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24</div>
25<div class="section" lang="en">
26<div class="titlepage"><div><div><h3 class="title">
27<a name="lambda.using_library"></a>Using the library</h3></div></div></div>
28<div class="toc"><dl>
29<dt><span class="section"><a href="using_library.html#lambda.introductory_examples">Introductory Examples</a></span></dt>
30<dt><span class="section"><a href="using_library.html#lambda.parameter_and_return_types">Parameter and return types of lambda functors</a></span></dt>
31<dt><span class="section"><a href="using_library.html#lambda.actual_arguments_to_lambda_functors">About actual arguments to lambda functors</a></span></dt>
32<dt><span class="section"><a href="using_library.html#lambda.storing_bound_arguments">Storing bound arguments in lambda functions</a></span></dt>
33</dl></div>
34<p>
35The purpose of this section is to introduce the basic functionality of the library.
36There are quite a lot of exceptions and special cases, but discussion of them is postponed until later sections.
37
38
39    </p>
40<div class="section" lang="en">
41<div class="titlepage"><div><div><h4 class="title">
42<a name="lambda.introductory_examples"></a>Introductory Examples</h4></div></div></div>
43<p>
44        In this section we give basic examples of using BLL lambda expressions in STL algorithm invocations.
45        We start with some simple expressions and work up.
46        First, we initialize the elements of a container, say, a <code class="literal">list</code>, to the value <code class="literal">1</code>:
47
48
49        </p>
50<pre class="programlisting">
51list&lt;int&gt; v(10);
52for_each(v.begin(), v.end(), _1 = 1);</pre>
53<p>
54
55        The expression <code class="literal">_1 = 1</code> creates a lambda functor which assigns the value <code class="literal">1</code> to every element in <code class="literal">v</code>.<sup>[<a name="id2707880" href="#ftn.id2707880">1</a>]</sup></p>
56<p>
57        Next, we create a container of pointers and make them point to the elements in the first container <code class="literal">v</code>:
58
59        </p>
60<pre class="programlisting">
61vector&lt;int*&gt; vp(10);
62transform(v.begin(), v.end(), vp.begin(), &amp;_1);</pre>
63<p>
64
65The expression <code class="literal">&amp;_1</code> creates a function object for getting the address of each element in <code class="literal">v</code>.
66The addresses get assigned to the corresponding elements in <code class="literal">vp</code>.
67      </p>
68<p>
69        The next code fragment changes the values in <code class="literal">v</code>.
70        For each element, the function <code class="literal">foo</code> is called.
71The original value of the element is passed as an argument to <code class="literal">foo</code>.
72The result of <code class="literal">foo</code> is assigned back to the element:
73
74
75        </p>
76<pre class="programlisting">
77int foo(int);
78for_each(v.begin(), v.end(), _1 = bind(foo, _1));</pre>
79<p>
80        The next step is to sort the elements of <code class="literal">vp</code>:
81       
82        </p>
83<pre class="programlisting">sort(vp.begin(), vp.end(), *_1 &gt; *_2);</pre>
84<p>
85
86        In this call to <code class="literal">sort</code>, we are sorting the elements by their contents in descending order.
87      </p>
88<p>
89        Finally, the following <code class="literal">for_each</code> call outputs the sorted content of <code class="literal">vp</code> separated by line breaks:
90
91</p>
92<pre class="programlisting">
93for_each(vp.begin(), vp.end(), cout &lt;&lt; *_1 &lt;&lt; '\n');
94</pre>
95<p>
96
97Note that a normal (non-lambda) expression as subexpression of a lambda expression is evaluated immediately. 
98This may cause surprises.
99For instance, if the previous example is rewritten as
100</p>
101<pre class="programlisting">
102for_each(vp.begin(), vp.end(), cout &lt;&lt; '\n' &lt;&lt; *_1);
103</pre>
104<p>
105the subexpression <code class="literal">cout &lt;&lt; '\n'</code> is evaluated immediately and the effect is to output a single line break, followed by the elements of <code class="literal">vp</code>.
