scope is used
U* pu;
// error, type name only exists in class C scope
u_t* pu2; // C2065
// error, class defined above so class C scope
V* pv; // C2065
// okay, fully qualified name
C::V* pv2;
}