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author David Bateman <dbateman@free.fr>
date Thu, 16 Oct 2008 09:20:58 +0100
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1 @c Copyright (C) 2008 David Bateman
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2 @c
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3 @c This file is part of Octave.
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4 @c
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5 @c Octave is free software; you can redistribute it and/or modify it
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6 @c under the terms of the GNU General Public License as published by the
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7 @c Free Software Foundation; either version 3 of the License, or (at
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8 @c your option) any later version.
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9 @c
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10 @c Octave is distributed in the hope that it will be useful, but WITHOUT
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11 @c ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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12 @c FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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13 @c for more details.
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14 @c
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15 @c You should have received a copy of the GNU General Public License
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16 @c along with Octave; see the file COPYING. If not, see
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17 @c <http://www.gnu.org/licenses/>.
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18
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19 @c FIXME
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20 @c For now can't include "@" character in the path name, and so name
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21 @c the example directory without the "@"!!
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22
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23 @macro classfile{class, file}
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24 @example
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25 @group
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26 @verbatiminclude @value{abs_top_srcdir}/examples/\class\/\file\
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27 @end group
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28 @end example
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29 @end macro
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30
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31 @macro polynomialfile{file}
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32 @classfile{polynomial,\file\}
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33 @end macro
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34
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35 @node Object Oriented Programming
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36 @chapter Object Oriented Programming
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37
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38 Octave includes the capability to include user classes, including the
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39 features of operator and function overloading. Equally a user class
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40 can be used to encapsulate certain properties of the class so that
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41 they can not be altered accidentally and can be set up to address the
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42 issue of class precedence in mixed class operations.
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43
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44 This chapter discussions the means of constructing a user class with
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45 the example of a polynomial class, how to query and set the properties
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46 of this class, together with the means to overload operators and
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47 functions.
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48
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49 @menu
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50 * Creating a Class::
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51 * Manipulating Classes::
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52 * Indexing Objects::
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53 * Overloading Objects::
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54 @end menu
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55
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56 @node Creating a Class
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57 @section Creating a Class
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58
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59 We use in the following text a polynomial class to demonstrate the use
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60 of object oriented programming within Octave. This class was chosen as
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61 it is simple, and so doesn't distract unnecessarily from the
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62 discussion of the programming features of Octave. However, even still
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63 a small understand of the polynomial class itself is necessary to
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64 fully grasp the techniques described.
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65
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66 The polynomial class is used to represent polynomials of the form
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67
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68 @example
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69 @iftex
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70 @tex
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71 $a_0 + a_1 x + a_2 x^2 + \ldots a_n x^n$
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72 @end tex
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73 @end iftex
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74 @ifnottex
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75 a0 + a1 * x + a2 * x^2 + @dots{} + an * x^n
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76 @end ifnottex
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77 @end example
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78
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79 @noindent
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80 where
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81 @iftex
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82 @tex
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83 $a_0$, $a_1$, etc are elements of $\Re$.
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84 @end tex
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85 @end iftex
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86 @ifnottex
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87 a0, a1, etc are real scalars.
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88 @end ifnottex
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89 Thus the polynomial can be represented by a vector
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90
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91 @example
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92 a = [a0, a1, a2, @dots{}, an];
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93 @end example
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94
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95 We therefore now have sufficient information about the requirements of
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96 the class constructor for our polynomial class to write it. All object
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97 oriented classes in Octave, must be contained with a directory taking
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98 the name of the class, prepended with the @@ symbol. For example, with
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99 our polynomial class, we would place the methods defining the class in
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100 the @@polynomial directory.
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101
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102 The constructor of the class, must have the name of the class itself
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103 and so in our example the constructor with have the name
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104 @file{@@polynomial/polynomial.m}. Also ideally when the constructor is
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105 called with no arguments to should return a value object. So for example
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106 our polynomial might look like
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107
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108 @polynomialfile{polynomial.m}
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109
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110 Note that the return value of the constructor must be the output of
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111 the @code{class} function called with the first argument being a
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112 structure and the second argument being the class name. An example of
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113 the call to this constructor function is then
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114
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115 @example
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116 p = polynomial ([1, 0, 1]);
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117 @end example
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118
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119 Note that methods of a class can be documented. The help for the
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120 constructor itself can be obtained with the constructor name, that is
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121 for the polynomial constructor @code{help polynomial} will return the
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122 help string. Also the help can be obtained by restricting the search
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123 for the help to a particular class, for example @code{help
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124 @@polynomial/polynomial}. This second method is the only means of
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125 getting help for the overloaded methods and functions of the class.
