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| 26 | <div class="titlepage"><div><div><h3 class="title"> |
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| 27 | <a name="lambda.extending"></a>Extending return type deduction system</h3></div></div></div> |
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| 28 | <p> |
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| 29 | |
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| 30 | In this section, we explain how to extend the return type deduction system |
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| 31 | to cover user defined operators. |
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| 32 | |
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| 33 | In many cases this is not necessary, |
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| 34 | as the BLL defines default return types for operators. |
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| 35 | |
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| 36 | For example, the default return type for all comparison operators is |
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| 37 | <code class="literal">bool</code>, and as long as the user defined comparison operators |
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| 38 | have a bool return type, there is no need to write new specializations |
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| 39 | for the return type deduction classes. |
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| 40 | |
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| 41 | Sometimes this cannot be avoided, though. |
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| 42 | |
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| 43 | </p> |
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| 44 | <p> |
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| 45 | The overloadable user defined operators are either unary or binary. |
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| 46 | |
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| 47 | For each arity, there are two traits templates that define the |
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| 48 | return types of the different operators. |
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| 49 | |
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| 50 | Hence, the return type system can be extended by providing more |
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| 51 | specializations for these templates. |
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| 52 | |
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| 53 | The templates for unary functors are |
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| 54 | |
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| 55 | <code class="literal"> |
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| 56 | plain_return_type_1<Action, A> |
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| 57 | </code> |
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| 58 | |
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| 59 | and |
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| 60 | |
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| 61 | <code class="literal"> |
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| 62 | return_type_1<Action, A> |
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| 63 | </code>, and |
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| 64 | |
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| 65 | <code class="literal"> |
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| 66 | plain_return_type_2<Action, A, B> |
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| 67 | </code> |
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| 68 | |
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| 69 | and |
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| 70 | |
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| 71 | <code class="literal"> |
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| 72 | return_type_2<Action, A, B> |
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| 73 | </code> |
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| 74 | |
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| 75 | respectively for binary functors. |
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| 76 | |
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| 77 | </p> |
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| 78 | <p> |
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| 79 | The first parameter (<code class="literal">Action</code>) to all these templates |
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| 80 | is the <span class="emphasis"><em>action</em></span> class, which specifies the operator. |
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| 81 | |
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| 82 | Operators with similar return type rules are grouped together into |
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| 83 | <span class="emphasis"><em>action groups</em></span>, |
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| 84 | and only the action class and action group together define the operator |
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| 85 | unambiguously. |
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| 86 | |
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| 87 | As an example, the action type |
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| 88 | <code class="literal">arithmetic_action<plus_action></code> stands for |
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| 89 | <code class="literal">operator+</code>. |
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| 90 | |
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| 91 | The complete listing of different action types is shown in |
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| 92 | <a href="extending.html#table:actions" title="Table 6.2. Action types">Table 6.2, “Action types”</a>. |
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| 93 | </p> |
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| 94 | <p> |
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| 95 | The latter parameters, <code class="literal">A</code> in the unary case, |
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| 96 | or <code class="literal">A</code> and <code class="literal">B</code> in the binary case, |
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| 97 | stand for the argument types of the operator call. |
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| 98 | |
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| 99 | The two sets of templates, |
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| 100 | <code class="literal">plain_return_type_<em class="parameter"><code>n</code></em></code> and |
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| 101 | <code class="literal">return_type_<em class="parameter"><code>n</code></em></code> |
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| 102 | (<em class="parameter"><code>n</code></em> is 1 or 2) differ in the way how parameter types |
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| 103 | are presented to them. |
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| 104 | |
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| 105 | For the former templates, the parameter types are always provided as |
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| 106 | non-reference types, and do not have const or volatile qualifiers. |
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| 107 | |
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| 108 | This makes specializing easy, as commonly one specialization for each |
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| 109 | user defined operator, or operator group, is enough. |
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| 110 | |
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| 111 | On the other hand, if a particular operator is overloaded for different |
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| 112 | cv-qualifications of the same argument types, |
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| 113 | and the return types of these overloaded versions differ, a more fine-grained control is needed. |
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| 114 | |
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| 115 | Hence, for the latter templates, the parameter types preserve the |
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| 116 | cv-qualifiers, and are non-reference types as well. |
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| 117 | |
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| 118 | The downside is, that for an overloaded set of operators of the |
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| 119 | kind described above, one may end up needing up to |
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| 120 | 16 <code class="literal">return_type_2</code> specializations. |
