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| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="id1992665"></a>Introduction</h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="s03.html#id1992670">Motivation</a></span></dt> |
| <dt><span class="section"><a href="s03.html#id1992932">Introduction to lambda expressions</a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id1992670"></a>Motivation</h3></div></div></div> |
| <p>The Standard Template Library (STL) |
| <a class="xref" href="../lambda.html#cit:stepanov:94" title="The Standard Template Library">[<abbr class="abbrev">STL94</abbr>]</a>, now part of the C++ Standard Library <a class="xref" href="../lambda.html#cit:c++:98" title="International Standard, Programming Languages – C++">[<abbr class="abbrev">C++98</abbr>]</a>, is a generic container and algorithm library. |
| Typically STL algorithms operate on container elements via <span class="emphasis"><em>function objects</em></span>. These function objects are passed as arguments to the algorithms. |
| </p> |
| <p> |
| Any C++ construct that can be called with the function call syntax |
| is a function object. |
| The STL contains predefined function objects for some common cases (such as <code class="literal">plus</code>, <code class="literal">less</code> and <code class="literal">not1</code>). |
| As an example, one possible implementation for the standard <code class="literal">plus</code> template is: |
| |
| </p> |
| <pre class="programlisting"> |
| template <class T> |
| struct plus : public binary_function<T, T, T> { |
| T operator()(const T& i, const T& j) const { |
| return i + j; |
| } |
| }; |
| </pre> |
| <p> |
| |
| The base class <code class="literal">binary_function<T, T, T></code> contains typedefs for the argument and return types of the function object, which are needed to make the function object <span class="emphasis"><em>adaptable</em></span>. |
| </p> |
| <p> |
| In addition to the basic function object classes, such as the one above, |
| the STL contains <span class="emphasis"><em>binder</em></span> templates for creating a unary function object from an adaptable binary function object by fixing one of the arguments to a constant value. |
| For example, instead of having to explicitly write a function object class like: |
| |
| </p> |
| <pre class="programlisting"> |
| class plus_1 { |
| int _i; |
| public: |
| plus_1(const int& i) : _i(i) {} |
| int operator()(const int& j) { return _i + j; } |
| }; |
| </pre> |
| <p> |
| |
| the equivalent functionality can be achieved with the <code class="literal">plus</code> template and one of the binder templates (<code class="literal">bind1st</code>). |
| E.g., the following two expressions create function objects with identical functionalities; |
| when invoked, both return the result of adding <code class="literal">1</code> to the argument of the function object: |
| |
| </p> |
| <pre class="programlisting"> |
| plus_1(1) |
| bind1st(plus<int>(), 1) |
| </pre> |
| <p> |
| |
| The subexpression <code class="literal">plus<int>()</code> in the latter line is a binary function object which computes the sum of two integers, and <code class="literal">bind1st</code> invokes this function object partially binding the first argument to <code class="literal">1</code>. |
| As an example of using the above function object, the following code adds <code class="literal">1</code> to each element of some container <code class="literal">a</code> and outputs the results into the standard output stream <code class="literal">cout</code>. |
| |
| </p> |
| <pre class="programlisting"> |
| transform(a.begin(), a.end(), ostream_iterator<int>(cout), |
| bind1st(plus<int>(), 1)); |
| </pre> |
| <p> |
| |
| </p> |
| <p> |
| To make the binder templates more generally applicable, the STL contains <span class="emphasis"><em>adaptors</em></span> for making |
| pointers or references to functions, and pointers to member functions, |
| adaptable. |
| |
| Finally, some STL implementations contain function composition operations as |
| extensions to the standard <a class="xref" href="../lambda.html#cit:sgi:02" title="The SGI Standard Template Library">[<abbr class="abbrev">SGI02</abbr>]</a>. |
| </p> |
| <p> |
| All these tools aim at one goal: to make it possible to specify |
| <span class="emphasis"><em>unnamed functions</em></span> in a call of an STL algorithm, |
| in other words, to pass code fragments as an argument to a function. |
| |
| However, this goal is attained only partially. |
| The simple example above shows that the definition of unnamed functions |
| with the standard tools is cumbersome. |
| |
| Complex expressions involving functors, adaptors, binders and |
| function composition operations tend to be difficult to comprehend. |
| |
| In addition to this, there are significant restrictions in applying |
| the standard tools. E.g. the standard binders allow only one argument |
| of a binary function to be bound; there are no binders for |
| 3-ary, 4-ary etc. functions. |
| </p> |
| <p> |
| The Boost Lambda Library provides solutions for the problems described above: |
| |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| <p> |
| Unnamed functions can be created easily with an intuitive syntax. |
| |
| The above example can be written as: |
