Unit

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A unit is a source code file (or the binary compiled from that file) which was written using the Pascal programming language, and that is designed to be a single module in an application or an object module.

Aim

A unit may be used where certain functionality is to be provided to an application program or to other units, and allowing code that performs that functionality to be created once and used in many places. This can reduce the possibility of errors and increase the chance of code reusage.

A binary unit may be used where a unit author wishes to provide certain functionality to be used in a Pascal program but does not wish to provide the source code that performs that functionality.

Units were also used on older versions of Pascal when it was nececessary on computers with limited resources to be able to load routines as needed rather than keeping every routine of the executable program in memory all of the time. A unit that needs to access e. g. procedures and data types in another unit must specify those units it needs to access in a Uses statement but linking is done without the need to write a makefile as in C.

A unit may also be used to declare a series of global constants or variables for use by the entire application, without actually containing any executable code. This is similar to the common keyword in the Fortran Programming Language.

Format

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 (but doing nothing) is:

unit minimalunit;
interface
	(* here comes stuff that the unit publicly offers *)
implementation
	(* here comes the implementation of offered stuff and *)
	(* optionally internal stuff (only known in the unit) *)
end.

where the part after interface corresponds to the public-part of other languages and implementation does so to private.

A more advanced basic structure is

unit advancedunit;
interface

implementation

initialization
	(* here may be placed code that is *)
	(* executed as the unit gets loaded *)

finalization
	(* code executed at program end *)

end.

The optional initialization and finalization blocks may be followed by code that is executed on program start/end.

Described unit-example

A step-by-step example:

unit randomunit;
(* this unit does something *)

(* public *)
interface

type
	(* the type TRandomNumber gets globally known *)
	(* since it is included somewhere (uses-statement) *)
	TRandomNumber = integer;

(* of course the const- and var-blocks are possible, too *)

(* a list of procedure/function signatures makes *)
(* them useable from outside of the unit *)
function getRandomNumber(): TRandomNumber;

(* an implementation of a function/procedure *)
(* must not be in the interface-part *)

(* private *)
implementation

var
	(* var in private-part *)
	(* => only modifiable inside from this unit *)
	chosenRandomNumber: TRandomNumber;

function getRandomNumber(): TRandomNumber;
begin
	(* return value *)
	getRandomNumber := chosenRandomNumber;
end;

(* initialization is the part executed *)
(* when the unit is loaded/included *)
initialization
begin
	(* choose our random number *)
	chosenRandomNumber := 3;
	(* chosen by fair-dice-roll *)
	(* guaranteed to be random *)
end;

(* finalization is worked off at program end *)
finalization
begin
	(* this unit says 'bye' at program halt *)
	writeln('bye');
end;
end.

To include a unit just use the uses-statement.

program chooseNextCandidate;
uses
	(* include a unit *)
	randomunit;

begin
	writeln('next candidate: no. ' + getRandomNumber());
end.

Note that fpc checks on compiling this program for unit-dependencies. It has to be able to find the unit ”randomunit”. The simplest way is a unit which name matches the filename it is written in (appending .pas is no problem). The unit-file may be in the same directory where the program-source is in or in the unit-path FPC looks for units.

Read more

• Unit scope (FPC html doc)
• Compiling a unit (FPC html doc)