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| 121 | </p> |
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| 122 | <p> |
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| 123 | Suppose the user has overloaded the following operators for some user defined |
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| 124 | types <code class="literal">X</code>, <code class="literal">Y</code> and <code class="literal">Z</code>: |
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| 125 | |
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| 126 | </p> |
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| 127 | <pre class="programlisting"> |
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| 128 | Z operator+(const X&, const Y&); |
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| 129 | Z operator-(const X&, const Y&); |
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| 130 | </pre> |
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| 131 | <p> |
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| 132 | |
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| 133 | Now, one can add a specialization stating, that if the left hand argument |
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| 134 | is of type <code class="literal">X</code>, and the right hand one of type |
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| 135 | <code class="literal">Y</code>, the return type of all such binary arithmetic |
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| 136 | operators is <code class="literal">Z</code>: |
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| 137 | |
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| 138 | </p> |
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| 139 | <pre class="programlisting"> |
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| 140 | namespace boost { |
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| 141 | namespace lambda { |
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| 142 | |
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| 143 | template<class Act> |
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| 144 | struct plain_return_type_2<arithmetic_action<Act>, X, Y> { |
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| 145 | typedef Z type; |
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| 146 | }; |
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| 147 | |
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| 148 | } |
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| 149 | } |
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| 150 | </pre> |
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| 151 | <p> |
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| 152 | |
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| 153 | Having this specialization defined, BLL is capable of correctly |
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| 154 | deducing the return type of the above two operators. |
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| 155 | |
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| 156 | Note, that the specializations must be in the same namespace, |
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| 157 | <code class="literal">::boost::lambda</code>, with the primary template. |
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| 158 | |
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| 159 | For brevity, we do not show the namespace definitions in the examples below. |
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| 160 | </p> |
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| 161 | <p> |
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| 162 | It is possible to specialize on the level of an individual operator as well, |
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| 163 | in addition to providing a specialization for a group of operators. |
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| 164 | Say, we add a new arithmetic operator for argument types <code class="literal">X</code> |
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| 165 | and <code class="literal">Y</code>: |
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| 166 | |
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| 167 | </p> |
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| 168 | <pre class="programlisting"> |
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| 169 | X operator*(const X&, const Y&); |
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| 170 | </pre> |
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| 171 | <p> |
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| 172 | |
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| 173 | Our first rule for all arithmetic operators specifies that the return |
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| 174 | type of this operator is <code class="literal">Z</code>, |
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| 175 | which obviously is not the case. |
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| 176 | Hence, we provide a new rule for the multiplication operator: |
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| 177 | |
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| 178 | </p> |
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| 179 | <pre class="programlisting"> |
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| 180 | template<> |
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| 181 | struct plain_return_type_2<arithmetic_action<multiply_action>, X, Y> { |
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| 182 | typedef X type; |
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| 183 | }; |
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| 184 | </pre> |
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| 185 | <p> |
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| 186 | The specializations can define arbitrary mappings from the argument types |
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| 187 | to the return type. |
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| 188 | |
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| 189 | Suppose we have some mathematical vector type, templated on the element type: |
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| 190 | |
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| 191 | </p> |
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| 192 | <pre class="programlisting"> |
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| 193 | template <class T> class my_vector; |
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| 194 | </pre> |
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| 195 | <p> |
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| 196 | |
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| 197 | Suppose the addition operator is defined between any two |
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| 198 | <code class="literal">my_vector</code> instantiations, |
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| 199 | as long as the addition operator is defined between their element types. |
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| 200 | |
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| 201 | Furthermore, the element type of the resulting <code class="literal">my_vector</code> |
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| 202 | is the same as the result type of the addition between the element types. |
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| 203 | |
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| 204 | E.g., adding <code class="literal">my_vector<int></code> and |
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| 205 | <code class="literal">my_vector<double></code> results in |
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| 206 | <code class="literal">my_vector<double></code>. |
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| 207 | |
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| 208 | The BLL has traits classes to perform the implicit built-in and standard |
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| 209 | type conversions between integral, floating point, and complex classes. |
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| 210 | |
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| 211 | Using BLL tools, the addition operator described above can be defined as: |
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| 212 | |
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| 213 | </p> |
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| 214 | <pre class="programlisting"> |
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| 215 | template<class A, class B> |
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| 216 | my_vector<typename return_type_2<arithmetic_action<plus_action>, A, B>::type> |
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| 217 | operator+(const my_vector<A>& a, const my_vector<B>& b) |
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| 218 | { |
