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The Free Component Library (FCL) consists of a collection of units, providing components (mostly classes) for common tasks. It intends to be compatible with Delphi's Visual Component Library (VCL), but the FCL is restricted to non-visual components. On the other hand, the FCL also goes beyond the VCL.

See Free Component Library for the current development status and an overview of the available components (though this seems inconsistent with Reference for 'fcl' in Lazarus). You can also check the source repository. Note that there are also some platform-specific files in the FCL.

After the example, you can find a list of some FCL components.


To use an FCL component you need to include its name in a uses clause of your program or unit (see example below). The default compiler configuration is set up to search the fcl directory for such units. You can also set the appropriate search path with a command-line compiler option of the form -Fu<path-to-fcl-units>.


Currently (future 2.2.2+) the FCL is divided into several subpackages:

  • fcl-base The base units
  • fcl-async Asynchronous I/O (serial?)
  • fcl-db Generic database support + prepackaged drivers
  • fcl-fpcunit Unit testing framework
  • fcl-image Raster Image reading and writing (aka fpimage)
  • fcl-json Routines for javascript object streaming
  • fcl-net Network related units
  • fcl-passrc Pascal language parsing and transformation
  • fcl-process Process controll
  • fcl-registry Registry
  • fcl-web Helper for web development
  • fcl-xml XML (DOM) unit and related units.
  • paszlib A Pascal conversion (thus without dependancies) of the standard zlib library.
  • fpvectorial Library to read, modify and write vectorial images


Currently, the FCL is not documented (feel free to contribute; also take a look at Reference for 'fcl'). For Delphi compatible units, you could consult the Delphi documentation. You can always take a look at the source files in the source repository.


The following program illustrates the use of the class TObjectList in the FCL unit Contnrs (providing various containers, including lists, stacks, and queues):

<delphi> program TObjectListExample;

  Classes, { from RTL for TObject }
  Contnrs; { from FCL for TObjectList }

   TMyObject = class(TObject)  { just some application-specific class }
     FName: String; { with a string field }
     constructor Create(AName: String); { and a constructor to create it with a given name }
     property Name: String read FName; { and a property to read the name }

constructor TMyObject.Create(AName: String);
  inherited Create;
  FName := AName;

  VObjectList: TObjectList; { for a list of objects; it is a reference to such a list! }

  VObjectList := TObjectList.Create  { create an empty list }
  with VObjectList do
    Add(TMyObject.Create('Thing One'));
    Writeln((Last as TMyObject).Name);
    Add(TMyObject.Create('Thing Two'));
    Writeln((Last as TMyObject).Name);

This program must be compiled in an object-oriented mode, such as -Mobjfpc or -Mdelphi.

FCL Components

This is not an exhaustive list (to avoid duplication of effort). It only mentions some important components, or components that are otherwise not easy to find.

Base classes for Object Pascal extensions in Delphi mode
Some common container classes
Unit testing framework (based on Kent Beck's unit testing framework, e.g. cf. JUnit), documenting article about FPCUnit
XMLRead, XMLWrite and DOM 
Detailed at the XML Tutorial