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<concept id="GUID-E81D72B2-BA77-5F4E-8742-3812A60A4DAC" xml:lang="en"><title>How to use modifiable buffer descriptor — TBuf&lt;TInt&gt;</title><shortdesc>Modifiable buffer descriptors are useful for holding strings

or data and providing safe ways to access and modify that data.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<ul>

<li id="GUID-F276E9FE-B839-54ED-9F50-8E45CB094922"><p>For text data,

it is usual to construct a <codeph>TBuf&lt;TInt&gt;</codeph> type and

allow the appropriate variant, either a <codeph>TBuf8&lt;TInt&gt;</codeph> or a <codeph>TBuf16&lt;TInt&gt;</codeph> to be selected at build time.</p> </li>

<li id="GUID-9860DB57-62C3-5295-B00E-5F629771FED9"><p>For binary data,

an explicit <codeph>TBuf8&lt;TInt&gt;</codeph> is used.</p> </li>

<li id="GUID-34C95FAB-0F58-5B61-B9FC-A31588855F96"><p>It is rare to

use an explicit <codeph>TBuf16&lt;TInt&gt;</codeph>.</p> </li>

</ul>

<p>Although, the following notes refer to the build independent types;

they are equally valid for the explicit 8 bit and 16 bit types.</p>

<section id="GUID-824524FB-98B7-4CDA-A3FD-233C7C0DDD5F"><title>Constructing

a TBuf&lt;TInt&gt;</title> <p>A modifiable buffer descriptor can be constructed

in a number of ways:</p> <ul>

<li id="GUID-BCFAA769-D038-5A09-B6BF-586C93F08B0B"><p>as an empty

buffer descriptor.</p> </li>

<li id="GUID-9DE61358-D6B9-54CA-9FED-11AC29443093"><p>as an empty

buffer descriptor but giving it a length.</p> </li>

<li id="GUID-50FB7967-79C8-5F8C-8569-36B40D14894E"><p>by copying data

from any other type of descriptor.</p> </li>

<li id="GUID-26E1FDDC-F0BB-5441-8F81-EF2CFA61787A"><p>by copying data

from another modifiable buffer descriptor of the same size.</p> </li>

</ul> <p>The following code fragment constructs a <codeph>TBuf&lt;16&gt;</codeph> object. The buffer descriptor is uninitialised, i.e. it contains

no data. The assignment operator or the <codeph>Copy()</codeph> function

can be used to put data into the buffer descriptor after construction:</p> <codeblock id="GUID-01CA390C-ECC4-56C2-A1F6-F62C2AA75E57" xml:space="preserve">_LIT(KText,"Hello World!");

...

TBuf<16> buf1; // length of buf1 is 0

...

buf1 = KText;  // data assigned</codeblock> <p>The source descriptor

is a literal which is converted to descriptor type.</p> <p>The following

code fragment constructs a <codeph>TBuf&lt;16&gt;</codeph> object and

sets it length to 12. No data is assigned into the descriptor.</p> <codeblock id="GUID-FA8A1FE8-7809-5E28-B3EB-420B405DDC32" xml:space="preserve">...

TBuf<16> buf1(12); // length of buf1 is 12

...</codeblock> <p>The following code fragment constructs a <codeph>TBuf&lt;16&gt;</codeph> object, initialised with the 12 characters making

up the English language phrase "Hello World!".</p> <codeblock id="GUID-74B4614D-4273-5F9E-8A2D-1E9A8A3960D8" xml:space="preserve">_LIT(KText,"Hello World!");

...

TBuf&lt;16&gt; buf1(KText);</codeblock> <p>The following code fragment

constructs a <codeph>TBuf&lt;16&gt;</codeph> object from another <codeph>TBuf&lt;16&gt;</codeph> object. This is, in effect, copy construction.</p> <codeblock id="GUID-BE55436F-FDB8-5768-947F-801D02FDA631" xml:space="preserve">_LIT(KText,"Hello World!");

...

TBuf<16> buf1(KText);

TBuf&lt;16&gt; buf2(buf1);   // buf2 constructed from the data in buf1</codeblock> <p>In both of these cases, the resulting length of the descriptor

is 12.</p> <p>A non-modifiable buffer descriptor can also be constructed

from 'C' style zero terminated string. However, this is rarely necessary

but may make it easier to port legacy 'C' code.</p> </section>

<section id="GUID-8A534041-1730-43FB-8FCD-4C5BFDAAE730"><title>Replacing

data</title> <p>Data within a modifiable buffer descriptor can be

completely replaced through the assignment operator or by using the <codeph>Copy()</codeph> function.</p> <codeblock id="GUID-D990C115-DD84-5E04-89DA-F483A63D03FA" xml:space="preserve">_LIT(KText,"Hello World!");

_LIT(KNewText,"New text");

_LIT(KReplaced,"Replaced");

...

TBuf<16> buf1(KText);

TBuf<16> buf2;

...

buf2 = buf1;               // buf2 now contains "Hello World!"

...

buf2 = KNewText;           // buf2 now contains "New text".

                           // the literal is converted to a descriptor

                           // type.

buf2.Copy(KReplaced);      // buf2 content replaced using Copy()</codeblock> </section>

<section id="GUID-E49444E0-457E-4645-A1D5-C350FF998F9A"><title>Accessing

and changing data</title> <p>Once a modifiable buffer descriptor has

been constructed, the functions in the base classes, <codeph>TDesC</codeph> and <codeph>TDes</codeph>, are available to be access and change

the data.</p> <codeblock id="GUID-386035F7-1C0F-5CB3-BA5B-5E0EF18DF6F8" xml:space="preserve">_LIT(KText,"Hello World!");

...

TBufC<16> buf1(KText);

...

buf1.Length();</codeblock> <p>and</p> <codeblock id="GUID-7BCC4118-4294-5607-A92B-5DBA9E68CBEE" xml:space="preserve">_LIT(KText,"Hello World!");

...

TBufC<16> buf1(KText);   // length is 12 

...

buf1.Delete(6,6);        // length is now 6, leaving "Hello" in

                         // the buffer</codeblock> </section>

<section id="GUID-970BCE5F-CC4D-4F60-91A9-6107975E9C3F"><title>Illegal

access causing an exception</title> <p>The following code fragment

raises a panic because of an attempt to assign too much data. The

maximum length of the buffer descriptor is 16 but the length of the

data to be copied is 31:</p> <codeblock id="GUID-38DC46A1-E2A0-52DD-A821-F0D6B2C45430" xml:space="preserve">_LIT(KText,"Hello World! The sun is shining");

...

TBufC&lt;16&gt; buf1(KText);</codeblock> <p>The following code fragment

raises a panic because of an attempt to delete data outside the boundary

defined by the descriptor:</p> <codeblock id="GUID-71FB3EFC-BFBA-5072-ADD2-3471E2A08D0C" xml:space="preserve">_LIT(KText,"Hello World!");

...

TBufC<16> buf1(KText);

buf1.Delete(99,1);</codeblock> </section>

</conbody><related-links>

<link href="GUID-7CB11EAD-260E-551A-85F1-FEAC975FE722.dita">

<linktext>Literal Descriptors</linktext></link>

</related-links></concept>