Code Conversion Guide

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Delphi to Lazarus Code Conversion Guide

About

This page is about how to convert existing code to work with the Free Pascal Compiler and Lazarus IDE. While Lazarus and the Free Pascal Compiler have aspects in common with Delphi and Kylix, they are not clones. There are a number of library call and convention differences... and in some areas, FPC is enhanced and can be more demanding about correct syntax. Please see the Lazarus For Delphi Users guide for a description of some of the functional differences.

The purpose of this guide is to document some of the specific differences that are frequently encountered during the code conversion process when translating existing code from Delphi to Lazarus.

This document was placed into the wiki knowledge-base area so it could be easily extended by anyone who had encountered a unique problem and would like to post it for others to be aware of.

Selecting a component or library for conversion

Where to find code to convert

There is a LOT of code available on the net that can be converted for use with FPC and Lazarus. Here is a [PageOfCodeSites|] that is just a start. Please add to it if you know of any other good locations. Turbo-Power Software has recently released their entire commercial offering under the MPL. A list of available packages can be found [1].

To avoid duplicating effort, packages that have already been converted are listed on the [CodeAndComponents|] page. If you've converted a package or are working on one, please add a note on the [CurrentProjects|] page.

Licensing

Licenses for existing code range from freeware/public domain to restrictive versions that prohibit modification, re-distribution and commercial use. Before converting any package, it's a good idea to examine its licensing and make sure it's going to be compatible with Lazarus and the Free Pascal Compiler. License selection is especially important with components since dropping one on a form can impose an unwanted or incompatible license on an entire application.

When converting components, please respect the wishes of the original author and retain all copyright and licensing headers along with email addresses and url's. It's courteous and often useful to inform the author that their component is being converted... especially if the component is under a restrictive license. New interest in an old or forgotten component can sometimes inspire authors to revise their original and overly restrictive licensing.

In general, Public Domain (freeware), and the LGPL/MPL are the the most flexible for distributing components. For more information, the Open Source Definition is a good place to start. There are also several comparisons available to help clarify how the various types of licenses work and what impact they'll have on code they're linked to. Search for "open source license comparison"

Dependencies

Another step before starting to work on a conversion is to verify that the code doesn't have deep dependancies on other packages that might not be available or represent a considerable conversion challenge. Some freeware offerings are bound to or extend proprietary packages that are frequently no longer available or come with inappropriate licenses.

Compiler Issues

Currently the stable 1.0.x version of the Free Pascal Compiler doesn't support interfaces, threads or Variant records. If the code you're converting needs these constructs, you'll have to use the 1.1 development branch of the compiler... which isn't always guaranteed to work with Lazarus.

Platform and OS Issues

Lazarus and the Free Pascal Compiler are cross-platform and cross-architecture development tools. In contrast, most existing Delphi code was specifically designed to run on an Intel processor under Win32. If your candidate component has a lot of Win32 specific code, it might be wise to try and find a less platform dependant alternative. But don't let this stop you... it's genuinely amazing what the LCL supports!

Doing the conversion

Setting up the Lazarus environment for a conversion project

Create a test project

  • Place code to be converted into a subdirectory (ie: convertdir)
  • Bring up Lazarus
  • File->Save All to the convertdir subdirectory. Meaningful names for the Project and default unit are optional.
  • Open the "main" unit to be converted in convertdir
  • Add it to the project: Project->Add Active Unit to Project
  • Run Tools->Quick Syntax Check or Run Build All to get started.

Initial items to watch out for

  • Filenames are case sensitive with the 1.0.x series compilers. If you're working with this version, make all your filenames lower case. You'll get "File not found" errors if you don't.

Delphi VCL, Kylix CLX Sources in Lazarus

When converting Delphi/Kylix sources, it is often useful to do a find declaration to see, what a specific function is doing. The Lazarus IDE can parse the Delphi/Kylix sources. To do this it needs some searchpaths and compiler settings. You can easily setup this in Environment->CodeTools Defines Editor->Insert Template

Conversion problems and solutions

Converting Delphi forms/units to Lazarus

Many existing Delphi forms can be converted to work with Lazarus by using the IDE's built in DFM to LFM form converter. It can be found under the Tools menu item as "Convert DFM file to LFM". Bring up the file dialog, select the dfm and the converter will do the rest.

