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Classes can be defined inside other classes. Classes that are defined inside
other classes are called
nested classes. Nested classes
are used in situations where the nested class has a close conceptual
relationship to its surrounding class. For example, with the class string
a type
string::iterator is available which will provide all characters
that are stored in the string. This string::iterator type could be
defined as an object
iterator, defined as nested class in the class
string.
A class can be nested in every part of the surrounding class: in the
public, protected or private section. Such a nested class can be
considered a member
of the surrounding class. The
normal access and rules in classes apply to nested classes. If a
class is nested in the public section of a class, it is
visible outside the surrounding class. If
it is nested in the protected section it is visible in subclasses, derived
from the surrounding class (see chapter 13), if it is nested in
the private section, it is only visible for the members of the surrounding
class.
The surrounding class has no special privileges with respect to the nested class. So, the nested class still has full control over the accessibility of its members by the surrounding class. For example, consider the following class definition:
class Surround
{
public:
class FirstWithin
{
int d_variable;
public:
FirstWithin();
int var() const;
};
private:
class SecondWithin
{
int d_variable;
public:
SecondWithin();
int var() const;
};
};
inline int Surround::FirstWithin::var() const
{
return d_variable;
}
inline int Surround::SecondWithin::var() const
{
return d_variable;
}
In this definition access to the members is defined as follows:
FirstWithin is visible both outside and inside
Surround. The class FirstWithin therefore has global scope.
FirstWithin() and the member function
var() of the class FirstWithin are also globally visible.
int d_variable datamember is only visible to the members
of the class FirstWithin. Neither the members of Surround nor the
members of SecondWithin can access d_variable of the class
FirstWithin directly.
SecondWithin is only visible inside
Surround. The public members of the class SecondWithin can also be
used by the members of the class FirstWithin, as nested classes can be
considered members of their surrounding class.
SecondWithin() and the member function
var() of the class SecondWithin can also only be reached by the
members of Surround (and by the members of its nested classes).
int d_variable datamember of the class SecondWithin
is only visible to the members of the class SecondWithin. Neither the
members of Surround nor the members of FirstWithin can access
d_variable of the class SecondWithin directly.
friend classes (see section 16.3).
The nested classes can be considered members of the surrounding class, but
the
members of nested classes are not members of the surrounding
class. So, a member of the class Surround may not access
FirstWithin::var() directly. This is understandable considering the fact
that a Surround object is not also a FirstWithin or SecondWithin
object. In fact, nested classes are just typenames. It is not implied that
objects of such classes automatically exist in the surrounding class. If a
member of the surrounding class should use a (non-static) member of a nested
class then the surrounding class must define a nested class object, which can
thereupon be used by the members of the surrounding class to use members of
the nested class.
For example, in the following class definition there is a surrounding
class Outer and a nested class Inner. The class Outer contains a
member function caller() which uses the inner object that is composed
in Outer to call the infunction() member function of Inner:
class Outer
{
public:
void caller();
private:
class Inner
{
public:
void infunction();
};
Inner d_inner; // class Inner must be known
};
void Outer::caller()
{
d_inner.infunction();
}
The mentioned function Inner::infunction() can be called as part
of the inline definition of Outer::caller(), even though the definition of
the class Inner is yet to be seen by the compiler. On the other hand, the
compiler must have seen the definition of the class Inner before a data
member of that class can be defined.
Outer::caller() would have been defined outside of the class Outer,
the full class definition (including the definition of the class Inner)
would have been available to the compiler. In that situation the function is
perfectly compilable. Inline functions can be compiled accordingly: they can
be defined and they can use any nested class. Even if it appears later in the
class interface.
As shown, when (nested) member functions are defined inline, their definition
should be put below their class interface. Static nested data members
are also normally defined outside of their classes.
If the class FirstWithin would have a static size_t datamember
epoch, it could be initialized as follows:
size_t Surround::FirstWithin::epoch = 1970;
Furthermore, multiple
scope resolution
operators are needed to refer to public static members in code outside of the
surrounding class:
void showEpoch()
{
cout << Surround::FirstWithin::epoch = 1970;
}
Inside the members of the class Surround only the FirstWithin::
scope must be used; inside the members of the class FirstWithin there is
no need to refer explicitly to the scope.