| |
| </p> |
| <pre class="programlisting"> |
| transform(a.begin(), a.end(), ostream_iterator<int>(cout), |
| 1 + _1); |
| </pre> |
| <p> |
| |
| or even more intuitively: |
| |
| </p> |
| <pre class="programlisting"> |
| for_each(a.begin(), a.end(), cout << (1 + _1)); |
| </pre> |
| <p> |
| </p> |
| </li> |
| <li class="listitem"><p> |
| Most of the restrictions in argument binding are removed, |
| arbitrary arguments of practically any C++ function can be bound. |
| </p></li> |
| <li class="listitem"><p> |
| Separate function composition operations are not needed, |
| as function composition is supported implicitly. |
| |
| </p></li> |
| </ul></div> |
| <p> |
| |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id1992932"></a>Introduction to lambda expressions</h3></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="s03.html#lambda.partial_function_application">Partial function application</a></span></dt> |
| <dt><span class="section"><a href="s03.html#lambda.terminology">Terminology</a></span></dt> |
| </dl></div> |
| <p> |
| Lambda expression are common in functional programming languages. |
| Their syntax varies between languages (and between different forms of lambda calculus), but the basic form of a lambda expressions is: |
| |
| |
| </p> |
| <pre class="programlisting"> |
| lambda x<sub>1</sub> ... x<sub>n</sub>.e |
| </pre> |
| <p> |
| |
| |
| A lambda expression defines an unnamed function and consists of: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"><p> |
| the parameters of this function: <code class="literal">x<sub>1</sub> ... x<sub>n</sub></code>. |
| |
| </p></li> |
| <li class="listitem"><p>the expression e which computes the value of the function in terms of the parameters <code class="literal">x<sub>1</sub> ... x<sub>n</sub></code>. |
| </p></li> |
| </ul></div> |
| <p> |
| |
| A simple example of a lambda expression is |
| </p> |
| <pre class="programlisting"> |
| lambda x y.x+y |
| </pre> |
| <p> |
| Applying the lambda function means substituting the formal parameters with the actual arguments: |
| </p> |
| <pre class="programlisting"> |
| (lambda x y.x+y) 2 3 = 2 + 3 = 5 |
| </pre> |
| <p> |
| |
| |
| </p> |
| <p> |
| In the C++ version of lambda expressions the <code class="literal">lambda x<sub>1</sub> ... x<sub>n</sub></code> part is missing and the formal parameters have predefined names. |
| In the current version of the library, |
| there are three such predefined formal parameters, |
| called <span class="emphasis"><em>placeholders</em></span>: |
| <code class="literal">_1</code>, <code class="literal">_2</code> and <code class="literal">_3</code>. |
| They refer to the first, second and third argument of the function defined |
| by the lambda expression. |
| |
| For example, the C++ version of the definition |
| </p> |
| <pre class="programlisting">lambda x y.x+y</pre> |
| <p> |
| is |
| </p> |
| <pre class="programlisting">_1 + _2</pre> |
| <p> |
| </p> |
| <p> |
| Hence, there is no syntactic keyword for C++ lambda expressions. |
| The use of a placeholder as an operand implies that the operator invocation is a lambda expression. |
| However, this is true only for operator invocations. |
| Lambda expressions containing function calls, control structures, casts etc. require special syntactic constructs. |
| Most importantly, function calls need to be wrapped inside a <code class="literal">bind</code> function. |
| |
| As an example, consider the lambda expression: |
| |
| </p> |
| <pre class="programlisting">lambda x y.foo(x,y)</pre> |
| <p> |
| |
| Rather than <code class="literal">foo(_1, _2)</code>, the C++ counterpart for this expression is: |
| |
| </p> |
| <pre class="programlisting">bind(foo, _1, _2)</pre> |
| <p> |
| |
| We refer to this type of C++ lambda expressions as <span class="emphasis"><em>bind expressions</em></span>. |
| </p> |
| <p>A lambda expression defines a C++ function object, hence function application syntax is like calling any other function object, for instance: <code class="literal">(_1 + _2)(i, j)</code>. |
| |
| |
| </p> |
| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="lambda.partial_function_application"></a>Partial function application</h4></div></div></div> |
| <p> |
| A bind expression is in effect a <span class="emphasis"><em>partial function application</em></span>. |
| In partial function application, some of the arguments of a function are bound to fixed values. |
| The result is another function, with possibly fewer arguments. |
| When called with the unbound arguments, this new function invokes the original function with the merged argument list of bound and unbound arguments. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="lambda.terminology"></a>Terminology</h4></div></div></div> |
| <p> |
| A lambda expression defines a function. A C++ lambda expression concretely constructs a function object, <span class="emphasis"><em>a functor</em></span>, when evaluated. We use the name <span class="emphasis"><em>lambda functor</em></span> to refer to such a function object. |
| Hence, in the terminology adopted here, the result of evaluating a lambda expression is a lambda functor. |
| </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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