| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Rationale for some of the design decisions</title> |
| <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> |
| <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> |
| <link rel="up" href="../lambda.html" title="Chapter 11. Boost.Lambda"> |
| <link rel="prev" href="s09.html" title="Contributors"> |
| <link rel="next" href="../mpi.html" title="Chapter 12. Boost.MPI"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> |
| <td align="center"><a href="../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="s09.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../lambda.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../mpi.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="id1999367"></a>Rationale for some of the design decisions</h2></div></div></div> |
| <div class="toc"><dl><dt><span class="section"><a href="s10.html#lambda.why_weak_arity"> |
| Lambda functor arity |
| </a></span></dt></dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="lambda.why_weak_arity"></a> |
| Lambda functor arity |
| </h3></div></div></div> |
| <p> |
| The highest placeholder index in a lambda expression determines the arity of the resulting function object. |
| However, this is just the minimal arity, as the function object can take arbitrarily many arguments; those not needed are discarded. |
| Consider the two bind expressions and their invocations below: |
| |
| </p> |
| <pre class="programlisting"> |
| bind(g, _3, _3, _3)(x, y, z); |
| bind(g, _1, _1, _1)(x, y, z); |
| </pre> |
| <p> |
| |
| This first line discards arguments <code class="literal">x</code> and |
| <code class="literal">y</code>, and makes the call: |
| </p> |
| <pre class="programlisting"> |
| g(z, z, z) |
| </pre> |
| <p> |
| whereas the second line discards arguments <code class="literal">y</code> and |
| <code class="literal">z</code>, and calls: |
| </p> |
| <pre class="programlisting"> |
| g(x, x, x) |
| </pre> |
| <p> |
| In earlier versions of the library, the latter line resulted in a compile |
| time error. |
| |
| This is basically a tradeoff between safety and flexibility, and the issue |
| was extensively discussed during the Boost review period of the library. |
| The main points for the <span class="emphasis"><em>strict arity</em></span> checking |
| was that it might |
| catch a programming error at an earlier time and that a lambda expression that |
| explicitly discards its arguments is easy to write: |
| </p> |
| <pre class="programlisting"> |
| (_3, bind(g, _1, _1, _1))(x, y, z); |
| </pre> |
| <p> |
| This lambda expression takes three arguments. |
| The left-hand argument of the comma operator does nothing, and as comma |
| returns the result of evaluating the right-hand argument we end up with |
| the call |
| <code class="literal">g(x, x, x)</code> |
| even with the strict arity. |
| </p> |
| <p> |
| The main points against the strict arity checking were that the need to |
| discard arguments is commonplace, and should therefore be straightforward, |
| and that strict arity checking does not really buy that much more safety, |
| particularly as it is not symmetric. |
| For example, if the programmer wanted to write the expression |
| <code class="literal">_1 + _2</code> but mistakenly wrote <code class="literal">_1 + 2</code>, |
| with strict arity checking, the complier would spot the error. |
| However, if the erroneous expression was <code class="literal">1 + _2</code> instead, |
| the error would go unnoticed. |
| Furthermore, weak arity checking simplifies the implementation a bit. |
| Following the recommendation of the Boost review, strict arity checking |
| was dropped. |
| </p> |
| </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> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="s09.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../lambda.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../mpi.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
