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annotate docs/HowToSetUpLLVMStyleRTTI.rst @ 107:a03ddd01be7e

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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
parents 60c9769439b8
children
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1 ======================================================
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2 How to set up LLVM-style RTTI for your class hierarchy
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3 ======================================================
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4
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5 .. contents::
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6
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7 Background
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8 ==========
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9
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10 LLVM avoids using C++'s built in RTTI. Instead, it pervasively uses its
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11 own hand-rolled form of RTTI which is much more efficient and flexible,
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12 although it requires a bit more work from you as a class author.
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13
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14 A description of how to use LLVM-style RTTI from a client's perspective is
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15 given in the `Programmer's Manual <ProgrammersManual.html#isa>`_. This
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16 document, in contrast, discusses the steps you need to take as a class
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17 hierarchy author to make LLVM-style RTTI available to your clients.
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18
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19 Before diving in, make sure that you are familiar with the Object Oriented
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20 Programming concept of "`is-a`_".
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21
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22 .. _is-a: http://en.wikipedia.org/wiki/Is-a
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23
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24 Basic Setup
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25 ===========
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26
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27 This section describes how to set up the most basic form of LLVM-style RTTI
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28 (which is sufficient for 99.9% of the cases). We will set up LLVM-style
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29 RTTI for this class hierarchy:
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30
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31 .. code-block:: c++
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32
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33 class Shape {
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34 public:
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35 Shape() {}
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36 virtual double computeArea() = 0;
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37 };
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38
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39 class Square : public Shape {
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40 double SideLength;
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41 public:
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42 Square(double S) : SideLength(S) {}
83
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43 double computeArea() override;
0
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44 };
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45
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46 class Circle : public Shape {
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47 double Radius;
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48 public:
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49 Circle(double R) : Radius(R) {}
83
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50 double computeArea() override;
0
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51 };
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52
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53 The most basic working setup for LLVM-style RTTI requires the following
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54 steps:
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55
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56 #. In the header where you declare ``Shape``, you will want to ``#include
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57 "llvm/Support/Casting.h"``, which declares LLVM's RTTI templates. That
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58 way your clients don't even have to think about it.
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59
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60 .. code-block:: c++
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61
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62 #include "llvm/Support/Casting.h"
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63
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64 #. In the base class, introduce an enum which discriminates all of the
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65 different concrete classes in the hierarchy, and stash the enum value
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66 somewhere in the base class.
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67
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68 Here is the code after introducing this change:
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69
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70 .. code-block:: c++
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71
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72 class Shape {
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73 public:
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74 + /// Discriminator for LLVM-style RTTI (dyn_cast<> et al.)
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75 + enum ShapeKind {
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76 + SK_Square,
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77 + SK_Circle
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78 + };
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79 +private:
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80 + const ShapeKind Kind;
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81 +public:
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82 + ShapeKind getKind() const { return Kind; }
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83 +
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84 Shape() {}
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85 virtual double computeArea() = 0;
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86 };
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87
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88 You will usually want to keep the ``Kind`` member encapsulated and
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89 private, but let the enum ``ShapeKind`` be public along with providing a
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90 ``getKind()`` method. This is convenient for clients so that they can do
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91 a ``switch`` over the enum.
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92
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93 A common naming convention is that these enums are "kind"s, to avoid
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94 ambiguity with the words "type" or "class" which have overloaded meanings
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95 in many contexts within LLVM. Sometimes there will be a natural name for
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96 it, like "opcode". Don't bikeshed over this; when in doubt use ``Kind``.
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97
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98 You might wonder why the ``Kind`` enum doesn't have an entry for
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99 ``Shape``. The reason for this is that since ``Shape`` is abstract
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100 (``computeArea() = 0;``), you will never actually have non-derived
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101 instances of exactly that class (only subclasses). See `Concrete Bases
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102 and Deeper Hierarchies`_ for information on how to deal with
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103 non-abstract bases. It's worth mentioning here that unlike
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104 ``dynamic_cast<>``, LLVM-style RTTI can be used (and is often used) for
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105 classes that don't have v-tables.
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106
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107 #. Next, you need to make sure that the ``Kind`` gets initialized to the
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108 value corresponding to the dynamic type of the class. Typically, you will
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109 want to have it be an argument to the constructor of the base class, and
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110 then pass in the respective ``XXXKind`` from subclass constructors.
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111
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112 Here is the code after that change:
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113
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114 .. code-block:: c++
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115
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116 class Shape {
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117 public:
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118 /// Discriminator for LLVM-style RTTI (dyn_cast<> et al.)