106The BLL provides functions <code class="literal">constant</code> and <code class="literal">var</code> to turn constants and, respectively, variables into lambda expressions, and can be used to prevent the immediate evaluation of subexpressions:
107</p>
108<pre class="programlisting">
109for_each(vp.begin(), vp.end(), cout &lt;&lt; constant('\n') &lt;&lt; *_1);
110</pre>
111<p>
112These functions are described more thoroughly in <a href="le_in_details.html#lambda.delaying_constants_and_variables" title="Delaying constants and variables">the section called &#8220;Delaying constants and variables&#8221;</a></p>
113</div>
114<div class="section" lang="en">
115<div class="titlepage"><div><div><h4 class="title">
116<a name="lambda.parameter_and_return_types"></a>Parameter and return types of lambda functors</h4></div></div></div>
117<p>
118        During the invocation of a lambda functor, the actual arguments are substituted for the placeholders.
119        The placeholders do not dictate the type of these actual arguments.
120        The basic rule is that a lambda function can be called with arguments of any types, as long as the lambda expression with substitutions performed is a valid C++ expression.
121        As an example, the expression
122        <code class="literal">_1 + _2</code> creates a binary lambda functor.
123        It can be called with two objects of any types <code class="literal">A</code> and <code class="literal">B</code> for which <code class="literal">operator+(A,B)</code> is defined (and for which BLL knows the return type of the operator, see below).
124      </p>
125<p>
126        C++ lacks a mechanism to query a type of an expression.
127        However, this precise mechanism is crucial for the implementation of C++ lambda expressions.
128        Consequently, BLL includes a somewhat complex type deduction system which uses a set of traits classes for deducing the resulting type of lambda functions.
129        It handles expressions where the operands are of built-in types and many of the expressions with operands of standard library types.
130        Many of the user defined types are covered as well, particularly if the user defined operators obey normal conventions in defining the return types.
131      </p>
132<p>
133        There are, however, cases when the return type cannot be deduced. For example, suppose you have defined:
134
135        </p>
136<pre class="programlisting">C operator+(A, B);</pre>
137<p>
138
139        The following lambda function invocation fails, since the return type cannot be deduced:
140
141        </p>
142<pre class="programlisting">A a; B b; (_1 + _2)(a, b);</pre>
143<p>
144        There are two alternative solutions to this.
145        The first is to extend the BLL type deduction system to cover your own types (see <a href="extending.html" title="Extending return type deduction system">the section called &#8220;Extending return type deduction system&#8221;</a>).
146        The second is to use a special lambda expression (<code class="literal">ret</code>) which defines the return type in place (see <a href="le_in_details.html#lambda.overriding_deduced_return_type" title="Overriding the deduced return type">the section called &#8220;Overriding the deduced return type&#8221;</a>):
147
148        </p>
149<pre class="programlisting">A a; B b; ret&lt;C&gt;(_1 + _2)(a, b);</pre>
150<p>
151        For bind expressions, the return type can be defined as a template argument of the bind function as well:
152        </p>
153<pre class="programlisting">bind&lt;int&gt;(foo, _1, _2);</pre>
154</div>
155<div class="section" lang="en">
156<div class="titlepage"><div><div><h4 class="title">
157<a name="lambda.actual_arguments_to_lambda_functors"></a>About actual arguments to lambda functors</h4></div></div></div>
158<p>A general restriction for the actual arguments is that they cannot be non-const rvalues.
159        For example:
160
161</p>
162<pre class="programlisting">
163int i = 1; int j = 2;
164(_1 + _2)(i, j); // ok
165(_1 + _2)(1, 2); // error (!)
166</pre>
167<p>
168
169        This restriction is not as bad as it may look.
170        Since the lambda functors are most often called inside STL-algorithms,
171        the arguments originate from dereferencing iterators and the dereferencing operators seldom return rvalues.
172        And for the cases where they do, there are workarounds discussed in
173<a href="le_in_details.html#lambda.rvalues_as_actual_arguments" title="Rvalues as actual arguments to lambda functors">the section called &#8220;Rvalues as actual arguments to lambda functors&#8221;</a>.
174
175
176      </p>
177</div>
178<div class="section" lang="en">
179<div class="titlepage"><div><div><h4 class="title">
180<a name="lambda.storing_bound_arguments"></a>Storing bound arguments in lambda functions</h4></div></div></div>
181<p>
182
183By default, temporary const copies of the bound arguments are stored
184in the lambda functor.
185
186This means that the value of a bound argument is fixed at the time of the
187creation of the lambda function and remains constant during the lifetime
188of the lambda function object.
189For example:
190</p>
191<pre class="programlisting">
192int i = 1;
193(_1 = 2, _1 + i)(i);
194</pre>
195<p>
196The comma operator is overloaded to combine lambda expressions into a sequence;
197the resulting unary lambda functor first assigns 2 to its argument,
198then adds the value of <code class="literal">i</code> to it.
199The value of the expression in the last line is 3, not 4.
