CS 202 Fall 2013
Notes for Tuesday, September 24, 2013

Classes II: Some Details [14.1–14.3]

Terminology

Local

Declared inside a function.

Member

Declared inside a class.

Global

Neither.

Assignment

A C++ compiler will write an assignment operator (“=”) for a class as long as you do not declare one (we do not know how to do declare one, yet). This operator will do assignment on each member variable.

[C++]

class Ggg {
private:
    int _x;
   double _y;
    string _z;
};

Ggg a;
Ggg b;
a = b;

The assignment above (“a = b;”) is essentially the same as

[C++]

a._x = b._x;
a._y = b._y;
a._z = b._z;

except that these would not compile, since the member variables are private.

Static Member Variables

If a member variable is declared static, then there is only one copy of it (as opposed to a separate copy for each object).

[C++]

class Hhh {
private:
    int _x;
    static double _y;
};

When defining a class in separate header/source files, initialize a static member variable in the source file.

[C++]

// hhh.cpp
#include "hhh.h"

double Hhh::_y = 7.1;

Since a static member variable is not part of an object, it can be modified in a const member function.

[C++]

class Hhh {
public:
    void foo() const
    {
        _y = 3.2;
    }
private:
    static double _y;
};

Static Member Functions

Member functions can also be static. Such a function does not need to be called on a particular object. Note that a static member function cannot be const.

[C++]

class Iii {
public:
    static void bar()
    {
        ...
    }
};

We can call a static member function using an object.

[C++]

Iii x;
x.bar();

Or call it using the class name and “::”.

[C++]

Iii::bar();

Global Functions in a Package

To put a global function into the same package as a class, prototype it in the header, and define it in the source.

[C++]

// xyz.h
#ifndef XYZ_H
#define XYZ_H

class Xyz {
    ...
};

void foo();
#endif //#ifndef XYZ_H

[C++]

// xyz.cpp
#include "xyz.h"

void foo()
{
    blah_blah_blah();
}

Friend Functions

A global function can access private members of a class if it is a friend of the class. Make a function a friend by putting a prototype of the function inside the class definition, preceded by “friend”. Note that a friend will generally end up having two prototypes.

[C++]

// xyz.h
#ifndef XYZ_H
#define XYZ_H

class Xyz {
    friend void foo();
    ...
};

void foo();
#endif //#ifndef XYZ_H

Example Code

See sumup.h and sumup.cpp for the code for class SumUp, which, among other things, has a static data member and a friend.

See dosumup.cpp for a C++ program that uses class SumUp.

For today’s lab work, see the 9/24 Challenge.