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119 enum ShapeKind {
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120 SK_Square,
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121 SK_Circle
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122 };
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123 private:
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124 const ShapeKind Kind;
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125 public:
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126 ShapeKind getKind() const { return Kind; }
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127
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128 - Shape() {}
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129 + Shape(ShapeKind K) : Kind(K) {}
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130 virtual double computeArea() = 0;
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131 };
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132
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133 class Square : public Shape {
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parents:
diff changeset
134 double SideLength;
95c75e76d11b LLVM 3.4
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parents:
diff changeset
135 public:
95c75e76d11b LLVM 3.4
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parents:
diff changeset
136 - Square(double S) : SideLength(S) {}
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parents:
diff changeset
137 + Square(double S) : Shape(SK_Square), SideLength(S) {}
83
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parents: 77
diff changeset
138 double computeArea() override;
0
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parents:
diff changeset
139 };
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parents:
diff changeset
140
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parents:
diff changeset
141 class Circle : public Shape {
95c75e76d11b LLVM 3.4
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parents:
diff changeset
142 double Radius;
95c75e76d11b LLVM 3.4
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parents:
diff changeset
143 public:
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parents:
diff changeset
144 - Circle(double R) : Radius(R) {}
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parents:
diff changeset
145 + Circle(double R) : Shape(SK_Circle), Radius(R) {}
83
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parents: 77
diff changeset
146 double computeArea() override;
0
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parents:
diff changeset
147 };
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parents:
diff changeset
148
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parents:
diff changeset
149 #. Finally, you need to inform LLVM's RTTI templates how to dynamically
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
150 determine the type of a class (i.e. whether the ``isa<>``/``dyn_cast<>``
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
151 should succeed). The default "99.9% of use cases" way to accomplish this
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
152 is through a small static member function ``classof``. In order to have
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parents:
diff changeset
153 proper context for an explanation, we will display this code first, and
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
154 then below describe each part:
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parents:
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155
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parents:
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156 .. code-block:: c++
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parents:
diff changeset
157
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parents:
diff changeset
158 class Shape {
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parents:
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159 public:
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parents:
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160 /// Discriminator for LLVM-style RTTI (dyn_cast<> et al.)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
161 enum ShapeKind {
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parents:
diff changeset
162 SK_Square,
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
163 SK_Circle
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
164 };
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parents:
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165 private:
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
166 const ShapeKind Kind;
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parents:
diff changeset
167 public:
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parents:
diff changeset
168 ShapeKind getKind() const { return Kind; }
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parents:
diff changeset
169
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parents:
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170 Shape(ShapeKind K) : Kind(K) {}
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parents:
diff changeset
171 virtual double computeArea() = 0;
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parents:
diff changeset
172 };
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parents:
diff changeset
173
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parents:
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174 class Square : public Shape {
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parents:
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175 double SideLength;
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parents:
diff changeset
176 public:
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parents:
diff changeset
177 Square(double S) : Shape(SK_Square), SideLength(S) {}
83
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parents: 77
diff changeset
178 double computeArea() override;
0
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parents:
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179 +
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parents:
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180 + static bool classof(const Shape *S) {
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parents:
diff changeset
181 + return S->getKind() == SK_Square;
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parents:
diff changeset
182 + }
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parents:
diff changeset
183 };
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parents:
diff changeset
184
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parents:
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185 class Circle : public Shape {
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parents:
diff changeset
186 double Radius;
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parents:
diff changeset
187 public:
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parents:
diff changeset
188 Circle(double R) : Shape(SK_Circle), Radius(R) {}
83
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parents: 77
diff changeset
189 double computeArea() override;
0
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parents:
diff changeset
190 +
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parents:
diff changeset
191 + static bool classof(const Shape *S) {
95c75e76d11b LLVM 3.4
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parents:
diff changeset
192 + return S->getKind() == SK_Circle;
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parents:
diff changeset
193 + }
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
194 };
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parents:
diff changeset
195
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parents:
diff changeset
196 The job of ``classof`` is to dynamically determine whether an object of
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
197 a base class is in fact of a particular derived class. In order to
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
198 downcast a type ``Base`` to a type ``Derived``, there needs to be a
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
199 ``classof`` in ``Derived`` which will accept an object of type ``Base``.