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126
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127 The same is true for other Octave functions that take a function name
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128 as an argument. For example @code{type @@polynomial/display} will
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129 print the code of the display method of the polynomial class to the
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130 screen, and @code{dbstop @@polynomial/display} will set a breakpoint
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131 at the first executable line of the display method of the polynomial
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132 class.
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133
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134 To check where a variable is a user class, the @code{isobject} and
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135 @code{isa} functions can be used. for example
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136
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137 @example
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138 @group
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139 p = polynomial ([1, 0, 1]);
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140 isobject (p)
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141 @result{} 1
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142 isa (p, "polynomial")
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143 @result{} 1
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144 @end group
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145 @end example
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146
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147 @DOCSTRING(isobject)
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148
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149 @noindent
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150 The available methods of a class can be displayed with the
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151 @code{methods} function.
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152
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153 @DOCSTRING(methods)
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154
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155 @noindent
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156 To enquiry whether a particular method is available to a user class, the
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157 @code{ismethod} function can be used.
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158
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159 @DOCSTRING(ismethod)
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160
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161 @noindent
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162 For example
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163
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164 @example
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165 @group
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166 p = polynomial ([1, 0, 1]);
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167 ismethod (p, "roots")
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168 @result{} 1
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169 @end group
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170 @end example
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171
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172 @node Manipulating Classes
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173 @section Manipulating Classes
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174
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175 There are a number of basic classes methods that can be defined to allow
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176 the contents of the classes to be queried and set. The most basic of
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177 these is the @code{display} method. The @code{display} method is used
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178 by Octave when displaying a class on the screen, due to an expression
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179 that is not terminated with a semicolon. If this method is not defined,
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180 then Octave will printed nothing when displaying the contents of a class.
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181
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182 @DOCSTRING(display)
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183
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184 @noindent
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185 An example of a display method for the polynomial class might be
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186
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187 @polynomialfile{display.m}
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188
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189 @noindent
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190 Note that in the display method, it makes sense to start the method
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191 with the line @code{fprintf("%s =", inputname(1))} to be consistent
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192 with the rest of Octave and print the variable name to be displayed
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193 when displaying the class.
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194
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195 To be consistent with the Octave graphic handle classes, a class
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196 should also define the @code{get} and @code{set} methods. The
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197 @code{get} method should accept one or two arguments, and given one
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198 argument of the appropriate class it should return a structure with
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199 all of the properties of the class. For example
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200
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201 @polynomialfile{get.m}
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202
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203 @noindent
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204 Similarly, the @code{set} method should taken as its first argument an
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205 object to modify, and then take property/value pairs to be modified.
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206
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207 @polynomialfile{set.m}
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208
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209 @noindent
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210 Note that as Octave does not implement pass by reference, than the
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211 modified object is the return value of the @code{set} method and it
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212 must be called like
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213
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214 @example
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215 p = set (p, "a", [1, 0, 0, 0, 1]);
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216 @end example
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217
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218 @noindent
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219 Also the @code{set} method makes use of the @code{subsasgn} method of
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220 the class, and this method must be defined. The @code{subsasgn} method
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221 is discussed in the next section.
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222
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223 Finally, user classes can be considered as a special type of a
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224 structure, and so they can be saved to a file in the same manner as a
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225 structure. For example
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226
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227 @example
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228 @group
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229 p = polynomial ([1, 0, 1]);
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230 save userclass.mat p
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231 clear p
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232 load userclass.mat
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233 @end group
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234 @end example
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235
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236 @noindent
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237 All of the file formats supported by @code{save} and @code{load} are
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238 supported. In certain circumstances, a user class might either contain
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239 a field that it makes no sense to save or a field that needs to be
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240 initialized before it is saved. This can be done with the
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241 @code{saveobj} method of the class
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242
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243 @DOCSTRING(saveobj)
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244
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245 @noindent
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246 @code{saveobj} is called just prior to saving the class to a
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247 file. Likely, the @code{loadobj} method is called just after a class
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248 is loaded from a file, and can be used to ensure that any removed
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249 fields are reinserted into the user object.