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| 219 | typedef typename |
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| 220 | return_type_2<arithmetic_action<plus_action>, A, B>::type res_type; |
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| 221 | return my_vector<res_type>(); |
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| 222 | } |
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| 223 | </pre> |
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| 224 | <p> |
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| 225 | To allow BLL to deduce the type of <code class="literal">my_vector</code> |
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| 226 | additions correctly, we can define: |
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| 227 | |
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| 228 | </p> |
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| 229 | <pre class="programlisting"> |
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| 230 | template<class A, class B> |
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| 231 | class plain_return_type_2<arithmetic_action<plus_action>, |
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| 232 | my_vector<A>, my_vector<B> > { |
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| 233 | typedef typename |
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| 234 | return_type_2<arithmetic_action<plus_action>, A, B>::type res_type; |
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| 235 | public: |
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| 236 | typedef my_vector<res_type> type; |
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| 237 | }; |
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| 238 | </pre> |
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| 239 | <p> |
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| 240 | Note, that we are reusing the existing specializations for the |
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| 241 | BLL <code class="literal">return_type_2</code> template, |
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| 242 | which require that the argument types are references. |
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| 243 | </p> |
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| 244 | <div class="table"> |
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| 245 | <a name="table:actions"></a><p class="title"><b>Table 6.2. Action types</b></p> |
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| 246 | <table class="table" summary="Action types"> |
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| 247 | <colgroup> |
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| 248 | <col> |
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| 249 | <col> |
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| 250 | </colgroup> |
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| 251 | <tbody> |
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| 252 | <tr> |
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| 253 | <td><code class="literal">+</code></td> |
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| 254 | <td><code class="literal">arithmetic_action<plus_action></code></td> |
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| 255 | </tr> |
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| 256 | <tr> |
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| 257 | <td><code class="literal">-</code></td> |
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| 258 | <td><code class="literal">arithmetic_action<minus_action></code></td> |
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| 259 | </tr> |
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| 260 | <tr> |
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| 261 | <td><code class="literal">*</code></td> |
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| 262 | <td><code class="literal">arithmetic_action<multiply_action></code></td> |
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| 263 | </tr> |
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| 264 | <tr> |
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| 265 | <td><code class="literal">/</code></td> |
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| 266 | <td><code class="literal">arithmetic_action<divide_action></code></td> |
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| 267 | </tr> |
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| 268 | <tr> |
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| 269 | <td><code class="literal">%</code></td> |
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| 270 | <td><code class="literal">arithmetic_action<remainder_action></code></td> |
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| 271 | </tr> |
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| 272 | <tr> |
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| 273 | <td><code class="literal">+</code></td> |
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| 274 | <td><code class="literal">unary_arithmetic_action<plus_action></code></td> |
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| 275 | </tr> |
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| 276 | <tr> |
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| 277 | <td><code class="literal">-</code></td> |
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| 278 | <td><code class="literal">unary_arithmetic_action<minus_action></code></td> |
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| 279 | </tr> |
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| 280 | <tr> |
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| 281 | <td><code class="literal">&</code></td> |
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| 282 | <td><code class="literal">bitwise_action<and_action></code></td> |
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| 283 | </tr> |
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| 284 | <tr> |
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| 285 | <td><code class="literal">|</code></td> |
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| 286 | <td><code class="literal">bitwise_action<or_action></code></td> |
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| 287 | </tr> |
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| 288 | <tr> |
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| 289 | <td><code class="literal">~</code></td> |
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| 290 | <td><code class="literal">bitwise_action<not_action></code></td> |
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| 291 | </tr> |
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| 292 | <tr> |
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| 293 | <td><code class="literal">^</code></td> |
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| 294 | <td><code class="literal">bitwise_action<xor_action></code></td> |
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| 295 | </tr> |
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| 296 | <tr> |
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| 297 | <td><code class="literal"><<</code></td> |
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| 298 | <td><code class="literal">bitwise_action<leftshift_action_no_stream></code></td> |
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| 299 | </tr> |
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| 300 | <tr> |
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| 301 | <td><code class="literal">>></code></td> |
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| 302 | <td><code class="literal">bitwise_action<rightshift_action_no_stream></code></td> |
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| 303 | </tr> |
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| 304 | <tr> |
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| 305 | <td><code class="literal">&&</code></td> |
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| 306 | <td><code class="literal">logical_action<and_action></code></td> |
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| 307 | </tr> |
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| 308 | <tr> |
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| 309 | <td><code class="literal">||</code></td> |
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| 310 | <td><code class="literal">logical_action<or_action></code></td> |
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| 311 | </tr> |
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| 312 | <tr> |
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| 313 | <td><code class="literal">!</code></td> |
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| 314 | <td><code class="literal">logical_action<not_action></code></td> |
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| 315 | </tr> |
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| 316 | <tr> |
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| 317 | <td><code class="literal"><</code></td> |
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| 318 | <td><code class="literal">relational_action<less_action></code></td> |