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parents:
diff changeset
200
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parents:
diff changeset
201 To be concrete, consider the following code:
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parents:
diff changeset
202
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parents:
diff changeset
203 .. code-block:: c++
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parents:
diff changeset
204
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parents:
diff changeset
205 Shape *S = ...;
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
206 if (isa<Circle>(S)) {
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
207 /* do something ... */
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
208 }
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parents:
diff changeset
209
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
210 The code of the ``isa<>`` test in this code will eventually boil
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
211 down---after template instantiation and some other machinery---to a
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
212 check roughly like ``Circle::classof(S)``. For more information, see
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
213 :ref:`classof-contract`.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
214
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
215 The argument to ``classof`` should always be an *ancestor* class because
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
216 the implementation has logic to allow and optimize away
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
217 upcasts/up-``isa<>``'s automatically. It is as though every class
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
218 ``Foo`` automatically has a ``classof`` like:
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parents:
diff changeset
219
95c75e76d11b LLVM 3.4
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parents:
diff changeset
220 .. code-block:: c++
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parents:
diff changeset
221
95c75e76d11b LLVM 3.4
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parents:
diff changeset
222 class Foo {
95c75e76d11b LLVM 3.4
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parents:
diff changeset
223 [...]
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
224 template <class T>
95c75e76d11b LLVM 3.4
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parents:
diff changeset
225 static bool classof(const T *,
77
54457678186b LLVM 3.6
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents: 0
diff changeset
226 ::std::enable_if<
54457678186b LLVM 3.6
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents: 0
diff changeset
227 ::std::is_base_of<Foo, T>::value
0
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
228 >::type* = 0) { return true; }
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
229 [...]
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
230 };
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
231
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
232 Note that this is the reason that we did not need to introduce a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
233 ``classof`` into ``Shape``: all relevant classes derive from ``Shape``,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
234 and ``Shape`` itself is abstract (has no entry in the ``Kind`` enum),
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
235 so this notional inferred ``classof`` is all we need. See `Concrete
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
236 Bases and Deeper Hierarchies`_ for more information about how to extend
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
237 this example to more general hierarchies.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
238
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
239 Although for this small example setting up LLVM-style RTTI seems like a lot
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
240 of "boilerplate", if your classes are doing anything interesting then this
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
241 will end up being a tiny fraction of the code.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
242
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
243 Concrete Bases and Deeper Hierarchies
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
244 =====================================
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
245
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
246 For concrete bases (i.e. non-abstract interior nodes of the inheritance
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
247 tree), the ``Kind`` check inside ``classof`` needs to be a bit more
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
248 complicated. The situation differs from the example above in that
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
249
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
250 * Since the class is concrete, it must itself have an entry in the ``Kind``
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
251 enum because it is possible to have objects with this class as a dynamic
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
252 type.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
253
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
254 * Since the class has children, the check inside ``classof`` must take them
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
255 into account.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
256
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
257 Say that ``SpecialSquare`` and ``OtherSpecialSquare`` derive
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
258 from ``Square``, and so ``ShapeKind`` becomes:
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
259
95c75e76d11b LLVM 3.4
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parents:
diff changeset
260 .. code-block:: c++
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
261
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
262 enum ShapeKind {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
263 SK_Square,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
264 + SK_SpecialSquare,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
265 + SK_OtherSpecialSquare,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
266 SK_Circle
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
267 }
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268
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269 Then in ``Square``, we would need to modify the ``classof`` like so:
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270
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271 .. code-block:: c++
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272
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273 - static bool classof(const Shape *S) {
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274 - return S->getKind() == SK_Square;
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275 - }
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276 + static bool classof(const Shape *S) {
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277 + return S->getKind() >= SK_Square &&
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278 + S->getKind() <= SK_OtherSpecialSquare;
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279 + }
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280
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281 The reason that we need to test a range like this instead of just equality
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282 is that both ``SpecialSquare`` and ``OtherSpecialSquare`` "is-a"
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283 ``Square``, and so ``classof`` needs to return ``true`` for them.
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284
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285 This approach can be made to scale to arbitrarily deep hierarchies. The
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286 trick is that you arrange the enum values so that they correspond to a
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287 preorder traversal of the class hierarchy tree. With that arrangement, all
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288 subclass tests can be done with two comparisons as shown above. If you just
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289 list the class hierarchy like a list of bullet points, you'll get the
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290 ordering right::
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291
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292 | Shape
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293 | Square
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294 | SpecialSquare
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295 | OtherSpecialSquare
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296 | Circle
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297
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298 A Bug to be Aware Of
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299 --------------------
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300
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301 The example just given opens the door to bugs where the ``classof``\s are
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302 not updated to match the ``Kind`` enum when adding (or removing) classes to
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303 (from) the hierarchy.
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304
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305 Continuing the example above, suppose we add a ``SomewhatSpecialSquare`` as
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306 a subclass of ``Square``, and update the ``ShapeKind`` enum like so:
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307
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308 .. code-block:: c++
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309
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310 enum ShapeKind {
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311 SK_Square,
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312 SK_SpecialSquare,
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parents:
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313 SK_OtherSpecialSquare,
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314 + SK_SomewhatSpecialSquare,
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315 SK_Circle
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316 }
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317
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318 Now, suppose that we forget to update ``Square::classof()``, so it still
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319 looks like:
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320
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321 .. code-block:: c++
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322
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323 static bool classof(const Shape *S) {
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324 // BUG: Returns false when S->getKind() == SK_SomewhatSpecialSquare,
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325 // even though SomewhatSpecialSquare "is a" Square.
