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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="lambda.using_library"></a>Using the library</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="using_library.html#lambda.introductory_examples">Introductory Examples</a></span></dt>
<dt><span class="section"><a href="using_library.html#lambda.parameter_and_return_types">Parameter and return types of lambda functors</a></span></dt>
<dt><span class="section"><a href="using_library.html#lambda.actual_arguments_to_lambda_functors">About actual arguments to lambda functors</a></span></dt>
<dt><span class="section"><a href="using_library.html#lambda.storing_bound_arguments">Storing bound arguments in lambda functions</a></span></dt>
</dl></div>
<p>
The purpose of this section is to introduce the basic functionality of the library.
There are quite a lot of exceptions and special cases, but discussion of them is postponed until later sections.
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="lambda.introductory_examples"></a>Introductory Examples</h3></div></div></div>
<p>
In this section we give basic examples of using BLL lambda expressions in STL algorithm invocations.
We start with some simple expressions and work up.
First, we initialize the elements of a container, say, a <code class="literal">list</code>, to the value <code class="literal">1</code>:
</p>
<pre class="programlisting">
list&lt;int&gt; v(10);
for_each(v.begin(), v.end(), _1 = 1);</pre>
<p>
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>.<a href="#ftn.id-1.3.21.6.3.2.7" class="footnote" name="id-1.3.21.6.3.2.7"><sup class="footnote">[7]</sup></a>
</p>
<p>
Next, we create a container of pointers and make them point to the elements in the first container <code class="literal">v</code>:
</p>
<pre class="programlisting">
vector&lt;int*&gt; vp(10);
transform(v.begin(), v.end(), vp.begin(), &amp;_1);</pre>
<p>
The expression <code class="literal">&amp;_1</code> creates a function object for getting the address of each element in <code class="literal">v</code>.
The addresses get assigned to the corresponding elements in <code class="literal">vp</code>.
</p>
<p>
The next code fragment changes the values in <code class="literal">v</code>.
For each element, the function <code class="literal">foo</code> is called.
The original value of the element is passed as an argument to <code class="literal">foo</code>.
The result of <code class="literal">foo</code> is assigned back to the element:
</p>
<pre class="programlisting">
int foo(int);
for_each(v.begin(), v.end(), _1 = bind(foo, _1));</pre>
<p>
</p>
<p>
The next step is to sort the elements of <code class="literal">vp</code>:
</p>
<pre class="programlisting">sort(vp.begin(), vp.end(), *_1 &gt; *_2);</pre>
<p>
In this call to <code class="literal">sort</code>, we are sorting the elements by their contents in descending order.
</p>
<p>
Finally, the following <code class="literal">for_each</code> call outputs the sorted content of <code class="literal">vp</code> separated by line breaks:
</p>
<pre class="programlisting">
for_each(vp.begin(), vp.end(), cout &lt;&lt; *_1 &lt;&lt; '\n');
</pre>
<p>
Note that a normal (non-lambda) expression as subexpression of a lambda expression is evaluated immediately.
This may cause surprises.
For instance, if the previous example is rewritten as
</p>
<pre class="programlisting">
for_each(vp.begin(), vp.end(), cout &lt;&lt; '\n' &lt;&lt; *_1);
</pre>
<p>
the 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>.
The 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:
</p>
<pre class="programlisting">
for_each(vp.begin(), vp.end(), cout &lt;&lt; constant('\n') &lt;&lt; *_1);
</pre>
<p>
These functions are described more thoroughly in <a class="xref" href="le_in_details.html#lambda.delaying_constants_and_variables" title="Delaying constants and variables">the section called “Delaying constants and variables”</a>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="lambda.parameter_and_return_types"></a>Parameter and return types of lambda functors</h3></div></div></div>
<p>
During the invocation of a lambda functor, the actual arguments are substituted for the placeholders.
The placeholders do not dictate the type of these actual arguments.
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.
As an example, the expression
<code class="literal">_1 + _2</code> creates a binary lambda functor.
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).
</p>
<p>
C++ lacks a mechanism to query a type of an expression.
However, this precise mechanism is crucial for the implementation of C++ lambda expressions.
Consequently, BLL includes a somewhat complex type deduction system which uses a set of traits classes for deducing the resulting type of lambda functions.
It handles expressions where the operands are of built-in types and many of the expressions with operands of standard library types.
Many of the user defined types are covered as well, particularly if the user defined operators obey normal conventions in defining the return types.
</p>
<p>
There are, however, cases when the return type cannot be deduced. For example, suppose you have defined:
</p>
<pre class="programlisting">C operator+(A, B);</pre>
<p>
The following lambda function invocation fails, since the return type cannot be deduced:
</p>
<pre class="programlisting">A a; B b; (_1 + _2)(a, b);</pre>
<p>
</p>
<p>
There are two alternative solutions to this.
The first is to extend the BLL type deduction system to cover your own types (see <a class="xref" href="extending.html" title="Extending return type deduction system">the section called “Extending return type deduction system”</a>).