What about the members of the class SecondWithin? The classes
FirstWithin and SecondWithin are both nested within Surround, and
can be considered members of the surrounding class. Since members of a class
may directly refer to each other, members of the class SecondWithin can
refer to (public) members of the class FirstWithin. Consequently, members
of the class SecondWithin could refer to the epoch member of
FirstWithin as
FirstWithin::epoch
For example, the following class Outer contains two nested classes
Inner1 and Inner2. The class Inner1 contains a pointer to
Inner2 objects, and Inner2 contains a pointer to Inner1
objects. Such cross references require forward declarations. These forward
declarations must be specified in the same access-category as their actual
definitions. In the following example the Inner2 forward declaration must
be given in a private section, as its definition is also part of the
class Outer's private interface:
class Outer
{
private:
class Inner2; // forward declaration
class Inner1
{
Inner2 *pi2; // points to Inner2 objects
};
class Inner2
{
Inner1 *pi1; // points to Inner1 objects
};
};
friend keyword must be used. Consider the following
situation, in which a class Surround has two nested classes
FirstWithin and SecondWithin, while each class has a
static data member int s_variable:
class Surround
{
static int s_variable;
public:
class FirstWithin
{
static int s_variable;
public:
int value();
};
int value();
private:
class SecondWithin
{
static int s_variable;
public:
int value();
};
};
If the class Surround should be able to access FirstWithin and
SecondWithin's private members, these latter two classes must declare
Surround to be their friend. The function Surround::value() can
thereupon access the private members of its nested classes. For example (note
the friend declarations in the two nested classes):
class Surround
{
static int s_variable;
public:
class FirstWithin
{
friend class Surround;
static int s_variable;
public:
int value();
};
int value();
private:
class SecondWithin
{
friend class Surround;
static int s_variable;
public:
int value();
};
};
inline int Surround::FirstWithin::value()
{
FirstWithin::s_variable = SecondWithin::s_variable;
return (s_variable);
}
Now, to allow the nested classes access to the private members of
their surrounding class, the class Surround must declare its nested classes
as friends. The friend keyword may only be used when the class that is to
become a friend is already known as a class by the compiler, so either a
forward declaration of the nested classes is required, which is followed
by the friend declaration, or the friend declaration follows the definition of
the nested classes. The forward declaration followed by the friend declaration
looks like this:
class Surround
{
class FirstWithin;
class SecondWithin;
friend class FirstWithin;
friend class SecondWithin;
public:
class FirstWithin;
...
Alternatively, the friend declaration may follow the definition of the
classes. Note that a class can be declared a friend following its definition,
while the inline code in the definition already uses the fact that it will be
declared a friend of the outer class. When defining members within the class
interface implementations of nested class members may use members of the
surrounding class that have not yet been seen by the compiler. Finally note
that q`s_variable' which is
defined in the class Surround is
accessed in the nested classes as Surround::s_variable:
class Surround
{
static int s_variable;
public:
class FirstWithin
{
friend class Surround;
static int s_variable;
public:
int value();
};
friend class FirstWithin;
int value();
private:
class SecondWithin
{
friend class Surround;
static int s_variable;
public:
int value();
};
static void classMember();
friend class SecondWithin;
};
inline int Surround::value()
{
FirstWithin::s_variable = SecondWithin::s_variable;
return s_variable;
}
inline int Surround::FirstWithin::value()
{
Surround::s_variable = 4;
Surround::classMember();
return s_variable;
}
inline int Surround::SecondWithin::value()
{
Surround::s_variable = 40;
return s_variable;
}
Finally, we want to allow the nested classes access to each other's
private members. Again this requires some friend declarations. In order to
allow FirstWithin to access SecondWithin's private members nothing but
a friend declaration in SecondWithin is required. However, to allow
SecondWithin to access the private members of FirstWithin the
friend class SecondWithin declaration cannot plainly be given in the class
FirstWithin, as the definition of SecondWithin is as yet unknown. A
forward declaration of SecondWithin is required, and this forward
declaration must be provided by the class Surround, rather than by the
class FirstWithin.
Clearly, the forward declaration class SecondWithin in the class
FirstWithin itself makes no sense, as this would refer to an external
(global) class SecondWithin. Likewise, it is impossible to provide the
forward declaration of the nested class SecondWithin inside
FirstWithin as class Surround::SecondWithin, with the compiler issuing
a message like
`Surround' does not have a nested type named `SecondWithin'
The proper procedure here is to declare the class SecondWithin in the
class Surround, before the class FirstWithin is defined. Using this
procedure, the friend declaration of SecondWithin is accepted inside the
definition of FirstWithin. The following class definition allows full
access of the private members of all classes by all other classes:
class Surround
{
class SecondWithin;
static int s_variable;
public:
class FirstWithin
{
