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2 | <!-- --> |
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3 | <!-- [History] --> |
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4 | <!-- Converted to DocBook: Katsuya Amako, Aug-2006 --> |
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5 | <!-- Changed by: Katsuya Amako, 4-Aug-1998 --> |
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6 | <!-- Changed by: Dennis Wright, 27-Nov-2001 --> |
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7 | <!-- Proof read by: Joe Chuma, 30-Jun-1999 --> |
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8 | <!-- --> |
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9 | <!-- ******************************************************** --> |
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10 | |
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11 | <!-- ******************* Section (Level#1) ****************** --> |
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12 | <sect1 id="sect.ObjPers"> |
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13 | <title> |
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14 | Object Persistency |
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15 | </title> |
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16 | |
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17 | <!-- ******************* Section (Level#2) ****************** --> |
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18 | <sect2 id="sect.ObjPers.PersG4"> |
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19 | <title> |
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20 | Persistency in Geant4 |
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21 | </title> |
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22 | |
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23 | <para> |
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24 | Object persistency is provided by Geant4 as an optional category, |
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25 | so that the user may run Geant4 with or without an object database |
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26 | management system (ODBMS). |
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27 | </para> |
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28 | |
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29 | <para> |
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30 | When a usual (transient) object is created in C++, the object is |
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31 | placed onto the application heap and it ceases to exist when the |
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32 | application terminates. Persistent objects, on the other hand, live |
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33 | beyond the termination of the application process and may then be |
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34 | accessed by other processes (in some cases, by processes on other |
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35 | machines). |
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36 | |
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37 | <figure id="fig.ObjPers_1"> |
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38 | <title> |
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39 | Persistent object. |
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40 | </title> |
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41 | <mediaobject> |
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42 | <imageobject role="fo"> |
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43 | <imagedata fileref="./AllResources/Detector/persistency.src/pobject.jpg" |
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44 | format="JPG" contentwidth="7.0cm" align="center" /> |
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45 | </imageobject> |
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46 | <imageobject role="html"> |
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47 | <imagedata fileref="./AllResources/Detector/persistency.src/pobject.jpg" |
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48 | format="JPG" align="center" /> |
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49 | </imageobject> |
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50 | </mediaobject> |
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51 | </figure> |
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52 | </para> |
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53 | |
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54 | <para> |
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55 | C++ does not have, as an intrinsic part of the language, the |
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56 | ability to store and retrieve persistent objects. Geant4 provides |
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57 | an abstract framework for persistency of hits, digits and |
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58 | events. |
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59 | </para> |
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60 | |
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61 | <para> |
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62 | Two examples demonstrating an implementation of object |
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63 | persistency using one of the tools accessible through the available |
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64 | interface, is provided in |
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65 | <literal>examples/extended/persistency</literal>. |
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66 | </para> |
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67 | |
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68 | </sect2> |
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69 | |
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70 | <!-- ******************* Section (Level#2) ****************** --> |
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71 | <sect2 id="sect.ObjPers.Reflex"> |
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72 | <title> |
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73 | Using Reflex for persistency of Geant4 objects |
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74 | </title> |
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75 | |
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76 | <para> |
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77 | Object persistency of Geant4 objects is also possible by the |
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78 | mean of the <ulink url="http://cern.ch/seal-reflex/"> |
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79 | Reflex library</ulink>. |
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80 | Reflex provides, in a non-intrusive way, reflection capabilities to C++ |
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81 | classes by generating "dictionary information" for them. Those |
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82 | dictionaries can then be loaded in memory allowing direct |
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83 | persistency of the given objects without any instrumentation of the |
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84 | code. The Reflex library is also part of |
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85 | <ulink url="http://root.cern.ch/"> |
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86 | ROOT</ulink> (since release <literal>v5.08</literal>). |
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87 | </para> |
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88 | |
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89 | <para> |
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90 | The basic steps that one needs to do in order to use Reflex with |
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91 | ROOT I/O for arbitrary C++ classes is: |
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92 | |
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93 | <orderedlist spacing="compact"> |
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94 | <listitem><para> |
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95 | Generate the dictionary for the given classes using the |
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96 | <literal>genreflex</literal> tool from ROOT (this usually is done by adding |
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97 | the appropriate command to the makefile) |
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98 | </para></listitem> |
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99 | <listitem><para> |
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100 | Add initialization of ROOT I/O and loading of the generated |
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101 | dictionary for the given classes in the appropriate part of the |
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102 | code |
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103 | </para></listitem> |
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104 | <listitem><para> |
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105 | Whenever the objects to be persistified are available, call the |
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106 | <literal>WriteObject</literal> method of <literal>TFile</literal> with |
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107 | the pointer to the appropriate object as argument (usually it is some sort |
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108 | of container, like <literal>std::vector</literal> containing the collection |
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109 | of objects to be persistified) |
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110 | </para></listitem> |
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111 | </orderedlist> |
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112 | </para> |
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113 | |
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114 | <para> |
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115 | The two examples (<literal>P01</literal> and <literal>P02</literal>) provided |
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116 | in <literal>examples/extended/persistency</literal> |
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117 | demonstrate how to perform object persistency using the |
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118 | Reflex mechanism in ROOT I/O for storing hits and geometry description. |
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119 | </para> |
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120 | |
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121 | |
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122 | </sect2> |
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123 | </sect1> |
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