Re: comp.lang.pascal.delphi.misc miniFAQ (full version)
From: Joe (joen_at_spamintelcom.si)
Date: 04/05/04
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Date: Mon, 5 Apr 2004 20:11:01 +0200
My name should be mentioned for the inspiration of your faq publishing
name.
btw: you have "grown" taller since the last lesson. keep up the good work!
"Evil MiniFAQ Boss" <miniFAQ@mfw.dds.nl> wrote in message
news:406b75c4$0$568$e4fe514c@news.xs4all.nl...
> FAQ FOR NEWSGROUPS: COMP.LANG.PASCAL.DELPHI.MISC,
ALT.COMP.LANG.BORLAND-DELPHI
>
> Version: 2003-10-07
> First maintainer: Esther Michaels
> Current maintainer: Maarten Wiltink (Evil miniFAQ Boss)
> Posting schedule: Short version every Sunday, full version every first of
> the month
>
> Copyright 2003 the current maintainer, all rights reserved except the
right
> to re-distribute the current document. Even better: give out the URL.
>
> --------------------------------------------------------------------------
------
>
> Latest version:
> http://www.bancoems.com/CompLangPascalDelphiMisc-MiniFAQ.htm
> Plain text version:
> http://www.bancoems.com/CompLangPascalDelphiMisc-MiniFAQ.txt
> Examples page:
> http://www.bancoems.com/mini_faq_examples.htm
>
> --------------------------------------------------------------------------
------
>
> CONTENTS
> 1. How do I start another program from my program?
> 2. How to stop a second instance of my program from executing? (Also:
HPrevInst
> doesn't seem to work, why?)
> 3. Floating-point work - i.e. with single, real, real48, double,
extended -
> gives long decimals or wrong numbers. Why?
> 4. Is there a limit on the number of characters allowed in a tMemo?
> 5. Why do I get an "abstract error" when I try to use a tStrings variable?
> 6. How do I access a component/property/field of Form1 from Form2?
> 7. How do I change the background, font, general appearance of an entry in
a
> tListView, tTreeView, tListBox, tCombobox list, tStringGrid, etc.?
> 8. How do I change the formatting of a line/word/character in a tMemo?
> 9. Why does the debugger show my For loop variable counting down, not up?
> 10. How do I get the path of my executable? And, how do I retrieve the
command
> line parameters?
> 11. When my program is in a loop it doesn't respond to user input or
update its
> display. Can I change this?
> 12. How do I access the compilation date in my program?
> 13. How do I access the VersionInfo tab's data at runtime?
> 14. Does Delphi have an equivalent to the VB Split function?
> 15. I create a label/edit/some other control at run-time but it doesn't
appear
> on the form. Why?
> 16. How do I use streams?
> 17. How do I declare an event handler for an object that I create at
runtime?
> 18. When using the TRegistry component under Windows NT, Windows 2000, or
> Windows XP, a user with less than Administrator rights is unable to
> access information stored in the HKEY_LOCAL_MACHINE hive. How can I
> work around this?
> 19. When drawing text on a canvas, how do I determine the resulting text
size?
> 20. How can I loop through the components/labels/edit-boxes on my form?
(Also:
> Tag is unused, what is it good for?)
>
> USEFUL LINKS
>
> --------------------------------------------------------------------------
------
>
> 1. How do I start another program from my program?
>
> The most common methods of doing this employ either ShellExecute or
> CreateProcess, both of which are documented in the WindowsSDK help
> file. ShellExecute is quite versatile and has the advantage of being
> simple and able to open or print any file for which there is a file
> association registered on the system. CreateProcess is a little more
> complex but provides much better control over the process. Information
> returned by CreateProcess can be used to pause a program until the
> called program has completed (using WaitForSingleObject). See
> http://www.bancoems.com/mini_faq_examples.htm for example code showing
> how to call each of these procedures.
>
> --------------------------------------------------------------------------
------
>
> 2. How to stop a second instance of my program from executing? (Also:
> HPrevInst doesn't seem to work, why?)
>
> HPrevInst only works in Delphi 1. There are several methods of handling
> this in Delphi 2 and above. The most common recommendation is to use a
> mutex (see the Windows SDK help). See
> http://www.bancoems.com/mini_faq_examples.htm for example code.
>
> --------------------------------------------------------------------------
------
>
> 3. Floating-point work - i.e. with single, real, real48, double,
extended -
> gives long decimals or wrong numbers. Why?
>
> Because computers are finite, all floating point formats are
approximations.
> Values that have no exact representation are simply replaced by the value
> closest to it. It's like replacing "1/3" by "0.3333", and the computer has
> no thoughts about what that number means. Multiplying it by 3 again gives
> 0.9999, not 1. Also, Trunc(0.9999) will return 0, not 1.
