<?xml version="1.0" encoding="utf-8"?>

<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->

<!-- This component and the accompanying materials are made available under the terms of the License 

"Eclipse Public License v1.0" which accompanies this distribution, 

and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->

<!-- Initial Contributors:

    Nokia Corporation - initial contribution.

Contributors: 

-->

<!DOCTYPE concept

  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

<concept id="GUID-27340D18-A31D-512E-920A-B06C784A978A" xml:lang="en"><title>String

Pools </title><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>Many systems, components and applications deal with pre-defined, well known

string constants. For example, parsing and manipulating text containing structure

mostly requires comparisons against standard string constants. </p>

<p>In a complex system, composed of a large number of objects, there may also

be a need to pass strings between objects, and to route processing depending

on the value of some string. The implementation of the HTTP transport framework

and the XML framework are examples within the Symbian platform where such

intense string handling is required. </p>

<p>To improve efficiency, the Symbian platform uses the idea of the string

pool. </p>

<p>A string pool is referenced through an <xref href="GUID-4BF6ECD3-5FB3-3A6A-B9CE-98871BD0812B.dita"><apiname>RStringPool</apiname></xref>,

which is a handle like object. The following diagram illustrates the basic

idea: </p>

<fig id="GUID-756DB8BA-1D6F-56AE-9C93-66DD3512B4A7">

<title>           String pool representation          </title>

<image href="GUID-7F8F6ABB-8F46-5486-A116-6965787BC539_d0e223118_href.png" placement="inline"/>

</fig>

<p>A string pool is a mechanism for storing strings in a particular way that

makes the comparison of strings a very fast operation. It is particularly

efficient at handling strings that can be set up at program compile time.

For example, strings that identify lexical elements in a structured text.

Typically, they are well known strings that are likely to be used very often

in a given context. </p>

<p>Such strings are organised into tables, and each string within a table

can be referenced by an index value, which can be symbolised by an enum. Such

tables are referred to as <b>static string tables</b> (See <xref href="GUID-27340D18-A31D-512E-920A-B06C784A978A.dita#GUID-27340D18-A31D-512E-920A-B06C784A978A/GUID-8221E235-15F9-55E9-8C17-4015051F70CD">Static string tables</xref>). The basic algorithm used internally ensures

that the pool contains only one string of any particular value, and uses a

reference counting mechanism to keep track of usage. </p>

<p>The advantages of representing string constants in such a way are: </p>

<ul>

<li id="GUID-DE544092-1FA8-5537-8357-62510F42D2EA"><p>avoiding a proliferation

of duplicate strings throughout a component or an application; typically there

is one string pool per thread, and one copy of a string </p> </li>

<li id="GUID-55E5FD27-5E2B-51D6-91AA-435AE1A4421C"><p>allowing string constants

to be represented by integer values </p> </li>

<li id="GUID-3C5CA539-11E1-53EB-8A1F-69AEF0D9C1F0"><p>allowing strings to

be passed between objects by passing integer values, wrapped in a class (any

one of the <xref href="GUID-C117EB69-0B1E-3D16-88E1-B44349C716DE.dita"><apiname>String</apiname></xref> and <xref href="GUID-B886F0AA-DB9C-356C-9B96-33252820F93E.dita"><apiname>RStringF</apiname></xref> classes) </p> </li>

<li id="GUID-559BCA58-B426-5919-A948-DC15ED501397"><p>allowing strings to

be compared by comparing the integer values. </p> </li>

</ul>

<p>Internally, a string pool uses hash tables to reference strings. </p>

<p>Static string tables and string constants can be added dynamically to the

string pool, for example, at run time. However, there is always a performance

penalty while adding either a static or a dynamic string to the string pool

as the hash tables need to be updated. This means that it is better to add

static string tables at string pool initialisation time, as this is the best

time to absorb the overhead. </p>

<section id="GUID-B2F3D713-6BCA-4A47-99E4-EC4886F857EA"><title>Key features of string pool</title> <ul>

<li id="GUID-39F73513-7F37-57DA-AF7A-D3A7B2B28A76"><p>The string pool as supplied

by the Symbian platform supports any strings that can be represented

by a <xref href="GUID-445B19E5-E2EE-32E2-8D6C-C7D6A9B3C507.dita"><apiname>TDes8</apiname></xref> descriptor; this includes ASCII or UTF-8 encoded

strings. Note that a <xref href="GUID-445B19E5-E2EE-32E2-8D6C-C7D6A9B3C507.dita"><apiname>TDes8</apiname></xref> type can represent any type

of data , including binary data, and means that a string pool can easily handle

the extended characters of another encoding. </p> </li>

<li id="GUID-8F9BA578-B6B5-54FD-8C29-8C2996AED4D0"><p>Within the string pool,

strings are of two types - case sensitive and case insensitive. This affects

the way strings are compared. Case insensitivity implies that strings are

folded for the purpose of comparison. </p> </li>

<li id="GUID-D958763A-70F8-5452-A556-99F92B2B0246"><p>A string pool can contain

up to 4,096 static string tables, and each table can represent up 26,2144

strings. </p> </li>

</ul> </section>

<section id="GUID-8221E235-15F9-55E9-8C17-4015051F70CD"><title>Static string

tables</title> <p>Static string tables are defined and built at compile time.

