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In the previous chapters we have shown examples of classes where each object
of a class had its own set of public or private data. Each
public or private function could access the object's own
version of the data.
In some situations it may be desirable that one or more common data fields exist, which are accessible to all objects of the class. An example of such a situation is the name of the startup directory in a program which recursively scans the directory tree of a disk. A second example is a flag variable, which states whether some specific initialization has occurred: only the first object of the class would then perform the initialization and would then set the flag to `done'.
Such situations are analogous to C code, where several functions need to
access the same variable. A common solution in C is to define all these
functions in one source file and to declare the variable as a static:
the variable name is then not known beyond the scope of the source file. This
approach is quite valid, but doesn't stroke with our philosophy of one
function per source file. Another C-solution is to give the variable in
question an unusual name, e.g., _6uldv8, and then to hope that
other program parts won't use this name by accident. Neither the first, nor
the second C-like solution is elegant.
C++'s solution is to define static data and functions,
common to all objects of a class, and inaccessible outside of the class.
These functions and data will be discussed in this
chapter.
static; be it in the public
or private part of the class definition. Such a data member is created
and initialized only once, in contrast to non-static data members, which are
created again and again, for each separate object of the class.
A static data member is created once:
when the program starts executing. Nonetheless, it is still part of the class.
static data members which are declared public are like `normal'
global variables: they can be reached by all code of the program using
their name, together with their class name and the scope resolution operator.
This is illustrated in the following code fragment:
class Test
{
public:
static int
public_int;
private:
static int
private_int;
}
int main()
{
Test::public_int = 145; // ok
Test::private_int = 12; // wrong, don't touch
// the private parts
return (0);
}
This code fragment is not suitable for consumption by a C++ compiler: it
only illustrates the interface, and not the implementation of
static data members. We will discuss the implementation of such members
shortly.
static data member which is a private
variable in a class, consider the following code fragment:
class Directory
{
public:
// constructors, destructors, etc. (not shown)
...
private:
// data members
static char
path[];
};
The data member path[] is a private static variable. During the
execution of the program, only one Directory::path[] exists,
even though more than one object of the class Directory may
exist. This data member could be inspected or altered by the constructor,
destructor or by any other member function of the class Directory.
Since constructors are called for each new object of a class, static data
members are never initialized by constructors. At most they are
modified. The reason for this is that the static data members exist
before the constructor of the class is called for the very first time.
The static data members can be initialized during their
definition, outside of all member functions, in the same way as global
variables are initialized. The definition and initialization of a static
data member usually occurs in one of the source files of the class functions,
preferably in a source file dedicated to the definition of static data
members, called data.cc.
The data member path[] from the above class Directory could thus be
defined and initialized in the source file of the constructor (or in a
separate file data.cc):
// the static data member: definition and initialization
char
Directory::path [200] = "/usr/local";
// the default constructor
Directory::Directory()
{
...
}
It should be noted that the definition of the static data member can
occur in any source file; as long as it is defined only once. So, there
is no need to define it in, e.g., a source file in which also a
member function of the class is implemented.
In the class interface the static member is actually
only declared. At its implementation (definition)
its type and class name are explicitly stated. Note also that
the size specification can be left out of the interface, as is shown in
the above array path[]. However, its size is needed at its
implementation.
A second example of a useful private static data member is given
below. A class Graphics defines the communication of a program with a
graphics-capable device (e.g., a VGA screen). The initial preparing of the
device, which in this case would be to switch from text mode to graphics
mode, is an action of the constructor and depends on a static flag
variable nobjects. The variable nobjects simply counts the number of
Graphics objects which are present at one time. Similarly, the
destructor of the class may switch back from graphics mode to text mode when
the last Graphics object ceases to exist.
The class interface for this Graphics class might be:
class Graphics
{
public:
// constructor, destructor
Graphics();
~Graphics();
// other interface is not shown here,
// e.g. to draw lines or whatever
private:
// counter of # of objects
static int nobjects;
// hypothetical functions to switch to graphics
// mode or back to text mode
void setgraphicsmode();
void settextmode();
}
The purpose of the variable nobjects is to count the number of objects
which exist at one given time. When the first object is created, the graphics
device is initialized. At the destruction of the last Graphics
object, the switch from graphics mode to text mode is made:
// the static data member
int Graphics::nobjects = 0;
// the constructor
Graphics::Graphics()
{
if (! nobjects)
setgraphicsmode();
nobjects++;
}
// the destructor
Graphics::~Graphics()
{
nobjects--;
if (! nobjects)
settextmode();
}
It is obvious that when the class Graphics would define more than
one constructor, each constructor would need to increase the variable
nobjects and possibly would have to initialize the graphics mode.
public section of a class definition,
although this is not common practice (such a setup would violate the principle
of data hiding). E.g., when the static data member path[] from
chapter 9.1 would be declared in the public section of
the class definition, all program code could access this variable:
int main()
{
getcwd(Directory::path, 199);
return(0);
}
Note that the variable path would still have to be defined. As before,
the class interface would only declare the array path[].
This means that some source file would still need to contain the
implementation:
char
Directory::path[200];
static data, C++ allows the definition of static
functions. Similar to the concept of static data, in which these
variables are shared by all objects of the class, static functions apply
to all objects of the class.
The static functions can therefore address only the static data of a
class; non-static data are unavailable to these functions. If
non-static data could be addressed, to which object would they belong?
Similarly, static functions cannot call non-static functions of the
class. All this is caused by the fact that static functions have no
this pointer.
Functions which are static and which are declared in the public section
of a class interface can be called without specifying an object of the class.
This is illustrated in the following code fragment:
class Directory
{
public:
// constructors, destructors etc. not shown here
...
// here's the static public function
static void setpath(char const *newpath);
private:
// the static string
static char path [];
};
// implementation of the static variable
char Directory::path [199] = "/usr/local";
// the static function
void Directory::setpath(char const *newpath)
{
strncpy(path, newpath, 199);
}
// example of the usage
int main()
{
// Alternative (1): calling setpath() without
// an object of the class Directory
Directory::setpath("/etc");
// Alternative (2): with an object
Directory
dir;
dir.setpath("/etc");
return (0);
}
In the example above the function setpath() is a public static
function. C++ also allows private static functions: these functions
can only be called from other member functions of the class of which they are
themselves members, but not from other functions.
Note that such a private static function could only
(a) access static variables, or (b) call other static functions: non-static
code or data members would still be inaccessible to the static function.