>
> Comparing floating point numbers should not be done by testing for exact
> equality, but for a difference being smaller than the inaccuracy in the
> computed numbers. Errors start larger for shorter types and grow further
> as more calculations are performed with inexact numbers.
>
> If you must have precision use the Currency type (4 decimals) or use an
> integer type and scale the results. When it's just a question of
displaying
> a result, use one of the formatting functions that allow you to specify
> the number of decimal places to display - Format and FormatFloat. See
> http://www.efg2.com/Lab/Library/Delphi/MathInfo for more info on this and
> other math related questions.
>
> --------------------------------------------------------------------------
------
>
> 4. Is there a limit on the number of characters allowed in a tMemo?
>
> Yes. The limit depends on the operating system and can be as low as 32KB.
> If you need to have more than 32KB in a memo, use a tRichEdit instead.
> Set the rich edit's PlainText property to True and its MaxLength property
> to an appropriately large value, e.g. RichEdit1.MaxLength := MaxInt - 2;
> allows the RichEdit1 to work with ~2GB. (Setting MaxLength to 0 as
> suggested for tCustomEdit in the help doesn't work for rich edits.)
>
> --------------------------------------------------------------------------
------
>
> 5. Why do I get an "abstract error" when I try to use a tStrings variable?
>
> You should never create a tStrings object. Instead create one of its
> descendants like tStringList.
>
> --------------------------------------------------------------------------
------
>
> 6. How do I access a component/property/field of Form1 from Form2?
>
> Add the Form1 unit to the Uses clause in the implementation section of
Form2.
> (By default there is no uses clause so you may have to add one - do so
just
> after the '{$R *.DFM}' line.) You can then reference items belonging to
> Form1 by prefixing them with 'Form1.', e.g.,
> Form1.Label1.Caption := 'Hi there, I was set from Form2';
>
> --------------------------------------------------------------------------
------
>
> 7. How do I change the background, font, general appearance of an entry in
a
> tListView, tTreeView, tListBox, tCombobox list, tStringGrid, etc.?
>
> You have to use the owner draw event (typically OnDraw<something>) to
> draw the items. This means that you have to draw each item when the
> control asks you to do so. See
> http://www.bancoems.com/mini_faq_examples.htm for example code.
>
> --------------------------------------------------------------------------
------
>
> 8. How do I change the formatting of a line/word/character in a tMemo?
>
> You can't. Use a tRichEdit instead.
>
> --------------------------------------------------------------------------
------
>
> 9. Why does the debugger show my For loop variable counting down, not up?
>
> The Delphi optimizer may generate loop code that decrements an internal
> counter instead of your variable. This behavior is guaranteed not to alter
> the correctness of your program. Ignore the debugger value and consider
> the For loop variable "unavailable due to optimization".
>
> --------------------------------------------------------------------------
------
>
> 10. How do I get the path of my executable? And, how do I retrieve the
> command line parameters?
>
> Application.ExeName will return the full name of your executable. You can
> get the path portion using ExtractFilePath. ParamStr(0) will return the
> same value as Application.ExeName. To retrieve other command line
arguments
> use the ParamCount and ParamStr functions. See also FindCmdLineSwitch and
> CmdLine in the help.
>
> --------------------------------------------------------------------------
------
>
> 11. When my program is in a loop it doesn't respond to user input or
update
> its display. Can I change this?
>
> Yes. In your loop you need to add a call to the
Application.ProcessMessages
> method. This will allow your application to process Windows messages,
> including those generated by user actions. There are two significant
> caveats. First, since Windows messages often translate into calls to
> event handlers your program may begin to do things at inappropriate
> times. Make sure that the user can't initiate actions that will interfere
> with the loop while the loop is active. In particular, note the following
> sentence, taken from Delphi 3's help file on TApplication.Terminated:
> "For applications using calculation-intensive loops, call
> Application.ProcessMessages periodically, and also check
> Application.Terminated to determine whether or not to abort
> the calculation so that the application can terminate."
> The second caveat is that calling Application.ProcessMessages can be
> relatively expensive and may slow the program. In a fast (tight) loop
> you may not want to call the method on each iteration. If you only want
> to update the display and not handle user input you can use the Update
> method (Delphi 3 and up) of the control covering the part of the display
> you want to update. Remember that this will also slow down the loop!
>
> --------------------------------------------------------------------------
------
>
> 12. How do I access the compilation date in my program?
>
> There are a number of ways to achieve this, depending on the architecture
> of your program. If the program consists of a single executable whose
> creation date is guaranteed never to change after compilation, then the
> following code will be sufficient:
> DateToStr(FileDateToDateTime(FileAge(Application.ExeName))).