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250
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251 @DOCSTRING(loadobj)
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252
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253 @node Indexing Objects
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254 @section Indexing Objects
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255
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256 Objects in can be indexed with parenthesises, either like
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257 @code{@var{a} (@var{idx})} or like @code{@var{a} @{@var{idx}@}}, or even
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258 like @code{@var{a} (@var{idx}).@var{field}}. However, it is up to the user
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259 to decide what this indexing actually means. In the case of our polynomial
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260 class @code{@var{p} (@var{n})} might mean either the coefficient of the
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261 @var{n}-th power of the polynomial, or it might be the evaluation of the
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262 polynomial at @var{n}. The meaning of this subscripted referencing is
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263 determined by the @code{subsref} method.
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264
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265 @DOCSTRING(subsref)
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266
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267 For example we might decide that indexing with "()" evaluates the
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268 polynomial and indexing with "{}" returns the @var{n}-th coefficient.
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269 In this case the @code{subsref} method of our polynomial class might look like
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270
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271 @polynomialfile{subsref.m}
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272
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273 The equivalent functionality for subscripted asignments uses the
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274 @code{subsasgn} method.
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275
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276 @DOCSTRING(subsasgn)
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277
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278 If you wish to use the @code{end} keyword in subscripted expressions
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279 of an object. Then the user needs to define the @code{end} method for
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280 the class.
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281
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282 @DOCSTRING(end)
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283
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284 For example the @code{end} method for our polynomial class might look like
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285
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286 @polynomialfile{end.m}
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287
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288 @noindent
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289 which is a fairly generic @code{end} method that has a behavior similar to
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290 the @code{end} keyword for Octave Array classes. It can then be used for
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291 example like
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292
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293 @example
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294 @group
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295 p = polynomial([1,2,3,4]);
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296 p(end-1)
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297 @result{} 3
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298 @end group
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299 @end example
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300
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301 Objects can also be used as the index in a subscripted expression themselves
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302 and this is controlled with the @code{subsindex} function.
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303
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304 @DOCSTRING(subsindex)
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305
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306 Finally, objects can equally be used like ranges, using the @code{colon}
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307 method
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308
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309 @DOCSTRING(colon)
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310
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311 @node Overloading Objects
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312 @section Overloading Objects
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313
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314 @menu
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315 * Function Overloading::
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316 * Operator Overloading::
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317 * Precedence of Objects::
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318 @end menu
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319
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320 @node Function Overloading
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321 @subsection Function Overloading
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322
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323 Any Octave function can be overloaded, and allows a object specific
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324 version of this function to be called as needed. A pertinent example
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325 for our polynomial class might be to overload the @code{polyval} function
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326 like
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327
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328 @polynomialfile{polyval.m}
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329
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330 This function just hands off the work to the normal Octave @code{polyval}
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331 function. Another interesting example for an overloaded function for our
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332 polynomial class is the @code{plot} function.
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333
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334 @polynomialfile{plot.m}
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335
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336 @noindent
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337 which allows polynomials to be plotted in the domain near the region
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338 of the roots of the polynomial.
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339
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340 Functions that are of particular interest to be overloaded are the class
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341 conversion functions such as @code{double}. Overloading these functions
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342 allows the @code{cast} function to work with the user class and can aid
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343 in the use of methods of other classes with the user class. An example
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344 @code{double} function for our polynomial class might look like.
