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<!DOCTYPE concept
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<concept id="GUID-BD649ABC-4DBC-5E37-B80D-2BB86F94F26A" xml:lang="en"><title>How to
use the non-modifiable buffer descriptor — TBufC&lt;TInt&gt;</title><shortdesc>Non-modifiable buffer descriptors are useful for holding constant
strings or data and providing safe ways to access that data.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<ul>
<li id="GUID-74A075D5-195D-5BB7-BC40-D683635BE6F7"><p>For text data, it is
usual to construct a <codeph>TBufC&lt;TInt&gt;</codeph> type and allow the appropriate
variant, either a <codeph>TBufC8&lt;TInt&gt;</codeph> or a <codeph>TBufC16&lt;TInt&gt;</codeph> to
be selected at build time.</p> </li>
<li id="GUID-0AA8DD41-55DF-5448-A5D5-DC403E60C3A4"><p>For binary data, an
explicit <codeph>TBufC8&lt;TInt&gt;</codeph> is used.</p> </li>
<li id="GUID-441D5170-8C00-5BE6-AF0C-44DB149E87F1"><p>It is rare to use an
explicit <codeph>TBufC16&lt;TInt&gt;</codeph>.</p> </li>
</ul>
<p>Data cannot be changed through this descriptor although it can be replaced
using the assignment operators.</p>
<p>By using the <codeph>Des()</codeph> function to construct a <codeph>TPtr/TPtr8/TPtr16</codeph> modifiable
pointer descriptor for the buffer's data, it becomes possible to modify that
data.</p>
<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-5591AC50-485E-413D-B3A7-BCE59F3B634D"><title>Constructing a TBufC&lt;TInt&gt;</title> <p>A non-modifiable
buffer descriptor can be constructed in a number of ways:</p> <ul>
<li id="GUID-8328A45B-EB36-5C80-9B3D-969DA033B71E"><p>as an empty buffer descriptor.</p> </li>
<li id="GUID-D0CDC7BA-A2A2-5953-8700-F43FF5FD2AE2"><p>by copying data from
any other type of descriptor.</p> </li>
<li id="GUID-A7CC00D3-16AB-5D2F-B25E-011D689C3BA9"><p>by copying data from
another non-modifiable buffer descriptor of the same size.</p> </li>
</ul> <p>The following code fragment constructs a <codeph>TBufC&lt;16&gt;</codeph> object.
The buffer descriptor is uninitialised, i.e. it contains no data. The assignment
operator can be used to put data into the buffer descriptor after construction:</p> <codeblock id="GUID-BED8AC0B-4F0B-5859-994B-723F4B8F84A1" xml:space="preserve">_LIT(KText,"Hello World!");
...
TBufC&lt;16&gt; buf1; // length of buf1 is 0
...
buf1 = KText;   // data assigned</codeblock> <p>The following code fragment
constructs a <codeph>TBufC&lt;16&gt;</codeph> object, initialised with the 12
characters making up the English language phrase "Hello World!".</p> <p>The
source descriptor is a literal which is converted to descriptor type.</p> <codeblock id="GUID-737E0C5F-AE11-586D-B81C-0E3D1AD96414" xml:space="preserve">_LIT(KText,"Hello World!");
...
TBufC&lt;16&gt; buf1(KText);  // length of buf1 is 12</codeblock> <p>The following
code fragment constructs a <codeph>TBufC&lt;16&gt;</codeph> object, initialised
with the content of another <codeph>TBufC&lt;16&gt;</codeph> object.</p> <codeblock id="GUID-A8A21448-DBE4-5F68-8CA0-57298E04036C" xml:space="preserve">_LIT(KText,"Hello World!");
...
TBufC&lt;16&gt; buf1(KText);
TBufC&lt;16&gt; buf2(buf1);  // data copied from descriptor buf1
                       // length of buf2 is 12</codeblock> </section>
<section id="GUID-1C814D9C-5F53-4885-90B9-B83480CF623E"><title>Replacing data</title> <p>Data within a non-modifiable buffer
descriptor can be completely replaced by using the assignment operator:</p> <codeblock id="GUID-C3F90B27-EB1C-5864-A80A-0D0C77BB67FC" xml:space="preserve">_LIT(KText,"Hello World!");
_LIT(KNewText,"New text");
...
TBufC&lt;16&gt; buf1(KText);
TBufC&lt;16&gt; buf2;
...
buf2 = buf1;               // buf2 now contains "Hello World!"
...
buf2 = KNewText;           // buf2 now contains "New text".
                           // the literal is converted to a descriptor
                           // type.</codeblock> <p>To prevent data replacement,
declare <codeph>buf2</codeph> as const.</p> </section>
<section id="GUID-5F3BEAB3-B469-4673-A3D8-04DB697B8D0F"><title>Constructing a modifiable pointer descriptor to change the
data</title> <p>The data contained in non-modifiable buffer descriptor <codeph>TBufC&lt;TInt&gt;</codeph>  <i>can</i> be
changed by constructing a <codeph>TPtr</codeph> modifiable pointer descriptor
using the<codeph>Des()</codeph> member function and then changing the data
through that <codeph>TPtr</codeph>.</p> <p>The maximum length of the <codeph>TPtr</codeph> is
the value of the <codeph>TBufC&lt;TInt&gt;</codeph> template parameter.</p> <p>The
following code fragment shows data being changed:</p> <codeblock id="GUID-02B09EBC-3E58-51B1-8D14-E388250A674A" xml:space="preserve">_LIT(KText,"Hello World!");
_LIT(KExtraText," &amp; Hi");
...
TBufC&lt;16&gt; buf1(KText);
...
TPtr ptr = buf1.Des();
...
ptr.Delete((ptr.Length()-1),1);
ptr.Append(KExtraText);
...</codeblock> <p>This deletes the last character in <codeph>buf1</codeph> and
adds the characters " &amp; Hi" so that <codeph>buf1</codeph> now contains
the text "Hello World &amp; Hi" and its length is 16. Note that the length
of both <codeph>buf1</codeph> and <codeph>ptr</codeph> change to reflect the
data that they now both represent.</p> <p>Note that any attempt to append
more data raises a panic.</p> </section>
<section id="GUID-9DD4E5D1-B0A0-41BC-AE54-FC062D21173C"><title>Accessing data</title> <p>Once a non-modifiable buffer descriptor
has been constructed, the functions in the base class, <codeph>TDesC</codeph>,
are available to access the data.</p> <codeblock id="GUID-CE288FF9-26F7-533D-ADC4-E535035039B2" xml:space="preserve">_LIT(KText,"Hello World!");
...
TBufC&lt;16&gt; buf1(KText);
...</codeblock> <codeblock id="GUID-656150B0-C7DF-54B7-9127-6BE685EEEA83" xml:space="preserve">buf1.Length();</codeblock> </section>
</conbody><related-links>
<link>
<desc><xref href="GUID-7CB11EAD-260E-551A-85F1-FEAC975FE722.dita">Literal Descriptors</xref></desc>
</link>
</related-links></concept>