They are represented by a <xref href="GUID-91322CD6-6DA0-3868-A5D7-CA561A102074.dita"><apiname>TStringTable</apiname></xref> object. A string

table can be added to the string pool by passing the string table reference

to a call to: </p> <codeblock id="GUID-2FAB371D-74A4-5D7E-99F1-B7C02BEDA8D5" xml:space="preserve">void RStringTable::OpenL( const TStringTable&amp; aTable );</codeblock> <p>The following diagram illustrates a general picture. Note that the strings

in any given string table are deemed to be either case sensitive or case insensitive,

and this governs how comparisons are made. </p> <fig id="GUID-D7CEA86C-0643-5BDA-A60E-D80B97008BF2">

<image href="GUID-ECACB935-AD19-5BE6-AC27-A63465244087_d0e223225_href.png" placement="inline"/>

</fig> <p>As the name implies, a static string table is declared as a <codeph>const

          TStringTable</codeph> data member of a class with a user-defined

name. The class name is defined in a header file while the table itself is

implemented in a <codeph>.cpp</codeph> C++ source file. Both the header file

and the C++ source file are normally included in the project definition. Typically,

a set of enum values are also defined within the scope of the class, and each

value is associated with the strings in the table; code in other parts of

the program access the strings in the string pool using these enum values. </p> <p>The

Perl script, <codeph>stringtable.pl</codeph>, located in <filepath>...\syslibs\bafl\stringtools\</filepath>,

can be used to generate these <codeph>.cpp</codeph> and <codeph>.h</codeph> files

from a simple text definition. The text definition file simply lists the strings

and the enum symbols to be associated with them; the file itself is given

a <codeph>.st</codeph> file type. </p> <p>Following is a simple example of <codeph>ExampleStringTable.st</codeph> file: </p> <codeblock id="GUID-5D487C02-D710-55E6-BD8D-769BE4C4E754" xml:space="preserve"># Example String Table

fstringtable ExampleStringTable

!// Some types of fruit

# This comment won't appear in the .h file, but the one above will.

EApple apple

EOrange orange

EBanana banana

# Some animals

ECat cat

EDog dog

</codeblock> <p>The main points to note are: </p> <ul>

<li id="GUID-8FCB19B1-7033-5120-98DD-08EECA91C65B"><p>the keyword <i>fstringtable</i> is

used to define the name of the class that contains the string table declaration

and the enum value symbols. The class name itself follows the keyword. For

example, <codeph>ExampleStringTable</codeph>. </p> <p>Note that you can include

underscore characters in the class name. For example, <codeph>Example_StringTable</codeph>. </p> </li>

<li id="GUID-0E714FA5-859E-597F-9B33-2F0DC3372FCD"><p>the symbols <codeph>EApple</codeph> and <codeph>EOrange</codeph> form

the enum value symbols that correspond to the strings <codeph>apple</codeph> and <codeph>orange</codeph> respectively. </p> </li>

<li id="GUID-F9F62AD7-5209-5D01-9F60-413C7E161689"><p>all statements starting

with a <b>#</b> are comments and are completely ignored. However <b>#</b> characters

can appear in a string. For example <codeph>ap#ple</codeph> is a valid string

and is not interpreted as a comment. </p> </li>

<li id="GUID-F70C2B1A-2029-5B1A-A27D-A65A123A6CA9"><p>all statements starting

with a <b>!</b> are comments that are inserted into the generated header file. </p> </li>

</ul> <p>Running the Perl script with <codeph>ExampleStringTable.st</codeph> as

source generates the header file <codeph>ExampleStringTable.h</codeph> and

the C++ source file <codeph>ExampleStringTable.cpp</codeph> as illustrated

below: </p> <codeblock id="GUID-7E0263D1-D4A5-5B0A-B355-C2D958247590" xml:space="preserve">// Autogenerated from epoc32\build\generated\example\ExampleStringTable.st by the stringtable tool - Do not edit

#ifndef STRINGTABLE_ExampleStringTable

#define STRINGTABLE_ExampleStringTable

#include "StringPool.h"

struct TStringTable;

/** A String table */

class ExampleStringTable 

       {

    public:

           enum TStrings

              {

              // Some types of fruit

              /** apple */

              EApple,

              /** orange */

              EOrange,

              /** banana */

              EBanana,

              /** cat */

              ECat,

              /** dog */

              EDog

              };

       static const TStringTable Table;    

       };

#endif // STRINGTABLE_ExampleStringTable

</codeblock> <codeblock id="GUID-6447B06C-00EB-527C-8381-7239F2220D61" xml:space="preserve">// Autogenerated from epoc32\build\generated\example\ExampleStringTable.st by the stringtable tool - Do not edit

#include <e32std.h>

#include "StringPool.h"

#include "StringTableSupport.h"

#include "ExampleStringTable.h"

#ifdef _DEBUG

#undef _DEBUG

#endif

_STLIT8( K1, "apple" );

_STLIT8( K2, "orange" );

_STLIT8( K3, "banana" );

_STLIT8( K4, "cat" );

_STLIT8( K5, "dog" );

// Intermediate

const void * const KStringPointers[] =

       {

       ( const void* )&K1,

       ( const void* )&K2,

       ( const void* )&K3,

       ( const void* )&K4,

       ( const void* )&K5

       };

const TStringTable ExampleStringTable::Table = {5, KStringPointers, EFalse};

</codeblock> <p>The table itself is the static data member <codeph>Table</codeph> of

class <codeph>ExampleStringTable</codeph>. </p> </section>

</conbody><related-links>

<link href="GUID-24D509E8-CF46-58D3-85E9-27DA7AB22012.dita"><linktext>Constructing

a Static                 String Table</linktext></link>

<link href="GUID-2B6D7221-A23E-562E-9D9C-5EC197F1CB78.dita"><linktext>Using string

pools</linktext></link>

</related-links></concept>