> This returns a string containing the date of the .exe file, formatted
> using the host computer's current date format setting. However, these
> circumstances don't hold for most programs. In these cases one solution
> is to create a file, perhaps called Today.pas, which holds a single
> constant: const COMPILE_DATE = '2001-11-17'; This file should be updated
> daily, perhaps at boot time. In units of your program which need to refer
> to the compilation date, you add the file to the "uses" clause, and then
> simply refer to the constant as you would any other. For example, an About
> box might contain a TEdit whose job is to display the compilation date:
> Edit1.Text := COMPILE_DATE; This will then show the compilation date in
> your About box.
>
> --------------------------------------------------------------------------
------
>
> 13. How do I access the VersionInfo tab's data at runtime?
>
> See http://www.bancoems.com/mini_faq_examples.htm for example code and a
> component that does this.
>
>
> --------------------------------------------------------------------------
------
>
> 14. Does Delphi have an equivalent to the VB Split function?
>
> There is no direct equivalent. However one can use the CommaText property
> of a tStrings to achieve the same result.
>
> Example:
>
> procedure Split(const aString, Delimiter: string; Results: tStrings);
> begin
> Results.CommaText:='"' + StringReplace(aString, Delimiter, '","',
[rfReplaceAll]) + '"';
> end;
>
> (Note: the Results parameter is declared as tStrings. If you are using a
> variable as the actual parameter, (as opposed to a VCL control's property,
> e.g. tMemo.Lines, tListBox.Items, and tComboBox.Items), see question 5.)
>
> --------------------------------------------------------------------------
------
>
> 15. I create a label/edit/some other control at run-time but it doesn't
> appear on the form. Why?
>
> The most common cause of this problem is failure to assign, or correctly
> assign, the Parent property. When one creates a control at run-time the
> Owner (passed as a parameter to the constructor) should in almost all
> circumstances be the form. Once the control has been constructed one must
> then assign it a Parent. In most circumstances the Parent should also be
> the form. However, if one wants the control to be on a Panel or in a
> GroupBox then the Panel/GroupBox control should be made the Parent.
>
> --------------------------------------------------------------------------
------
>
> 16. How do I use streams?
>
> Alan Lloyd has written an excellent introduction to streams in the form of
> a help file, available from http://www.bancoems.com/mini_faq_examples.htm.
>
> --------------------------------------------------------------------------
------
>
> 17. How do I declare an event handler for an object that I create at
runtime?
>
> Declare the event handler with the appropriate type signature (e.g. an
> OnClick handler will be of type TNotifyEvent), then once you have created
> the object simply assign the handler to the OnClick property of the
object,
> like this: MyRuntimeButton.OnClick := MyRuntimeButtonClickHandler; See
> the examples file at http://www.bancoems.com/mini_faq_examples.htm for an
> example.
>
> --------------------------------------------------------------------------
------
>
> 18. When using the TRegistry component under Windows NT, Windows 2000, or
> Windows XP, a user with less than Administrator rights is unable to access
> information stored in the HKEY_LOCAL_MACHINE hive. How can I work around
this?
>
> The problem is caused by the fact that TRegistry (and the derived
TRegIniFile)
> always opens a key with KEY_ALL_ACCESS privileges, even if only KEY_READ
would
> be needed.
>
> In Delphi 5+, you can specify the access you require using the Access
property.
>
> You can also avoid this by going back to using the WinAPI registry
functions
> (RegOpenKey et. al.), or in Delphi versions that don't already have it,
add an
> Access property to a new class derived from TRegistry. Alternatiely, the
new
> class could just always open keys read-only.
>
> --------------------------------------------------------------------------
------
>
> 19. When drawing text on a canvas, how do I determine the resulting text
size?
>
> Create a bitmap and allocate a font to its canvas. To get the height of
> the font elements, call the WinAPI function GetTextMetrics using the
> bitmap's handle. Use TCanvas.TextWidth to get character widths. Some
> printing terms are used in GetTextMetrics:
>
> * Ascent is the pixel height of characters, including any diacritics,
> above the base height.
> * Descent is the pixel amount below the baseline, in letters like "y" and
"g".
> * Internal leading (pronounced "ledding") is how much higher diacritics go
> than capitals.
> * External leading is the minimum empty space between two lines above
> each other: between descent for the top line and capitals or diacritics
> for the bottom line.
>
> So the vertical margins in a TEdit are TEdit.Height - (Ascent + Descent),
> but this dimension is neither constant nor the same as External Leading.
>
> All values from GetTextMetrics will be in pixels. A call to GetDeviceCaps
> can tell you how many pixels there are in an inch on your output device.