If you need to convert the whole unit (with or without a form), Lazarus also includes a built in "Convert Delphi unit to Lazarus unit" which will do the following for you -

  1. renames the .pas and .dfm file to lowercase.
  2. converts .dfm file to .lfm file (currently without content check, just format)
  3. creates an empty .lrs file (the content will be created later)
  4. adds
    {$mode delphi}
    directive
  5. replaces windows unit with LCLIntf
  6. adds LResources unit if needed
  7. removes variants unit
  8. removes
    {$R *.dfm}
    directive
  9. adds initialization section and
    {$i unit.lrs}
    directive

This allows quick and easy conversion of most units from Delphi format to Lazarus format. It does not do any validity check, or automatic syntax changes, so any syntax changes you need to make, additional unit/unit name changes, or dfm/pas changes for control/component differences you must still do manually, though in time some wizards to help facilitate some of this, especially the repairing of converted forms(lfm), may become available.

Selecting the right compiler mode

The [FreePascal|] Compiler supports 5 different pascal modes. For example TP for turbo pascal, lets you compile turbo pascal units. There is also a DELPHI compatibility mode that can be set to make existing code easier to convert. Lazarus prefers the OBJFPC mode, which almost the DELPHI mode, but less ambigious than the Delphi syntax. Here are the important points:

The mode can be selected at command line or at the start of the source. Using the command line has the advantage, that you don't need to change the source, but the disadvantage, that others must be told.

Most Delphi units can be compiled by the Free Pascal compiler by adding

{$IFDEF FPC}
  {$MODE DELPHI}
{$ENDIF}

right after the unit name.

For more details about Free Pascal modes see the Free Pascal Documentation

Cross-Platform considerations

  • Inline assembler is always a problem because it will bind the code to the Intel architecture. Some developers do algorithm prototypes in Pascal and ifdef the their optimized assembler. Fortunately Turbo-Power did this in numerous places with their code. If this is the case with the package you're converting, throw the switch back to Pascal.
  • Don't reference specific memory location like the BIOS data area. Find out what the code needs and try to find a cross platform alternative.
  • Don't do processor specific tricks (like using the Intel TSC) without enclosing your code in an ifdef for the platform the code needs... and providing an alternative for environments that don't have the hardware capability.
  • If you need some OS specific code, than you can use IFDEFs. See below for a list of macros.

Useful compiler variables

To write code, that behaves on different systems differently, you can use the

{$IFDEF Name}

directives.

  • {$IfDef LCL}

This variable is defined, when using the LCL package. Useful to write code, that works with the LCL and Delphi.

  • {$IfDef FPC}

This variable is defined, when using the FPC compiler. Useful to write code, that works with FPC and Delphi.

  • {$IfDef Unix}
    ,
    {$IfDef Win32}
    , ...

Defined by FPC for the current Target OS. Delphi defines "Linux", "Win32" and "MSWindows". Free Pascal runs on much more platforms and so it is recommended to use the more general items. For example "Unix" is defined for Linux, FreeBSD, NetBSD and OpenBSD, where Lazarus already runs. Use

{$IfDef Linux}
  {$Define Unix}
{$EndIf}

to work around this for Kylix.

For more details see the Free Pascal Documentation.

Finding a missing identifier

There are differences in how the LCL is organized when compared to the Delphi VCL. If you get a "not found" compiler error about a major class or identifier, the chances are good that it's in a different unit. A complete cross reference can be found by grep'ing lazarus/docs/xml or the lcl subdirectory.

For example the commonly used tbutton typically throws an error in Delphi code because it's located in a unit named buttons.pp. The following command finds the correct unit very quickly (in the lazarus source directory):

 grep -in ' tbutton =' lcl/*


Major unit differences between Lazarus and Delphi

    • Please add to this topic! **
  • Windows->LCLIntf, LCLType, LCLProc, VCLGlobals, ...)

