|
1 <?xml version="1.0" encoding="utf-8"?> |
|
2 <!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. --> |
|
3 <!-- This component and the accompanying materials are made available under the terms of the License |
|
4 "Eclipse Public License v1.0" which accompanies this distribution, |
|
5 and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". --> |
|
6 <!-- Initial Contributors: |
|
7 Nokia Corporation - initial contribution. |
|
8 Contributors: |
|
9 --> |
|
10 <!DOCTYPE concept |
|
11 PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
|
12 <concept id="GUID-5829247E-2E8E-502F-9051-A59DA8EE71B0" xml:lang="en"><title>CBase-derived |
|
13 classes and two-phase construction</title><shortdesc>This document describes the need for two-phase construction and |
|
14 how to use second phase constructors.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody> |
|
15 <p>When the construction of an object cannot leave (except for out-of-memory |
|
16 for the allocation of the object itself), then it is appropriate to use the |
|
17 conventional C++ constructor, which is automatically invoked by <codeph>new</codeph>. </p> |
|
18 <p>When the construction of an object may leave, then the object must be pushed |
|
19 to the clean-up stack, or a pointer to the object must be stored in an object |
|
20 that would be cleaned up if a leave occurred, <i>before</i> any part of the |
|
21 construction function is invoked that may leave. To allow this to happen, |
|
22 construction steps that can leave, are performed not in the C++ constructor, |
|
23 but in another initialization functions, referred to as a second phase constructor.</p> |
|
24 <p>Thus, the general sequence for two-phase construction is:</p> |
|
25 <ol id="GUID-4EA8A66C-A0FA-58D4-BD86-7FD72029AA17"> |
|
26 <li id="GUID-38D6D313-2348-5E1A-95BB-205AF451CC5D"><p>allocate memory for |
|
27 the object (and leave if out of memory) using <codeph>new</codeph> </p> </li> |
|
28 <li id="GUID-1BC50131-75C9-5EFF-9439-030FB35750F9"><p>optionally define a |
|
29 C++ constructor to perform any construction that cannot leave</p> </li> |
|
30 <li id="GUID-DA1E047E-40E6-5B71-BB02-C42C7B3609C8"><p>push a pointer to the |
|
31 object, or store a pointer to it in a class with cleanup support</p> </li> |
|
32 <li id="GUID-6FDA6D4A-9ECD-509D-9D0D-3E7234961657"><p>use the second phase |
|
33 constructor to perform any part of the construction that might leave</p> </li> |
|
34 </ol> |
|
35 <p>Note that:</p> |
|
36 <ul> |
|
37 <li id="GUID-C67CA4EB-480A-5A5B-A4F5-B7206DBD3CFE"><p>The whole sequence is |
|
38 usually encapsulated in static member functions called <codeph>NewL()</codeph>, |
|
39 and <codeph>NewLC()</codeph> (which additionally leaves the created object |
|
40 on the clean-up stack).</p> </li> |
|
41 <li id="GUID-B0AC323F-EDF5-5BEE-B843-A46E5611B1F3"><p>Abstract classes are |
|
42 not intended to be instantiated, and so have no <codeph>NewLC()</codeph> or <codeph>NewL()</codeph>, |
|
43 only a second phase constructor.</p> </li> |
|
44 <li id="GUID-557838D0-10D0-55B3-9848-254065EFDFF4"><p>Step 2 is optional, |
|
45 because all such construction can be performed by the second phase constructor. |
|
46 The C++ constructor is only necessary when the class is immediately derived |
|
47 from a base class whose default C++ constructor cannot be used. In this case, |
|
48 the derived class must call the base class’s constructor with appropriate |
|
49 parameters.</p> </li> |
|
50 <li id="GUID-93707F75-0BB3-563F-B00C-35683DEE007C"><p>It is conventional for |
|
51 the second phase constructor to be called <codeph>ConstructL()</codeph>. </p> </li> |
|
52 </ul> |
|
53 </conbody></concept> |