>
> --------------------------------------------------------------------------
--- > > 20. How can I loop through the components/labels/edit-boxes on my form? > (Also: Tag is unused, what is it good for?) > > Loop through the Components array property of the form. To process only > one kind of component, test the class of each using the "is" operator. > To indicate special processing for any particular component, you can set > its Tag property in the Object Inspector. > > If you use Tag to signify that a component should be treated specially > in your loop, use zero to mean that it should be ignored, to prevent > surprises later when you may add components to the form. Tag is declared > as LongInt and its default value is zero. If a 4-byte LongInt is not > enough for your purposes, you can store an object reference or a pointer > to a record in in, but then be careful that you know what you are doing. > > See http://www.bancoems.com/mini_faq_examples.htm for example code. > > -------------------------------------------------------------------------- ------ > > Links > > These links may prove useful. Inclusion here should not necessarily be taken > as an endorsement. > > Newsgroups Etiquette > http://www.uwasa.fi/~ts/http/tsfaq.html > Borland Delphi Community - TI, FAQ, links, articles > http://community.borland.com/delphi/0,1419,1,00.html > Borland Delphi Newsgroups > http://newsgroups.borland.com/cgi-bin/dnewsweb?cmd=listall&group=borland.public.delphi.&utag= (but first read the posting guidelines, at http://www.borland.com/newsgroups/guide.html) > Google - Newsgroups archives > http://groups.google.com/groups?hl=en&lr=&safe=off&group=comp.lang.pascal.delphi.misc > http://groups.google.com/ > The Delphi Bug List - bugs and gotchas > http://www.jrsoftware.org/buglist/bugsall.htm > Undu - Delphi articles, tips, techniques, links > http://www.undu.com/ > Deborah Pate's Pages - Word, Excel, etc. automation > http://www.djpate.freeserve.co.uk/Automation.htm > Kyle Cordes BDE Alternatives Guide > http://www.kylecordes.com/bag/index.html > Delphi Printing Info and Links > http://www.efg2.com/Lab/OtherProjects/PrinterProjects.htm > BDE Support Page > http://www.bdesupport.com/ > Microsoft MSDN - Win API documentation > http://msdn.microsoft.com/ > Project JEDI - open source code library > http://www.delphi-jedi.org/ > Delphi Super Page (DSP) - components, links > http://delphi.icm.edu.pl/ > http://community.borland.com/homepages/dsp > Torry's Delphi Pages - components, links > http://www.torry.net/ > ICS (Internet Component Suite) - internet components > http://overbyte.delphicenter.com/frame_index.html > Indy (Internet Direct, was Winshoes) - internet components > http://www.nevrona.com/Indy > The Tutorial Times - links to tutorials > http://www.tutorialtimes.com/ > Timo Salmi's Pascal programming material (a little dated but good general Pascal knowledge) > http://www.uwasa.fi/~ts/http/http2.html#programming > Martin Harvey's tutorial on multi-threading > http://www.pergolesi.demon.co.uk/prog/threads/ToC.html > Henry Bartlett's page on Delphi-related newsgroups > http://www.hotkey.net.au/~hambar/habit/newsgroups.htm > > -------------------------------------------------------------------------- ------ > > VERSION HISTORY > > 2003-Oct-07: Updated copyright notice (with thanks to John Stockton). > 2003-Sep-21: HTML tidied. Added copyright notice. Removed dead link from Q12. > Rephrased Q20. > 2003-Jul-29: Example date in Q12 changed to an unambiguous value. Reordered > and slightly changed Q18. Rephrased Q19 and added note about units. > 2003-Jun-27: Rephrased Q3, again. Added Q18 (with thanks to Dan Donoghue), > Q19 (with thanks to Alan Lloyd), and Q20 (with thanks to Henry > Bartlett). Added a link to Henry Bartlett's Delphi newsgroups page. > Reordered document. Removed introduction. > 2002-Nov-16: Rephrased Q3. > 2002-Jul-27: Reformatted into HTML. > 2002-May-10: Minor corrections, added links to Borland Delphi newsgroups > and posting guidelines. > 2002-Apr-21: Added Q17. > 2002-Apr-11: Minor corrections, added link to Martin Harvey's multi-threading > tutorial. > 2002-Apr-06: Added Q16 (with thanks to Alan Lloyd for the referenced file) > 2002-Jan-13: Added Q14 and Q15 (with thanks to Bruce Roberts), and updated > introduction to fit with abbreviated-posting policy. > 2001-Nov-22: Added Q13 (with thanks to Daniel Rutten). > 2001-Nov-11: Added Q12 (with thanks to John Stockton). > 2001-Oct-30: Updated Links section. > 2001-Apr-26: Added reference to FindCmdLineSwitch and CmdLine in Q10. > 2001-Apr-25: Formatted to 80 characters per line, added preamble, made > html version. > 2001-Apr-04: Initial version (kindly contributed by Bruce Roberts). > > -------------------------------------------------------------------------- ------ > > END
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