As the LCL is not windows specific, the code that is in the Delphi Windows unit for directly accessing the Win32 API is abstracted into seperate interfaces, which can be accessed from the LCLIntf unit. Keep in mind, that Lazarus does not emulate win32, so many functions are missing and some do not work as their win32 counterparts. These functions only exist for Delphi compatibility and should only be used for quick & dirty porting. LCL also breaks out many of the types, so often LCLType, and sometimes VCLGlobals are required. LCLProc also contains a few functions which can be usefull for lower level handling such as "FreeThenNil" as is in Delphi 5 and higher, "DeleteAmpersands" to remove additional ampersands from a string for controls(& vs && etc).

  • Messages->LMessages

TControl Messages for win32 event callbacks of the format WM_CALLBACK and the structs associated with them are often found in the Messages unit in Delphi. In the LCL these types of messages and there structs are usually found in LMessages, usually with name changes of WM to LM, so for instance WM_MOUSEENTER becomes LM_MOUSEENTER, and TWMMouse becomes TLMMouse.

  • Graphics, Controls->GraphTypes, GraphMath, Graphics, Controls

To simplify some things and break complexity of circles between units, a few types have been abstracted into a shared unit called GraphType, which includes things, which in Delphi are located in Graphics or Controls, for instance the bvNone etc of panels. So sometimes you have to include it. Also a unit which, although incompatible with Delphi, adds other usefull functionality is GraphMath, which adds a TFloatPoint for precision, misc routines for dealing with beziers, lines, and arcs, as well as some operator overloading for use with TPoints and TRect, such as for instance Point1 := Point2 + Point3, and comparing two rects like if (rect1 = rect2) then ...

  • Mask->MaskEdit

For more intelligent naming considerations, the unit for TMaskEdit is called [MaskEdit|] instead of the slightly more nebulous Mask as in many versions of Delphi.

  • StdCtrls->StdCtrls,Buttons

In many version of Delphi TButton is located in StdCtrls, while TSpeedButton and TBitBtn are in Buttons. For consistency and simplicity the LCL puts all button types in Buttons, which can occasionally break code conversion, so it is always a good idea to include.


Property and method differences Delphi -> FPC/LCL

  • TBitmap contains a canvas in the LCL

Syntax differences

Please add to this topic!

Because of the inherent strictness in FPC, some syntax changes are necessary, even though !!;dir {$Mode Delphi}!!;! does allow more lazyness like Delphi does. For this reason complying as much with the syntax rules of !!;dir {$Mode ObjFPC}!!;! as possible is highly recommended, even when the codebase is still going to be shared between Delphi and the LCL. Some of these are simply better coding practises, and sometimes because occasionally Delphi mode is not entirely accurate, or in a few instances Delphi acceptible code does not function as expected with FPC, even though it might compile. To that end even though not all such are strictly required, the following list of changes should be considered mandatory :

Variables initialization in FPC 1.0.x

With Delphi it is possible (and quite common) to initialize a variable in its declaration, however this is not possible in FPC 1.0.x, const must be used instead, or prefereably in many situations, initialized elsewhere at some point before the variable is used (like unit initialization).

For example -

Delphi FPC
var
MyObject: TSomeObject=
nil
;
//More code...
implementation
//More code...
initialization
//More code...
end
.
var
MyObject
:
TSomeObject;
//More code...
implementation
//More code...
initialization
MyObject:=
nil
;
//More code...
end
.
When assigning an event handling entry point, prefix it with an "@"

For instance, you might assign a button callback manually

Delphi FPC
begin
if not
Assigned(MyButton.OnClick)
then
MyButton.OnClick:= SomeFunction;
//@ not required
//more code...
end
;
begin
if not
Assigned(MyButton.OnClick)
then
MyButton.OnClick:= @SomeFunction;
//@ IS required
//more code...
end
;
When calling a procedure variable use this syntax
theprocname()

In Delphi there is no difference between a procedure and a procedure variable, however there is in FPC, so to call a procedure, even if it has no paramaters, you must append parenthesis. For Example -

Delphi FPC
With
(SomeObject)
do begin
If
Assigned(OnMyCallback)
then
OnMyCallback;
//parenthesis not required
end
;
With
(SomeObject)
do begin
If
Assigned(OnMyCallback)
then
OnMyCallback();
//parenthesis required
end
;
When accessing values in a pointer to a record you must dereference first