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326 return S->getKind() >= SK_Square &&
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327 S->getKind() <= SK_OtherSpecialSquare;
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328 }
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329
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330 As the comment indicates, this code contains a bug. A straightforward and
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331 non-clever way to avoid this is to introduce an explicit ``SK_LastSquare``
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parents:
diff changeset
332 entry in the enum when adding the first subclass(es). For example, we could
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parents:
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333 rewrite the example at the beginning of `Concrete Bases and Deeper
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334 Hierarchies`_ as:
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335
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336 .. code-block:: c++
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337
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338 enum ShapeKind {
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339 SK_Square,
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340 + SK_SpecialSquare,
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parents:
diff changeset
341 + SK_OtherSpecialSquare,
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diff changeset
342 + SK_LastSquare,
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diff changeset
343 SK_Circle
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344 }
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345 ...
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346 // Square::classof()
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347 - static bool classof(const Shape *S) {
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348 - return S->getKind() == SK_Square;
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349 - }
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350 + static bool classof(const Shape *S) {
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351 + return S->getKind() >= SK_Square &&
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352 + S->getKind() <= SK_LastSquare;
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diff changeset
353 + }
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354
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355 Then, adding new subclasses is easy:
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356
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357 .. code-block:: c++
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358
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359 enum ShapeKind {
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360 SK_Square,
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parents:
diff changeset
361 SK_SpecialSquare,
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parents:
diff changeset
362 SK_OtherSpecialSquare,
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parents:
diff changeset
363 + SK_SomewhatSpecialSquare,
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parents:
diff changeset
364 SK_LastSquare,
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parents:
diff changeset
365 SK_Circle
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366 }
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367
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368 Notice that ``Square::classof`` does not need to be changed.
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369
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370 .. _classof-contract:
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371
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372 The Contract of ``classof``
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373 ---------------------------
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374
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375 To be more precise, let ``classof`` be inside a class ``C``. Then the
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diff changeset
376 contract for ``classof`` is "return ``true`` if the dynamic type of the
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parents:
diff changeset
377 argument is-a ``C``". As long as your implementation fulfills this
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diff changeset
378 contract, you can tweak and optimize it as much as you want.
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diff changeset
379
83
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diff changeset
380 For example, LLVM-style RTTI can work fine in the presence of
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diff changeset
381 multiple-inheritance by defining an appropriate ``classof``.
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diff changeset
382 An example of this in practice is
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diff changeset
383 `Decl <http://clang.llvm.org/doxygen/classclang_1_1Decl.html>`_ vs.
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diff changeset
384 `DeclContext <http://clang.llvm.org/doxygen/classclang_1_1DeclContext.html>`_
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diff changeset
385 inside Clang.
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386 The ``Decl`` hierarchy is done very similarly to the example setup
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diff changeset
387 demonstrated in this tutorial.
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diff changeset
388 The key part is how to then incorporate ``DeclContext``: all that is needed
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diff changeset
389 is in ``bool DeclContext::classof(const Decl *)``, which asks the question
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390 "Given a ``Decl``, how can I determine if it is-a ``DeclContext``?".
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diff changeset
391 It answers this with a simple switch over the set of ``Decl`` "kinds", and
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392 returning true for ones that are known to be ``DeclContext``'s.
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diff changeset
393
0
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394 .. TODO::
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parents:
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395
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396 Touch on some of the more advanced features, like ``isa_impl`` and
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parents:
diff changeset
397 ``simplify_type``. However, those two need reference documentation in
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parents:
diff changeset
398 the form of doxygen comments as well. We need the doxygen so that we can
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parents:
diff changeset
399 say "for full details, see http://llvm.org/doxygen/..."
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parents:
diff changeset
400
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401 Rules of Thumb
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402 ==============
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403
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404 #. The ``Kind`` enum should have one entry per concrete class, ordered
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405 according to a preorder traversal of the inheritance tree.
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406 #. The argument to ``classof`` should be a ``const Base *``, where ``Base``
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407 is some ancestor in the inheritance hierarchy. The argument should
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408 *never* be a derived class or the class itself: the template machinery
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409 for ``isa<>`` already handles this case and optimizes it.
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410 #. For each class in the hierarchy that has no children, implement a
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411 ``classof`` that checks only against its ``Kind``.
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412 #. For each class in the hierarchy that has children, implement a
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413 ``classof`` that checks a range of the first child's ``Kind`` and the
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414 last child's ``Kind``.