200In other words, the lambda expression that is created is
201<code class="literal">lambda x.(x = 2, x + 1)</code> rather than
202<code class="literal">lambda x.(x = 2, x + i)</code>.
203     
204</p>
205<p>
206
207As said, this is the default behavior for which there are exceptions.
208The exact rules are as follows:
209
210</p>
211<div class="itemizedlist"><ul type="disc">
212<li>
213<p>
214
215The programmer can control the storing mechanism with <code class="literal">ref</code> 
216and <code class="literal">cref</code> wrappers [<a href="../lambda.html#cit:boost::ref" title="[ref]"><span class="abbrev">ref</span></a>].
217
218Wrapping an argument with <code class="literal">ref</code>, or <code class="literal">cref</code>,
219instructs the library to store the argument as a reference,
220or as a reference to const respectively.
221
222For example, if we rewrite the previous example and wrap the variable
223<code class="literal">i</code> with <code class="literal">ref</code>,
224we are creating the lambda expression <code class="literal">lambda x.(x = 2, x + i)</code> 
225and the value of the expression in the last line will be 4:
226
227</p>
228<pre class="programlisting">
229i = 1;
230(_1 = 2, _1 + ref(i))(i);
231</pre>
232<p>
233
234Note that <code class="literal">ref</code> and <code class="literal">cref</code> are different
235from <code class="literal">var</code> and <code class="literal">constant</code>.
236
237While the latter ones create lambda functors, the former do not.
238For example:
239
240</p>
241<pre class="programlisting">
242int i;
243var(i) = 1; // ok
244ref(i) = 1; // not ok, ref(i) is not a lambda functor
245</pre>
246<p>
247
248The functions <code class="literal">ref</code> and <code class="literal">cref</code> mostly
249exist for historical reasons,
250and <code class="literal">ref</code> can always
251be replaced with <code class="literal">var</code>, and <code class="literal">cref</code> with
252<code class="literal">constant_ref</code>.
253See <a href="le_in_details.html#lambda.delaying_constants_and_variables" title="Delaying constants and variables">the section called &#8220;Delaying constants and variables&#8221;</a> for details.
254The <code class="literal">ref</code> and <code class="literal">cref</code> functions are
255general purpose utility functions in Boost, and hence defined directly
256in the <code class="literal">boost</code> namespace.
257
258</p>
259</li>
260<li><p>
261Array types cannot be copied, they are thus stored as const reference by default.
262</p></li>
263<li>
264<p> 
265For some expressions it makes more sense to store the arguments as references.
266
267For example, the obvious intention of the lambda expression
268<code class="literal">i += _1</code> is that calls to the lambda functor affect the
269value of the variable <code class="literal">i</code>,
270rather than some temporary copy of it.
271
272As another example, the streaming operators take their leftmost argument
273as non-const references.
274
275The exact rules are:
276
277</p>
278<div class="itemizedlist"><ul type="circle">
279<li><p>The left argument of compound assignment operators (<code class="literal">+=</code>, <code class="literal">*=</code>, etc.) are stored as references to non-const.</p></li>
280<li><p>If the left argument of <code class="literal">&lt;&lt;</code> or <code class="literal">&gt;&gt;</code>  operator is derived from an instantiation of <code class="literal">basic_ostream</code> or respectively from <code class="literal">basic_istream</code>, the argument is stored as a reference to non-const.
281For all other types, the argument is stored as a copy.
282</p></li>
283<li><p>
284In pointer arithmetic expressions, non-const array types are stored as non-const references.
285This is to prevent pointer arithmetic making non-const arrays const.
286
287</p></li>
288</ul></div>
289</li>
290</ul></div>
291</div>
292<div class="footnotes">
293<br><hr width="100" align="left">
294<div class="footnote"><p><sup>[<a name="ftn.id2707880" href="#id2707880">1</a>] </sup>
295Strictly taken, the C++ standard defines <code class="literal">for_each</code> as a <span class="emphasis"><em>non-modifying sequence operation</em></span>, and the function object passed to <code class="literal">for_each</code> should not modify its argument.
296The requirements for the arguments of <code class="literal">for_each</code> are unnecessary strict, since as long as the iterators are <span class="emphasis"><em>mutable</em></span>, <code class="literal">for_each</code> accepts a function object that can have side-effects on their argument.
297Nevertheless, it is straightforward to provide another function template with the functionality of<code class="literal">std::for_each</code> but more fine-grained requirements for its arguments.
298</p></div>
299</div>
300</div>
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302<td align="left"></td>
303<td align="right"><small>Copyright © 1999-2004 Jaakko Järvi, Gary Powell</small></td>
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