Chapter 6: Fake objects and structs (arrays)

6.1. Using an array as a context struct

Using the passed-by-reference nature of arrays, it is possible to use an array to represent a data structure (similar to a C struct passed by pointer) that is passed to functions that manipulate it:
example program ch6_ex1 
{ calculate the overall intensity of the color }
function color_update(CLR);
begin
	CLR['intensity'] := (CLR['red']/255 + CLR['green']/255 + CLR['blue']/255)/3;
end;

function main(ARGV);
begin
	CLR1['red'] := 15;
	CLR1['green'] := 200;
	CLR1['blue'] := 60;

	color_update(CLR1);

	fawk_print(CLR1['intensity']);
end;

This helps implementing data driven programming:

6.2. The "struct syntax" of array indexing

To ease using the above pattern, fawk implements the "." operator that is a shorthand syntax for an array indexing with the string version of the field name. The following two array references are equivalent: 

arrname.fieldname
arrname['fieldname']

Using this syntax the above example cn be rewritten into a more readable form:
example program ch6_ex2 
{ calculate the overall intensity of the color }
function color_update(CLR);
begin
	CLR['intensity'] := (CLR.red/255 + CLR.green/255 + CLR.blue/255)/3;
end;

function main(ARGV);
begin
	CLR1.red := 15;
	CLR1.green := 200;
	CLR1.blue := 60;

	color_update(CLR1);

	fawk_print(CLR1.intensity);
end;

6.3. Using an array as an object

The next logical step is to bundle the functions that can manipulate the data, to the same array. This is possible since function references can be saved in variables or in array members (see section 4.6.):
example program ch6_ex3 
{ load a color array with component values }
function color_load(CLR; r; g; b);
begin
	CLR.red := r;
	CLR.green := g;
	CLR.blue := b;
end;

{ calculate the overall intensity of the color }
function color_update(CLR);
begin
	CLR['intensity'] := (CLR.red/255 + CLR.green/255 + CLR.blue/255)/3;
end;

function main(ARGV);
begin
{ initialize the object }
	CLR1.update := color_update;
	CLR1.load := color_load;

{ load and update }
	CLR1.load(CLR1, 15, 200, 60);
	CLR1.update(CLR1);

	fawk_print(CLR1.intensity);
end;

6.4. Inheritance: "base class"

It is possible to set up a "base class" for the color structure so that different object instances can be created more easily:
example program ch6_ex4 
{ load a color array with component values }
function color_load(CLR; r; g; b);
begin
	CLR.red := r;
	CLR.green := g;
	CLR.blue := b;
end;

{ calculate the overall intensity of the color }
function color_update(CLR);
begin
	CLR['intensity'] := (CLR.red/255 + CLR.green/255 + CLR.blue/255)/3;
end;

function main(ARGV);
begin
{ initialize the "base class" }
	COLOR.update := color_update;
	COLOR.load := color_load;

{ create an instance of COLOR called CLR1 }
	for i in COLOR do
		CLR1[i] := COLOR[i];

{ load and update }
	CLR1.load(CLR1, 15, 200, 60);
	CLR1.update(CLR1);

	fawk_print(CLR1.intensity);
end;

Obviously "multiple inheritance" can be implemented the same way, simply using two loops for copying fields from two different "base classes":
example program ch6_ex5 
{ load a color array with component values }
function color_load(CLR; r; g; b);
begin
	CLR.red := r;
	CLR.green := g;
	CLR.blue := b;
end;

{ calculate the overall intensity of the color }
function color_update(CLR);
begin
	CLR['intensity'] := (CLR.red/255 + CLR.green/255 + CLR.blue/255)/3;
end;

{ convert a single digit to hex }
function hexdigit(digit);
begin
	hexdigit := '?';
	if digit < 10 then hexdigit := digit;
	if digit = 10 then hexdigit := 'A';
	if digit = 11 then hexdigit := 'B';
	if digit = 12 then hexdigit := 'C';
	if digit = 13 then hexdigit := 'D';
	if digit = 14 then hexdigit := 'E';
	if digit = 15 then hexdigit := 'F';
end;

{ convert a byte value to a 2-digit hex string }
function tohex2(val   ;d1;d2);
begin
	d1 := int(val / 16);
	d2 := val % 16;
	tohex2 := hexdigit(d1) @ hexdigit(d2);
end;

{ convert the color to html color code }
function html_print_color(CLR);
begin
	html_print_color := '#' @ tohex2(CLR.red) @ tohex2(CLR.green) @ tohex2(CLR.blue);
end;

function main(ARGV);
begin
{ initialize the color "base class" }
	COLOR.update := color_update;
	COLOR.load := color_load;

{ initialize the html "base class" }
	HTML.print_color := html_print_color;

{ create an instance of COLOR+HMTL called CLR1 }
	for i in COLOR do
		CLR1[i] := COLOR[i];
	for i in HTML do
		CLR1[i] := HTML[i];

{ load and update }
	CLR1.load(CLR1, 15, 200, 60);

	fawk_print(CLR1.print_color(CLR1));
end;

6.5. Why it is not real OOP

There are many major features of OOP missing: there are no automatisms, everything needs to be done by the user, in code. Most notably there is no constructor or destructor and passing the object as the first argument is done manually, there is no "this" in functions.