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<concept id="GUID-9F82E4D3-DDD4-50A5-AB12-50B216B73368" xml:lang="en"><title>How to use the heap descriptor - HBufC</title><shortdesc>Heap descriptors provide a buffer of fixed length, allocated on the heap. Descriptors are useful for holding constant strings or data, when the length of the data may not be known until run time.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
    
    <section id="CONTEXT_361D960801D14A01BDCA84C4583DBC07"><p>Some key points about heap descriptors:</p><ul id="UL_1B2EE288A79A45CDADC061A8AD013AA1"><li id="LI_593B247AD60649A3995B848E524423FF">For text data, it is usual to construct a &lt;code&gt;<codeph>HBufC</codeph>&lt;/code&gt; type and			 allow the appropriate variant, either a &lt;code&gt;<codeph>HBufC8</codeph>&lt;/code&gt; or a <codeph>HBufC16</codeph> to be selected at build time.</li><li id="LI_5FB940C2E42143B9BFE6961D6CD72B86">For binary data, an explicit <codeph>HBufC8</codeph> is used.</li><li id="LI_9D41DDF6F4544E90A79494F10D453DC4">It is rare to use an explicit <codeph>HBufC16</codeph>.</li><li id="LI_0C6D6B9242EC473B8754A321A68A1BB2">Data cannot be changed through this descriptor although it can be
			 replaced using the assignment operators.
</li><li id="LI_FFFA4A8EB811448BB63BED6865555087">If you need to pass an <codeph>HBufC</codeph> to a function that takes a <codeph>TDesC&amp;</codeph> parameter, simply dereference the <codeph>HBufC</codeph> pointer.</li><li id="LI_D010A4CD7E26494EA8AB123A6FD76BEA">If you need to modify the <codeph>HBufC</codeph>'s data, use its <codeph>Des()</codeph> function to construct a <codeph>TPtr/TPtr8/TPtr16</codeph> modifiable pointer descriptor for the buffer's data.</li><li id="LI_1317203B41FC4D1887AC3E4F9E62EEBC">The size of the heap descriptor buffer can be replaced by
			 reallocating it with a new length.
</li></ul><p>Although the following notes refer to the build independent types, they are equally valid for the 8 bit and 16 bit types.</p> 


</section>
    <section id="SECTION_C424B2F2D52D477AA71C739E4B27BBBC">
		<title>Constructing an <codeph>HBufC</codeph></title>
		<p>
		  A heap descriptor can be constructed in one of two ways:
		</p> 

		<ul id="UL_97F558DD94EA45068D1C8B885D092AB1"> 
		  <li id="LI_FAA60C6043CB42BF84558932764C1054"> 
			 <p>
				using the static member functions <codeph>New()</codeph>,
				<codeph>NewL()</codeph> or <codeph>NewLC()</codeph>
			 </p> 
		  </li> 
		  <li id="LI_3A20A3574E404335BFFE058BB3DEA02F"> 
			 <p>
				using the <codeph>Alloc()</codeph>, <codeph>AllocL()</codeph> or
				<codeph>AllocLC()</codeph> functions of an existing descriptor
			 </p> 
		  </li> 
		</ul>

		<p>
		  The following code fragment constructs a heap descriptor which can hold up to 15 data items. The current length is zero.
		</p> 
		<codeblock xml:space="preserve">HBufC* buf;
...
buf = HBufC::NewL(15);</codeblock> 
		<p>
		  The following code fragment constructs a heap descriptor from an existing descriptor. The new heap descriptor is initialised with the content of
		  <codeph>buf1</codeph>, i.e. the string: "Hello World!"
		</p> 
		<p>
		  The source descriptor is a literal which is converted to descriptor type.
		</p> 
		<codeblock xml:space="preserve">_LIT(KText,"Hello World!");
TBufC&lt;16&gt; buf1(KText);
...
HBufC* hptr;
hptr = buf1.AllocL();
...</codeblock> 
	 </section> 

	 <section id="SECTION_B06A1A56ECEE4F02BEB57288C759CF80">
		<title>Replacing data and re-allocating</title>
		<p>
		  Although existing data within a heap descriptor cannot be modified, the
		  assignment operator can be used to replace that data.
		</p> 
		<codeblock xml:space="preserve">_LIT(KText,"Hello World!");
...
HBufC* buf;
...
buf  = HBufC::NewL(15);
*buf = KText;</codeblock> 
		<p>
		  The source descriptor is a literal which is converted to descriptor
		  type.
		</p> 
		<p>
		  To allow more than 15 characters or data items to be assigned into the
		  heap descriptor, it must be reallocated:
		</p> 
		<codeblock xml:space="preserve">buf = buf-&gt;ReAllocL(20);</codeblock> 
		<p>
		  This permits the following assignment to be done without raising a
		  panic:
		</p> 
		<codeblock xml:space="preserve">_LIT(KNewText,"Hello World! Morning");
...
*buf = KNewText;</codeblock> 
		<p>
		  <codeph>buf</codeph> may or may not point to a different location in the
		  heap after reallocation. The location of the reallocated descriptor depends on
		  the heap fragmentation and the size of the new cell. It is always safer to
		  assume that the location changes.
		</p> 
	 </section> 

	 <section id="SECTION_27A2310F89E141159A3DF3169D860CDA">
		<title>Changing data through a modifiable pointer descriptor.</title>
		<p>
		  The data contained by a heap descriptor <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 determined from the size
		  of the cell allocated to the data area of the heap descriptor.
		</p> 
		<p>
		  The following code fragment changes the data in the heap descriptor and
		  the length of the heap descriptor.
		</p> 
		<codeblock xml:space="preserve">TPtr ptr = buf-&gt;Des();
...
ptr.Delete((ptr.Length()-9),9);
ptr.Append(_LIT(" &amp; Hi"));</codeblock> 
		<p>
		  Take particular care if a the heap descriptor is re-allocated after the
		  <codeph>TPtr</codeph> has been constructed. A <codeph>TPtr</codeph> created before
		  re-allocating the heap descriptor is not guaranteed to have a valid pointer
		  after re-allocation. Any attempt to modify data through the original
		  <codeph>TPtr</codeph> after re-allocation may have undefined consequences.
		</p><note>it is a common error to use <codeph>Des()</codeph> to create a
		  <codeph>TDesC&amp;</codeph> reference. While not incorrect, it is simpler and much
		  more efficient to simply dereference the heap descriptor.</note>
		 
	 </section>     
  </conbody></concept>