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345
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346 @polynomialfile{double.m}
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347
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348 @node Operator Overloading
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349 @subsection Operator Overloading
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350
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351 @float Table,tab:overload_ops
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352 @iftex
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353 @tex
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354 \vskip 6pt
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355 {\hbox to \hsize {\hfill\vbox{\offinterlineskip \tabskip=0pt
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356 \halign{
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357 \vrule height2.0ex depth1.ex width 0.6pt #\tabskip=0.3em &
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358 # \hfil & \vrule # & # \hfil & \vrule # & # \hfil & # \vrule
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359 width 0.6pt \tabskip=0pt\cr
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360 \noalign{\hrule height 0.6pt}
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361 & Operation && Method && Description &\cr
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362 \noalign{\hrule}
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363 & $a + b$ && plus (a, b) && Binary addition operator&\cr
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364 & $a - b$ && plus (a, b) && Binary subtraction operator&\cr
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365 & $+ a$ && uplus (a) && Unary addition operator&\cr
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366 & $- a$ && uminus (a) && Unary subtraction operator&\cr
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367 & $a .* b$ && times (a, b) && Element-wise multiplication operator&\cr
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368 & $a * b$ && mtimes (a, b) && Matrix multiplication operator&\cr
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369 & $a ./ b$ && rdivide (a, b) && Element-wise right division operator&\cr
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370 & $a / b$ && mrdivide (a, b) && Matrix right division operator&\cr
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371 & $a .\backslash b$ && ldivide (a, b) && Element-wise left division operator&\cr
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372 & $a \backslash b$ && mldivide (a, b) && Matrix left division operator&\cr
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373 & $a .\hat b$ && power (a, b) && Element-wise power operator&\cr
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374 & $a \hat b$ && mpower (a, b) && Matrix power operator&\cr
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375 & $a < b$ && lt (a, b) && Less than operator&\cr
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376 & $a <= b$ && le (a, b) && Less than or equal to operator&\cr
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377 & $a > b$ && gt (a, b) && Greater than operator&\cr
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378 & $a >= b$ && ge (a, b) && Greater than or equal to operator&\cr
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379 & $a == b$ && eq (a, b) && Equal to operator&\cr
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380 & $a != b$ && ne (a, b) && Not equal to operator&\cr
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381 & $a \& b$ && and (a, b) && Logical and operator&\cr
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382 & $a | b$ && or (a, b) && Logical or operator&\cr
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383 & $! b$ && not (a) && Logical not operator&\cr
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384 & $a'$ && ctranspose (a) && Complex conjugate transpose operator &\cr
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385 & $a.'$ && transpose (a) && Transpose operator &\cr
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386 & $a : b$ && colon (a, b) && Two element range operator &\cr
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387 & $a : b : c$ && colon (a, b, c) && Three element range operator &\cr
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388 & $[a, b]$ && horzcat (a, b) && Horizontal concatenation opertaor &\cr
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389 & $[a; b]$ && vertcat (a, b) && Vertical concatenation opertaor &\cr
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390 & $a(s_1, \ldots, s_n)$ && subsref (a, s) && Subscripted reference &\cr
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391 & $a(s_1, \ldots, s_n) = b$ && subsasgn (a, s, b) && Subscripted assignment &\cr
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392 & $b (a)$ && subsindex (a) && Convert to zero-based index &\cr
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393 & {\it display} && display (a) && Commandline display function &\cr
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394 \noalign{\hrule height 0.6pt}
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395 }}\hfill}}
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396 @end tex
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397 @end iftex
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398 @ifnottex
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399 @multitable @columnfractions .1 .20 .20 .40 .1
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400 @item @tab Operation @tab Method @tab Description @tab
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401 @item @tab a + b @tab plus (a, b) @tab Binary addition @tab
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402 @item @tab a - b$ @tab plus (a, b) @tab Binary subtraction operator @tab
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403 @item @tab + a$ @tab uplus (a) @tab Unary addition operator @tab
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404 @item @tab - a$ @tab uminus (a) @tab Unary subtraction operator @tab
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405 @item @tab a .* b$ @tab times (a, b) @tab Element-wise multiplication operator @tab
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406 @item @tab a * b$ @tab mtimes (a, b) @tab Matirx multiplication operator @tab
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407 @item @tab a ./ b$ @tab rdivide (a, b) @tab Element-wise right division operator @tab
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408 @item @tab a / b$ @tab mrdivide (a, b) @tab Matrix right division operator @tab
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409 @item @tab a .\ b$ @tab ldivide (a, b) @tab Element-wise left division operator @tab
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410 @item @tab a \ b$ @tab mldivide (a, b) @tab Matrix left division operator @tab
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411 @item @tab a .^ b$ @tab ldivide (a, b) @tab Element-wise power operator @tab
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412 @item @tab a ^ b$ @tab mldivide (a, b) @tab Matrix power operator @tab