In Delphi it is not required to dereference a pointer to a record to acces values within it, it can in fact be treated just like the record itself, or any other object. In FPC it must be first dereferenced. As an example,

Delphi FPC
Function
GetSomeValue(ARecord: PMyRecord)
:
Integer;
begin
If
Assigned(ARecord)
then
Result
:=
ARecord.SomeValue
else
Result:=
0
;
end
;
Function
GetSomeValue(ARecord: PMyRecord)
:
Integer;
begin
If
Assigned(ARecord)
then
Result
:=
ARecord^.SomeValue
else
Result:=
0
;
end
;
When accessing chars of an indexed string Property of an object, it must be enclosed in parenthesis

With Delphi it is possible to treat a Property exactly like some other const or var, even to accessing for instance individual chars of a string directly, while this is not always possible in FPC, specifically for indexed properties. Instead it must be enclosed in parenthesis, to make distinct. While this may not always hold true it is probably a good practice to consider anyway. For example

Delphi FPC
Type
TSomeComponent=
class
(TComponent)
//More code...
Published
Property
MyString:
String
index
3
read
GetMyString;
//More code...
End
;
var
MyChar
:
char;
begin
If
Length(MyString)>
2
then
//no parenthesis needed
MyChar:= MyString[
3
];
//More code...
end
;
Type
TSomeComponent=
class
(TComponent)
//More code...
Published
Property
MyString:
String
index
3
read
GetMyString;
//More code...
End
;
var
MyChar
:
char;
begin
If
Length(MyString)>
2
then
//parenthesis sometimes needed
MyChar:= (MyString)[
3
];
//More code...
end
;


You must typecast pointers to actual type when using with var or function of that type

Sometimes in Delphi you will have a null pointer variable representing an object. While it might seem a complex situation, it is oddly quite common especially in large component packs as a method of preventing too many circular includes between objects in different units. In Delphi it is then possible to send this null pointer to a function expecting that object, without bothering to typecast to actual type, in fpc you must typecast.

For example -

Delphi FPC
Unit 1
Type
TSomeObject=
class
(TComponent)
//More code...
End
;
Procedure
DoSomething(Value: TSomeObject);
Function
GetSomeObject: TSomeObject;
Unit 2
Type
TSomeComponent=
class
(TComponent)
//More code...
Published
SomeObject: Pointer
;
//More code...
End
;
Application
var
MyComponent: TSomeComponent
;
begin
MyComponent.SomeObject
:=
GetSomeObject;
//More code...
DoSomething(MyComponent.SomeObject)
;
end
;
Unit 1
Type
TSomeObject=
class
(TComponent)
//More code...
End
;
Procedure
DoSomething(Value: TSomeObject);
Function
GetSomeObject: TSomeObject;
Unit 2
Type
TSomeComponent=
class
(TComponent)
//More code...
Published
SomeObject: Pointer
;
//More code...
End
;
Application
var
MyComponent: TSomeComponent
;
begin
MyComponent.SomeObject
:=
Pointer(GetSomeObject);
//More code...
DoSomething(TSomeObject(MyComponent.SomeObject))
;
end
;

Resources

Delphi resource files are win32 specific and not compatible with Lazarus, so you'll have to recreate and compile them using the lazres. Lazres can be found in the lazarus/tools subdirectory. If you've downloaded the Lazarus sources, you'll need to compile it first.

  • cd lazarus/tools
  • make install

To add a resource to your application:

  • lazres myresource.lrs mypix.xpm anotherpix.xpm
  • Add the LResources unit to your Uses clause
  • Include the .lrs file you created under the initialization block

Example:

function
TForm1.LoadGlyph(
const
GlyphName:
String
): TBitMap;
begin
Result:= TPixmap.Create
;
Result.LoadFromLazarusResource(GlyphName)
;
end
;
//More code...
begin
Speedbutton1.glyph:= LoadGlyph('mypix')
;
//More code...
end
;
initialization
{$I unit1.lrs}
{$I myresource.lrs}
end
.

Getting Help

If you encounter a problem during conversion that you just can't solve, there are a wide variety of places to get help. For pure Object Pascal and FPC issues, the best place to start is the Free Pascal Documentation by Michaël Van Canneyt and Florian Klämpfl. For more Lazarus oriented problems, the Lazarus Project Documentation in the Lazarus-CCR Knowledgebase [Main Page] is the next place to look. Finally you can post a question on any of the mailing lists for the Free Pascal Compiler or the FPC forums where a lot of experts are subscribed.