friend class Surround;
friend class SecondWithin;
static int s_variable;
public:
int value();
};
friend class FirstWithin;
int value();
private:
class SecondWithin
{
friend class Surround;
friend class FirstWithin;
static int s_variable;
public:
int value();
};
friend class SecondWithin;
};
inline int Surround::value()
{
FirstWithin::s_variable = SecondWithin::s_variable;
return s_variable;
}
inline int Surround::FirstWithin::value()
{
Surround::s_variable = SecondWithin::s_variable;
return s_variable;
}
inline int Surround::SecondWithin::value()
{
Surround::s_variable = FirstWithin::s_variable;
return s_variable;
}
ios we've seen values like
ios::beg and
ios::cur. In the current
Gnu C++ implementation these values are
defined as values in the
seek_dir enumeration:
class ios: public _ios_fields
{
public:
enum seek_dir
{
beg,
cur,
end
};
};
For illustration purposes, let's assume that a class DataStructure
may be traversed in a forward or backward direction. Such a class can define
an enumeration Traversal having the values forward and
backward. Furthermore, a member function setTraversal() can be defined
requiring either of the two enumeration values. The class can be defined as
follows:
class DataStructure
{
public:
enum Traversal
{
forward,
backward
};
setTraversal(Traversal mode);
private:
Traversal
d_mode;
};
Within the class DataStructure the values of the Traversal
enumeration can be used directly. For example:
void DataStructure::setTraversal(Traversal mode)
{
d_mode = mode;
switch (d_mode)
{
forward:
break;
backward:
break;
}
}
Ouside of the class DataStructure the name of the enumeration type is
not used to refer to the values of the enumeration. Here the classname is
sufficient. Only if a variable of the enumeration type is required the name of
the enumeration type is needed, as illustrated by the following piece of code:
void fun()
{
DataStructure::Traversal // enum typename required
localMode = DataStructure::forward; // enum typename not required
DataStructure ds;
// enum typename not required
ds.setTraversal(DataStructure::backward);
}
Again, only if DataStructure defines a nested class Nested, in
turn defining the enumeration Traversal, the two class scopes are
required. In that case the latter example should have been coded as follows:
void fun()
{
DataStructure::Nested::Traversal
localMode = DataStructure::Nested::forward;
DataStructure ds;
ds.setTraversal(DataStructure::Nested::backward);
}
Enum types usually have values. However, this is not required. In
section 14.5.1 the
std::bad_cast type was introduced. A
std::bad_cast is thrown by the
dynamic_cast<>() operator when a
reference to a
base class object cannot be cast to a
derived class
reference. The std::bad_cast could be caught as type, irrespective of any
value it might represent.
Actually, it is not even necessary for a
type to
contain values. It is possible to define an
empty enum, an enum
without any values, whose name may thereupon be used as a legitimate type name
in, e.g. a
catch clause defining an
exception handler.
An empty enum is defined as follows (often, but not necessarily within
a
class):
enum EmptyEnum
{};
Now an EmptyEnum may be thrown (and caught) as an exception:
#include <iostream>
enum EmptyEnum
{};
using namespace std;
int main()
try
{
throw EmptyEnum();
}
catch (EmptyEnum)
{
cout << "Caught empty enum\n";
}
/*
Generated output:
Caught empty enum
*/
Base was used as an abstract base class. A class
Clonable was thereupon defined to manage Base class pointers in
containers like vectors.
As the class Base is a very small class, hardly requiring any
implementation, it can well be defined as a nested class in Clonable. This
will emphasize the close relationship that exists between Clonable and
Base, as shown by the way classes are derived from Base. One no longer
writes:
class Derived: public Base
but rather:
class Derived: public Clonable::Base
Other than defining Base as a nested class, and deriving from
Clonable::Base rather than from Base, nothing needs to be
modified. Here is the program shown earlier in section 14.10, but now
using nested classes:
#include <iostream>
#include <vector>
#include <typeinfo>
class Clonable
{
public:
class Base
{
public:
virtual ~Base();
virtual Base *clone() const = 0;
};
private:
Base *d_bp;
public:
Clonable();
~Clonable();
Clonable(Clonable const &other);
Clonable &operator=(Clonable const &other);
// New for virtual constructions:
Clonable(Base const &bp);
Base &get() const;
private:
void copy(Clonable const &other);
};
inline Clonable::Base::~Base()
{}
inline Clonable::Clonable()
:
d_bp(0)
{}
inline Clonable::~Clonable()
{
delete d_bp;
}
inline Clonable::Clonable(Clonable const &other)
{
copy(other);
}
inline Clonable &Clonable::operator=(Clonable const &other)
{
if (this != &other)
{
delete d_bp;
copy(other);
}
return *this;
}
inline Clonable::Clonable(Base const &bp)
{
d_bp = bp.clone(); // allows initialization from
} // Base and derived objects
inline Clonable::Base &Clonable::get() const
{
return *d_bp;
}
inline void Clonable::copy(Clonable const &other)
{
if ((d_bp = other.d_bp))
d_bp = d_bp->clone();
}
class Derived1: public Clonable::Base
{
public:
~Derived1();
virtual Clonable::Base *clone() const;
};
inline Derived1::~Derived1()
{
std::cout << "~Derived1() called\n";
}
inline Clonable::Base *Derived1::clone() const
{
return new Derived1(*this);
}
using namespace std;
int main()
{
vector<Clonable> bv;
bv.push_back(Derived1());
cout << "==\n";
cout << typeid(bv[0].get()).name() << endl;
cout << "==\n";
vector<Clonable> v2(bv);
cout << typeid(v2[0].get()).name() << endl;
cout << "==\n";
}