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413 @item @tab a < b$ @tab lt (a, b) @tab Less than operator @tab
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414 @item @tab a <= b$ @tab le (a, b) @tab Less than or equal to operator @tab
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415 @item @tab a > b$ @tab gt (a, b) @tab Greater than operator @tab
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416 @item @tab a >= b$ @tab ge (a, b) @tab Greater than or equal to operator @tab
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417 @item @tab a == b$ @tab eq (a, b) @tab Equal to operator @tab
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418 @item @tab a != b$ @tab ne (a, b) @tab Not equal to operator @tab
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419 @item @tab a \& b$ @tab and (a, b) @tab Logical and operator @tab
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420 @item @tab a | b$ @tab or (a, b) @tab Logical or operator @tab
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421 @item @tab ! b$ @tab not (a) @tab Logical not operator @tab
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422 @item @tab a'$ @tab ctranspose (a) @tab Complex conjugate transpose operator @tab
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423 @item @tab a.'$ @tab transpose (a) @tab Transpose operator @tab
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424 @item @tab a : b$ @tab colon (a, b) @tab Two element range operator @tab
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425 @item @tab a : b : c$ @tab colon (a, b, c) @tab Three element range operator @tab
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426 @item @tab [a, b]$ @tab horzcat (a, b) @tab Horizontal concatenation opertaor @tab
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427 @item @tab [a; b]$ @tab vertcat (a, b) @tab Vertical concatenation opertaor @tab
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428 @item @tab a(s_1, \ldots, s_n)$ @tab subsref (a, s) @tab Subscripted reference @tab
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429 @item @tab a(s_1, \ldots, s_n) = b$ @tab subsasgn (a, s, b) @tab Subscripted assignment @tab
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430 @item @tab b (a)$ @tab subsindex (a) @tab Convert to zero-based index @tab
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431 @item @tab @dfn{display} @tab display (a) @tab Commandline display function @tab
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432 @end multitable
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433 @end ifnottex
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434 @caption{Available overloaded operators and their corresponding class method}
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435 @end float
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436
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437 An example @code{mtimes} method for our polynomial class might look like
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438
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439 @polynomialfile{mtimes.m}
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440
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441 @node Precedence of Objects
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442 @subsection Precedence of Objects
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443
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444 Many functions and operators take two or more arguments and so the
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445 case can easily arise that these functions are called with objects of
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446 different classes. It is therefore necessary to determine the precedence
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447 of which method of which class to call when there are mixed objects given
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448 to a function or operator. To do this the @code{superiorto} and
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449 @code{inferiorto} functions can be used
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450
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451 @DOCSTRING(superiorto)
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452
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453 @DOCSTRING(inferiorto)
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454
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455 For example with our polynomial class consider the case
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456
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457 @example
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458 2 * polynomial ([1, 0, 1]);
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459 @end example
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460
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461 @noindent
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462 That mixes an object of the class "double" with an object of the class
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David Bateman <dbateman@free.fr>
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463 "polynomial". In this case we like to ensure that the return type of
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David Bateman <dbateman@free.fr>
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464 the above is of the type "polynomial" and so we use the
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David Bateman <dbateman@free.fr>
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465 @code{superiorto} function in the class constructor. In particular our
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David Bateman <dbateman@free.fr>
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466 polynomial class constructor would be modified to be
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David Bateman <dbateman@free.fr>
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467
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David Bateman <dbateman@free.fr>
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468 @polynomialfile{polynomial_superiorto.m}
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David Bateman <dbateman@free.fr>
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469
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David Bateman <dbateman@free.fr>
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470 Note that user classes always have higher precedence than built-in
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David Bateman <dbateman@free.fr>
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471 Octave types. So in fact marking our polynomial class higher than the
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David Bateman <dbateman@free.fr>
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472 "double" class is in fact not necessary.
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David Bateman <dbateman@free.fr>
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473
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David Bateman <dbateman@free.fr>
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474