There are some outstanding search and knowledge bases online that can also be a great help for learning new techniques and solving problems. Tamarack Associates operates a fast search engine specifically for the Borland usenet archives. Mer Systems Inc. provides a similar search engine. Another outstanding source of information along with a sitewide search capability is Earl F. Glynn's Computer Lab and Reference Library.

Packaging and Releasing your component

Creating a Lazarus package for your component(s)

Creating a package makes installing the code you've converted a much easier process... especially if you're providing more then one component. Mattias Gärtner has written an overview of Lazarus Packages that should be read before beginning this process.

Documentation

The purpose of this site and the wiki format is to make the generation of professional documentation an easy and quick process. The wiki also makes it possible to see the results of your posting immediately and make any changes you'd like in real time.

Using the Lazarus-CCR wiki to create nice looking documentation is very easy. If you've never used wiki markup before, you can get familiar with it in the Sand Box practice area.

Creating a Code Release Page

The Code Release Page contains vital information about your component that a potential downloader will need to know, such as license, intended platform, status (alpha, beta, stable...), where to download it, who wrote it, is support available... etc.

The following procedure will let you create a Code Release Page with your browser:

  • Edit the Code And Components page and add a project name wiki link entry for your component in the "Released Components" section. Save the modified page.
  • Go to the Component Release Template, select all and copy. Hit the back button on your browser to return to the Code And Components page.
  • Click on your new wiki component name entry and paste the release template into the blank edit box.
  • Edit the template accordingly and hit save.
  • Do edit-saves until your document looks the way you want it to.

Creating a Comment Page for your component

When your component is posted to the Sourceforge File Release System, the Lazarus-CCR admins will lock your code release page to prevent the unlikely event of someone changing your license, name or other release info.

While you're building your code release page, you have the option to create another wiki link to a "news and comments" page for your component that will always be writable. This is called a ComponentCommentTemplate. The link to it should be added to the bottom of the code release template where there's a topic heading and a brief howto. The wiki name link should be your wiki component name with a Comments suffix like EpikTimer Comments or JpegLazarus Comments . The comments page will remain writable to collect feedback from users and for you to post updated information.

A comment page can be added like this:

  • Edit your component release page and add the wiki comment link (in the format described above). Save the modified page.
  • Go to the Component Comment Template , select all and copy. Hit the back button on your browser to return to your component release page.
  • Click on the comment entry entry you created and paste the comment template into the edit box.
  • Edit the template accordingly and hit save.
  • Do edit-saves until your comment page looks the way you want it to.

While the Lazarus-CCR admins initially lock code release pages, any member of the project can use their shell acount to unlock, edit and re-lock their pages.

Submitting the component

If you're a release technician on the project, upload your component to the SourceForge File Release System and add it to the list of release packages. Otherwise send it to one of the project administrators and we'll add it to the repository. We'll also put it into CVS so you'll continue to have access to it.

Contributors and Changes

This page has been converted from the epikwiki version.

  • Initial version by Tom Lisjac and Mattias Gärtner - 9/22/2003 VlxAdmin
  • Moved Getting help from the main page. T. Lisjac - 9/24/2003 VlxAdmin
  • Added documentation templates, procedure and links. 9/25/2003 VlxAdmin
  • LCLLinux was renamed to LCLIntf, Jesus Reyes, 9/27/2003
  • added more information on Unit changes, AndrewJohnson 9/27/2003
  • Updated Syntax differences, including some examples, AndrewJohnson 9/27/2003
  • FPC 1.0.x doesn't support interfaces, Vincent Snijders 9/28/2003
  • Fixed some of the examples per new WikiWord definition, 9/28/2003 VlxAdmin
  • Made code more consistant to remove last accidental Pascal WikiWord definitions, AndrewJohnson 9/27/2003
  • Use tables for code examples for nice blocks, and easy side by side view of Delphi->FPC differences, AndrewJohnson 10/17/2003
  • Use pascal stylesheet to make example code more readable, AndrewJohnson 10/18/2003