The second is to use a special lambda expression (<code class="literal">ret</code>) which defines the return type in place (see <a class="xref" href="le_in_details.html#lambda.overriding_deduced_return_type" title="Overriding the deduced return type">the section called “Overriding the deduced return type”</a>):
</p>
<pre class="programlisting">A a; B b; ret&lt;C&gt;(_1 + _2)(a, b);</pre>
<p>
</p>
<p>
For bind expressions, the return type can be defined as a template argument of the bind function as well:
</p>
<pre class="programlisting">bind&lt;int&gt;(foo, _1, _2);</pre>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="lambda.actual_arguments_to_lambda_functors"></a>About actual arguments to lambda functors</h3></div></div></div>
<p>A general restriction for the actual arguments is that they cannot be non-const rvalues.
For example:
</p>
<pre class="programlisting">
int i = 1; int j = 2;
(_1 + _2)(i, j); // ok
(_1 + _2)(1, 2); // error (!)
</pre>
<p>
This restriction is not as bad as it may look.
Since the lambda functors are most often called inside STL-algorithms,
the arguments originate from dereferencing iterators and the dereferencing operators seldom return rvalues.
And for the cases where they do, there are workarounds discussed in
<a class="xref" href="le_in_details.html#lambda.rvalues_as_actual_arguments" title="Rvalues as actual arguments to lambda functors">the section called “Rvalues as actual arguments to lambda functors”</a>.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="lambda.storing_bound_arguments"></a>Storing bound arguments in lambda functions</h3></div></div></div>
<p>
By default, temporary const copies of the bound arguments are stored
in the lambda functor.
This means that the value of a bound argument is fixed at the time of the
creation of the lambda function and remains constant during the lifetime
of the lambda function object.
For example:
</p>
<pre class="programlisting">
int i = 1;
(_1 = 2, _1 + i)(i);
</pre>
<p>
The comma operator is overloaded to combine lambda expressions into a sequence;
the resulting unary lambda functor first assigns 2 to its argument,
then adds the value of <code class="literal">i</code> to it.
The value of the expression in the last line is 3, not 4.
In other words, the lambda expression that is created is
<code class="literal">lambda x.(x = 2, x + 1)</code> rather than
<code class="literal">lambda x.(x = 2, x + i)</code>.
</p>
<p>
As said, this is the default behavior for which there are exceptions.
The exact rules are as follows:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p>
The programmer can control the storing mechanism with <code class="literal">ref</code>
and <code class="literal">cref</code> wrappers <a class="xref" href="../lambda.html#cit:boost::ref" title="Boost ref">[<abbr class="abbrev">ref</abbr>]</a>.
Wrapping an argument with <code class="literal">ref</code>, or <code class="literal">cref</code>,
instructs the library to store the argument as a reference,
or as a reference to const respectively.
For example, if we rewrite the previous example and wrap the variable
<code class="literal">i</code> with <code class="literal">ref</code>,
we are creating the lambda expression <code class="literal">lambda x.(x = 2, x + i)</code>
and the value of the expression in the last line will be 4:
</p>
<pre class="programlisting">
i = 1;
(_1 = 2, _1 + ref(i))(i);
</pre>
<p>
Note that <code class="literal">ref</code> and <code class="literal">cref</code> are different
from <code class="literal">var</code> and <code class="literal">constant</code>.
While the latter ones create lambda functors, the former do not.
For example:
</p>
<pre class="programlisting">
int i;
var(i) = 1; // ok
ref(i) = 1; // not ok, ref(i) is not a lambda functor
</pre>
<p>
The functions <code class="literal">ref</code> and <code class="literal">cref</code> mostly
exist for historical reasons,
and <code class="literal">ref</code> can always
be replaced with <code class="literal">var</code>, and <code class="literal">cref</code> with
<code class="literal">constant_ref</code>.
See <a class="xref" href="le_in_details.html#lambda.delaying_constants_and_variables" title="Delaying constants and variables">the section called “Delaying constants and variables”</a> for details.
The <code class="literal">ref</code> and <code class="literal">cref</code> functions are
general purpose utility functions in Boost, and hence defined directly
in the <code class="literal">boost</code> namespace.
</p>
</li>
<li class="listitem"><p>
Array types cannot be copied, they are thus stored as const reference by default.
</p></li>
<li class="listitem">
<p>
For some expressions it makes more sense to store the arguments as references.
For example, the obvious intention of the lambda expression
<code class="literal">i += _1</code> is that calls to the lambda functor affect the
value of the variable <code class="literal">i</code>,
rather than some temporary copy of it.
As another example, the streaming operators take their leftmost argument
as non-const references.
The exact rules are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem"><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>
<li class="listitem"><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.
For all other types, the argument is stored as a copy.
</p></li>
<li class="listitem"><p>
In pointer arithmetic expressions, non-const array types are stored as non-const references.
This is to prevent pointer arithmetic making non-const arrays const.
</p></li>
</ul></div>
<p>
</p>
</li>
</ul></div>
<p>
</p>
</div>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.id-1.3.21.6.3.2.7" class="footnote"><p><a href="#id-1.3.21.6.3.2.7" class="para"><sup class="para">[7] </sup></a>
Strictly 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.
The 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.
Nevertheless, 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.
</p></div>
</div>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 1999-2004 Jaakko Järvi, Gary Powell<p>Use, modification and distribution is subject to the Boost
Software License, Version 1.0. (See accompanying file
<code class="filename">LICENSE_1_0.txt</code> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</p>
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