Strings - memory usage
lazarus / 9-Editor

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LazaruX · 2009-02-27 18:27

Hi, the Pascal language has got the following string types:

Code: [Select]var
   Str1 : Char;        // Holds a single character, small alphabet
   Str2 : WideChar;    // Holds a single character, International
alphabet
   Str3 : AnsiChar;    // Holds a single character, small alphabet
   Str4 : ShortString; // Holds a string of up to 255 Char's
   Str5 : String;      // Holds strings of Char's of any size desired
   Str6 : AnsiString;  // Holds strings of AnsiChar's any size desired
   Str7 : WideString;  // Holds strings of WideChar's of any size
desired

My question is:
does a shortstring consume less memory than a string?

My doubt is that a shortstring reservs the space for a 255 charachter
string in memory, so even if its of 2 chars, still 255 chars were
reserved in memory.
But a string can be of any size, so if a string is like this:
"thestring"
Would it reserve it just the space for 9 chars or for more??

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Marc · 2009-02-27 19:36

A short string allocates always 256 bytes. You can check this with
Code: [Select]WriteLn(SizeOf(ShortString));
An AnsiString allocates at least 8 bytes (reference count + size) +
the chars in your string. Further each reference to it holds a
pointer.

Besides this, when assigning, ShortSting := Shortstring, the contents
is copied and you use 512 bytes. For a AnsiString := AnsiString you
have your string once, and 2 pointers pointing to it (and a refcount
of 2)

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LazaruX · 2009-02-28 21:52

Sooo...(I am not sure I have understood correctly)...
If I need a string who's size is
not defined in the beginning
that uses less memory
that is able to store at least 256 charachters (more are not needed)

I should use a: ??

shortstring
or a string??

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Marc · 2009-03-01 13:19

AnsiString is uses less memory.

Note that I write AnsiString, since depending on the compiler mode, a
String is either a ShortString ($mode fpc or objfpc) or AnsiString
($mode objfpc or delphi)

edit: corrected objfpc mode

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Vincent Snijders · 2009-03-02 09:01

In $mode objfpc string is shortstring too.

That is why the Lazarus unit template doesCode: [Select]{$mode
objfpc}{$H+}to enable 'string means ansistring'.

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Marc · 2009-03-02 10:58

You're right, I forgot that we put the $H+ in each unit