14.6 The Meaning of static

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14.6 The Meaning of static

The keyword static has many different meanings in C++. Table 14-1 is a complete list of the various ways static can be used.

Table 14-1. The meanings of static

Usage	Meaning
Variable outside the body of any function	The scope of the variable is limited to the file in which it is declared.
Variable declaration inside a function	The variable is permanent. It is initialized once, and only one copy is created even if the function is called recursively.
Function declaration	The scope of the function is limited to the file in which it is declared.
Member variable	One copy of the variable is created per class (not one per object).
Member function	Function can only access static members of the class.

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14.7 Programming Exercises

Exercies 14-1: Two classes share a file. Other areas of the program need to know when this file is busy. Create a function that returns 1 when the file is being used by either of these two classes.

Exercies 14-2: You are asked to write a booking program for the veterinarian; Dr. Able Smith, PHD (Pigs, Horses, Dogs). Define a class type for each animal. Each class should keep track in a private static variable of the number of animals that have been defined using that class. Define a function that returns the total number of animals (all three types combined).

Exercies 14-3: Write a class in which each instance of the class can access a stack ”not one stack per instance, but one stack, period. Any instance of the class can lock the stack for its own exclusive use and unlock it later. Define member functions to perform the lock and unlock functions.

As an added attraction, make the unlock function check to see that the current instance of the class was the same instance that locked the stack in the first place.

Exercies 14-4: You need to supply some I/O routines for handling lines in a file. The basic definition of the line-number class is:

class line_number {
    public:
        void goto_line(int line);
        int get_current_line(  );
        long int get_char_pos(  );
}

The member functions are defined as:

void goto_line(int line);: Positions the input file at specified line.
int get_current_line( );: Returns the current line number (as set by goto_line ).
long int get_char_pos( );: Returns the character position of the current line. (This is the tricky one.)

Several line_number classes may be in use at any time. The class maintains its own internal list so that it knows which line_number classes are in use. When goto_line is called, the function will scan the list of line_number classes to find the one nearest the given line number and use it to start scanning for the given line number.

For example, suppose there are four active line_number variables :

Variable	Position
beginning	Line 0
chapter_start	Line 87
current_heading	Line 112
current_location	Line 52

You wish to move current_location to line 90. The goto_line function would search the list for the line nearest the new location (in this case chapter_start ) and use it to jump to line 87. It then would read the file, character by character, until it saw three end-of-line characters to position itself at line 90.

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