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| 319 | </tr> |
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| 320 | <tr> |
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| 321 | <td><code class="literal">></code></td> |
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| 322 | <td><code class="literal">relational_action<greater_action></code></td> |
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| 323 | </tr> |
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| 324 | <tr> |
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| 325 | <td><code class="literal"><=</code></td> |
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| 326 | <td><code class="literal">relational_action<lessorequal_action></code></td> |
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| 327 | </tr> |
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| 328 | <tr> |
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| 329 | <td><code class="literal">>=</code></td> |
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| 330 | <td><code class="literal">relational_action<greaterorequal_action></code></td> |
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| 331 | </tr> |
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| 332 | <tr> |
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| 333 | <td><code class="literal">==</code></td> |
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| 334 | <td><code class="literal">relational_action<equal_action></code></td> |
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| 335 | </tr> |
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| 336 | <tr> |
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| 337 | <td><code class="literal">!=</code></td> |
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| 338 | <td><code class="literal">relational_action<notequal_action></code></td> |
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| 339 | </tr> |
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| 340 | <tr> |
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| 341 | <td><code class="literal">+=</code></td> |
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| 342 | <td><code class="literal">arithmetic_assignment_action<plus_action></code></td> |
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| 343 | </tr> |
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| 344 | <tr> |
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| 345 | <td><code class="literal">-=</code></td> |
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| 346 | <td><code class="literal">arithmetic_assignment_action<minus_action></code></td> |
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| 347 | </tr> |
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| 348 | <tr> |
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| 349 | <td><code class="literal">*=</code></td> |
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| 350 | <td><code class="literal">arithmetic_assignment_action<multiply_action></code></td> |
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| 351 | </tr> |
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| 352 | <tr> |
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| 353 | <td><code class="literal">/=</code></td> |
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| 354 | <td><code class="literal">arithmetic_assignment_action<divide_action></code></td> |
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| 355 | </tr> |
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| 356 | <tr> |
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| 357 | <td><code class="literal">%=</code></td> |
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| 358 | <td><code class="literal">arithmetic_assignment_action<remainder_action></code></td> |
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| 359 | </tr> |
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| 360 | <tr> |
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| 361 | <td><code class="literal">&=</code></td> |
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| 362 | <td><code class="literal">bitwise_assignment_action<and_action></code></td> |
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| 363 | </tr> |
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| 364 | <tr> |
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| 365 | <td><code class="literal">=|</code></td> |
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| 366 | <td><code class="literal">bitwise_assignment_action<or_action></code></td> |
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| 367 | </tr> |
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| 368 | <tr> |
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| 369 | <td><code class="literal">^=</code></td> |
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| 370 | <td><code class="literal">bitwise_assignment_action<xor_action></code></td> |
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| 371 | </tr> |
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| 372 | <tr> |
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| 373 | <td><code class="literal"><<=</code></td> |
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| 374 | <td><code class="literal">bitwise_assignment_action<leftshift_action></code></td> |
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| 375 | </tr> |
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| 376 | <tr> |
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| 377 | <td><code class="literal">>>=</code></td> |
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| 378 | <td><code class="literal">bitwise_assignment_action<rightshift_action></code></td> |
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| 379 | </tr> |
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| 380 | <tr> |
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| 381 | <td><code class="literal">++</code></td> |
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| 382 | <td><code class="literal">pre_increment_decrement_action<increment_action></code></td> |
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| 383 | </tr> |
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| 384 | <tr> |
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| 385 | <td><code class="literal">--</code></td> |
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| 386 | <td><code class="literal">pre_increment_decrement_action<decrement_action></code></td> |
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| 387 | </tr> |
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| 388 | <tr> |
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| 389 | <td><code class="literal">++</code></td> |
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| 390 | <td><code class="literal">post_increment_decrement_action<increment_action></code></td> |
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| 391 | </tr> |
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| 392 | <tr> |
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| 393 | <td><code class="literal">--</code></td> |
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| 394 | <td><code class="literal">post_increment_decrement_action<decrement_action></code></td> |
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| 395 | </tr> |
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| 396 | <tr> |
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| 397 | <td><code class="literal">&</code></td> |
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| 398 | <td><code class="literal">other_action<address_of_action></code></td> |
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| 399 | </tr> |
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| 400 | <tr> |
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| 401 | <td><code class="literal">*</code></td> |
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| 402 | <td><code class="literal">other_action<contents_of_action></code></td> |
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| 403 | </tr> |
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| 404 | <tr> |
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| 405 | <td><code class="literal">,</code></td> |
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| 406 | <td><code class="literal">other_action<comma_action></code></td> |
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| 407 | </tr> |
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| 408 | </tbody> |
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| 409 | </table> |
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| 410 | </div> |
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| 411 | </div> |
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| 412 | <table width="100%"><tr> |
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| 413 | <td align="left"></td> |
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| 414 | <td align="right"><small>Copyright © 1999-2004 Jaakko Järvi, Gary Powell</small></td> |
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