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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> |
| <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<int> 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>.<sup>[<a name="id1993242" href="#ftn.id1993242" class="footnote">1</a>]</sup> |
| </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<int*> vp(10); |
| transform(v.begin(), v.end(), vp.begin(), &_1);</pre> |
| <p> |
| |
| The expression <code class="literal">&_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 > *_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 << *_1 << '\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 << '\n' << *_1); |
| </pre> |
| <p> |
| the subexpression <code class="literal">cout << '\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 << constant('\n') << *_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<C>(_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<int>(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" 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" 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"><<</code> or <code class="literal">>></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 width="100" align="left"> |
| <div class="footnote"><p><sup>[<a name="ftn.id1993242" href="#id1993242" class="para">1</a>] </sup> |
| 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> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <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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