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  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-5829247E-2E8E-502F-9051-A59DA8EE71B0" xml:lang="en"><title>CBase-derived
classes and two-phase construction</title><shortdesc>This document describes the need for two-phase construction and
how to use second phase constructors.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>When the construction of an object cannot leave (except for out-of-memory
for the allocation of the object itself), then it is appropriate to use the
conventional C++ constructor, which is automatically invoked by <codeph>new</codeph>. </p>
<p>When the construction of an object may leave, then the object must be pushed
to the clean-up stack, or a pointer to the object must be stored in an object
that would be cleaned up if a leave occurred, <i>before</i> any part of the
construction function is invoked that may leave. To allow this to happen,
construction steps that can leave, are performed not in the C++ constructor,
but in another initialization functions, referred to as a second phase constructor.</p>
<p>Thus, the general sequence for two-phase construction is:</p>
<ol id="GUID-4EA8A66C-A0FA-58D4-BD86-7FD72029AA17">
<li id="GUID-38D6D313-2348-5E1A-95BB-205AF451CC5D"><p>allocate memory for
the object (and leave if out of memory) using <codeph>new</codeph> </p> </li>
<li id="GUID-1BC50131-75C9-5EFF-9439-030FB35750F9"><p>optionally define a
C++ constructor to perform any construction that cannot leave</p> </li>
<li id="GUID-DA1E047E-40E6-5B71-BB02-C42C7B3609C8"><p>push a pointer to the
object, or store a pointer to it in a class with cleanup support</p> </li>
<li id="GUID-6FDA6D4A-9ECD-509D-9D0D-3E7234961657"><p>use the second phase
constructor to perform any part of the construction that might leave</p> </li>
</ol>
<p>Note that:</p>
<ul>
<li id="GUID-C67CA4EB-480A-5A5B-A4F5-B7206DBD3CFE"><p>The whole sequence is
usually encapsulated in static member functions called <codeph>NewL()</codeph>,
and <codeph>NewLC()</codeph> (which additionally leaves the created object
on the clean-up stack).</p> </li>
<li id="GUID-B0AC323F-EDF5-5BEE-B843-A46E5611B1F3"><p>Abstract classes are
not intended to be instantiated, and so have no <codeph>NewLC()</codeph> or <codeph>NewL()</codeph>,
only a second phase constructor.</p> </li>
<li id="GUID-557838D0-10D0-55B3-9848-254065EFDFF4"><p>Step 2 is optional,
because all such construction can be performed by the second phase constructor.
The C++ constructor is only necessary when the class is immediately derived
from a base class whose default C++ constructor cannot be used. In this case,
the derived class must call the base class’s constructor with appropriate
parameters.</p> </li>
<li id="GUID-93707F75-0BB3-563F-B00C-35683DEE007C"><p>It is conventional for
the second phase constructor to be called <codeph>ConstructL()</codeph>. </p> </li>
</ul>
</conbody></concept>