Unit: what changed

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Title: Unit

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1	+	A `unit` is a [[Source code\|source code]] file (or the [[Binary\|binary]] compiled from that [[File\|file]]) which was written using the [[Pascal]] programming language, and that is designed to be a single module in an [[Application\|application]] or an [[Object module\|object module]]. `Unit` is a [[Reserved word\|reserved word]]. It must appear before anything in the unit except comments.
2	+
3	+	## Purpose
4	+
5	+	A unit may be used where functionality needs to be provided to an application program or to other units. It allows writing code that performs that functionality once and have it used in many places. This can reduce the possibility of errors and increase code reusage.
6	+
7	+	A binary unit may be used where a unit author wishes to provide certain functionality to be used in a Pascal [[Program\|program]] but does not wish to provide the source code that performs that functionality.
8	+
9	+	Units were also used on older versions of Pascal when it was necessary on computers with limited resources to be able to load [[Routine\|routines]] as needed rather than keeping every routine of the [[Executable program\|executable program]] in memory all of the time.
10	+
11	+	A unit or program that needs to access the [[Procedure\|procedures]] and [[Data type\|data types]] in another unit must specify the units it needs to access them in a [[Uses\|uses statement]] (but linking is done without the need to write a makefile as in C).
12	+
13	+	A unit may also be used to declare a series of global [[Const\|constants]] or [[Global variables\|variables]] for use by the entire application, without actually containing any executable code. This is similar to the `common` [[Keyword\|keyword]] in the [[Fortran]] programming language.
14	+
15	+	## Format
16	+
17	+	A unit is defined with the `unit` keyword followed by the unit-identifier. The unit-identifier (in the following example the unit's name is “minimalunit”) should match the filename it is written in. A minimal working example (which does nothing) is:
18	+
19	+	``` pascal
20	+	unit minimalunit;
21	+	interface
22	+	// here comes stuff that the unit publicly offers
23	+	implementation
24	+	// here comes the implementation of offered stuff and
25	+	// optionally internal stuff (only known in the unit)
26	+	end.
27	+	```
28	+
29	+	where the part after [[Interface\|interface corresponds]] to the `public`-part of other languages and [[Implementation\|implementation]] does so to `private`.
30	+
31	+	A more advanced basic structure is:
32	+
33	+	``` pascal
34	+	unit advancedunit;
35	+	interface
36	+
37	+	implementation
38	+
39	+	initialization
40	+	// here may be placed code that is
41	+	// executed as the unit gets loaded
42	+
43	+	finalization
44	+	// code executed at program end
45	+
46	+	end.
47	+	```
48	+
49	+	The optional [[Initialization\|initialization]] and [[Finalization\|finalization]] blocks may be followed by code that is executed on program start/end.
50	+
51	+	### Detailed unit example
52	+
53	+	A step-by-step example:
54	+
55	+	``` pascal
56	+	unit randomunit;
57	+	// this unit does something
58	+
59	+	// public - - - - - - - - - - - - - - - - - - - - - - - - -
60	+	interface
61	+
62	+	type
63	+	// the type TRandomNumber gets globally known
64	+	// since it is included somewhere (uses-statement)
65	+	TRandomNumber = integer;
66	+
67	+	// of course the const- and var-sections are possible, too
68	+
69	+	// a list of procedure/function signatures makes
70	+	// them usable from outside of the unit
71	+	function getRandomNumber(): TRandomNumber;
72	+
73	+	// an implementation of a function/procedure
74	+	// must not be in the interface-part
75	+
76	+	// private - - - - - - - - - - - - - - - - - - - - - - - - -
77	+	implementation
78	+
79	+	var
80	+	// var in private-part
81	+	// => only modifiable inside from this unit
82	+	chosenRandomNumber: TRandomNumber;
83	+
84	+	function getRandomNumber(): TRandomNumber;
85	+	begin
86	+	// return value
87	+	getRandomNumber := chosenRandomNumber;
88	+	end;
89	+
90	+	// initialization is the part executed
91	+	// when the unit is loaded/included
92	+	initialization
93	+	begin
94	+	// choose our random number
95	+	chosenRandomNumber := 3;
96	+	// chosen by fair-dice-roll
97	+	// guaranteed to be random
98	+	end;
99	+
100	+	// finalization is executed at program end
101	+	finalization
102	+	begin
103	+	// this unit says 'bye' at program halt
104	+	writeln('bye');
105	+	end;
106	+
107	+	// initialization and finalization
108	+	// are executed at most _once_
109	+	// during the entire runtime of a program
110	+	end.
111	+	```
112	+
113	+	To include a unit just use the `uses`-statement.
114	+
115	+	``` pascal
116	+	program chooseNextCandidate;
117	+	uses
118	+	// include a unit
119	+	randomunit;
120	+
121	+	begin
122	+	writeln('next candidate: no. ' + getRandomNumber());
123	+	end.
124	+	```
125	+
126	+	When compiling, FPC checks this program for unit dependencies. It has to be able to find the unit “randomunit”.
127	+
128	+	The simplest way to satisfy this is to have a unit whose name matches the file name it is written in (appending `.pas` is OK and encouraged). The unit file may be in the same directory where the program source is in or in the unit path FPC looks for units.
129	+
130	+	## Unit precedence
131	+
132	+	When multiple units are described in a use clause, conflicts can occur with identifiers (procedures, types, functions etc.) that have the same name in multiple units. In FreePascal, the last unit “wins” and provides the code for the unit.
133	+
134	+	If you want to achieve a different behavior, you can explicitly prepend `unitname.``identifier` to specify the unit you want to use, or reorder the units. The former is often the clearest option.
135	+
136	+	``` pascal
137	+	unit interestCalculations;
138	+
139	+	interface
140	+
141	+	type
142	+	basisType = currency;
143	+
144	+	implementation
145	+
146	+	end.
147	+	```
148	+
149	+	Specifying the unit of declaration explicitly:
150	+
151	+	``` pascal
152	+	program interestCalculator;
153	+
154	+	uses
155	+	interestCalculations;
156	+
157	+	type
158	+	// we already loaded a declaration of "basisType"
159	+	// from the interestCalculations unit, but we're
160	+	// re-declaring it here again ("shadowing")
161	+	basisType = extended;
162	+
163	+	var
164	+	// last declaration wins: originalCapital is an extended
165	+	originalCapital: basisType;
166	+	// specify a scope to use the declaration valid there
167	+	monthlyMargin: interestCalculations.basisType;
168	+
169	+	begin
170	+	end.
171	+	```
172	+
173	+	Note, the `interestCalculations` unit will still perform its own calculations with its own `basisType` (here [[Currency\|currency]]). You can only alter (“shadow”) declarations in the current [[Scope\|scope]] (and descending).
174	+
175	+	## See also
176	+
177	+	- [“Unit scope” in the “reference guide”](https://www.freepascal.org/docs-html/ref/refsu102.html) (FPC HTML documentation)
178	+	- [“Compiling a unit” in the “users's guide”](https://www.freepascal.org/docs-html/user/userse11.html) (FPC HTML documentation)
179	+	- [§ “Units” in the WikiBook “Pascal Programming”](https://en.wikibooks.org/wiki/Pascal_Programming/Units)
180	+	- [[Unit not found - How to find units]]
181	+
182	+	---
183	+	From the Free Pascal wiki, page "Unit", as it stood on 2022-01-02 after a change by Kai Burghardt, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).

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