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| author | Anas Nashif <anas.nashif@intel.com> | 2012-10-30 12:57:26 -0700 |
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| committer | Anas Nashif <anas.nashif@intel.com> | 2012-10-30 12:57:26 -0700 |
| commit | 1a78a62555be32868418fe52f8e330c9d0f95d5a (patch) | |
| tree | d3765a80e7d3b9640ec2e930743630cd6b9fce2b /doc/html/boost_staticassert.html | |
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Imported Upstream version 1.49.0upstream/1.49.0
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diff --git a/doc/html/boost_staticassert.html b/doc/html/boost_staticassert.html new file mode 100644 index 0000000000..03651e18ba --- /dev/null +++ b/doc/html/boost_staticassert.html @@ -0,0 +1,277 @@ +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> +<title>Chapter 26. Boost.StaticAssert</title> +<link rel="stylesheet" href="../../doc/src/boostbook.css" type="text/css"> +<meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> +<link rel="home" href="index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> +<link rel="up" href="libraries.html" title="Part I. The Boost C++ Libraries (BoostBook Subset)"> +<link rel="prev" href="signals2/tests.html" title="Testsuite"> +<link rel="next" href="boost_staticassert/how.html" title="How it works"> +</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="signals2/tests.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.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="boost_staticassert/how.html"><img src="../../doc/src/images/next.png" alt="Next"></a> +</div> +<div class="chapter"> +<div class="titlepage"><div> +<div><h2 class="title"> +<a name="boost_staticassert"></a>Chapter 26. Boost.StaticAssert</h2></div> +<div><div class="author"><h3 class="author"> +<span class="firstname">John</span> <span class="surname">Maddock</span> +</h3></div></div> +<div><div class="author"><h3 class="author"> +<span class="firstname">Steve</span> <span class="surname">Cleary</span> +</h3></div></div> +<div><p class="copyright">Copyright © 2000, 2005 Steve Cleary and John Maddock</p></div> +<div><div class="legalnotice"> +<a name="boost_staticassert.legal"></a><p> + Distributed under the Boost Software License, Version 1.0. (See accompanying + file LICENSE_1_0.txt 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></div> +</div></div> +<div class="toc"> +<p><b>Table of Contents</b></p> +<dl> +<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.intro">Overview and Tutorial</a></span></dt> +<dd><dl> +<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.namespace">Use at namespace scope.</a></span></dt> +<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.function">Use at function scope</a></span></dt> +<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.class">Use at class scope</a></span></dt> +<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.templates">Use in templates</a></span></dt> +</dl></dd> +<dt><span class="section"><a href="boost_staticassert/how.html">How it works</a></span></dt> +<dt><span class="section"><a href="boost_staticassert/test.html">Test Programs</a></span></dt> +</dl> +</div> +<p> + This manual is also available in <a href="http://sourceforge.net/projects/boost/files/boost-docs/" target="_top">printer + friendly PDF format</a>. + </p> +<div class="section"> +<div class="titlepage"><div><div><h2 class="title" style="clear: both"> +<a name="boost_staticassert.intro"></a>Overview and Tutorial</h2></div></div></div> +<div class="toc"><dl> +<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.namespace">Use at namespace scope.</a></span></dt> +<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.function">Use at function scope</a></span></dt> +<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.class">Use at class scope</a></span></dt> +<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.templates">Use in templates</a></span></dt> +</dl></div> +<p> + The header <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">static_assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> + supplies two macros: + </p> +<pre class="programlisting"><span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> +<span class="identifier">BOOST_STATIC_ASSERT_MSG</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">msg</span><span class="special">)</span> +</pre> +<p> + Both generate a compile time error message if the integral-constant-expression + <code class="computeroutput"><span class="identifier">x</span></code> is not true. In other words, + they are the compile time equivalent of the assert macro; this is sometimes + known as a "compile-time-assertion", but will be called a "static + assertion" throughout these docs. Note that if the condition is <code class="computeroutput"><span class="keyword">true</span></code>, then the macros will generate neither + code nor data - and the macros can also be used at either namespace, class + or function scope. When used in a template, the static assertion will be evaluated + at the time the template is instantiated; this is particularly useful for validating + template parameters. + </p> +<p> + If the C++0x <code class="computeroutput"><span class="identifier">static_assert</span></code> + feature is available, both macros will use it. For <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code>, + the error message with be a stringized version of <code class="computeroutput"><span class="identifier">x</span></code>. + For <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT_MSG</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> + <span class="identifier">msg</span><span class="special">)</span></code>, + the error message will be the <code class="computeroutput"><span class="identifier">msg</span></code> + string. + </p> +<p> + If the C++0x <code class="computeroutput"><span class="identifier">static_assert</span></code> + feature is not available, <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT_MSG</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> + <span class="identifier">msg</span><span class="special">)</span></code> + will be treated as <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code>. + </p> +<p> + The material that follows assumes the C++0x <code class="computeroutput"><span class="identifier">static_assert</span></code> + feature is not available. + </p> +<p> + One of the aims of <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> + is to generate readable error messages. These immediately tell the user that + a library is being used in a manner that is not supported. While error messages + obviously differ from compiler to compiler, but you should see something like: + </p> +<pre class="programlisting"><span class="identifier">Illegal</span> <span class="identifier">use</span> <span class="identifier">of</span> <span class="identifier">STATIC_ASSERTION_FAILURE</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span> +</pre> +<p> + Which is intended to at least catch the eye! + </p> +<p> + You can use <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> + at any place where you can place a declaration, that is at class, function + or namespace scope, this is illustrated by the following examples: + </p> +<div class="section"> +<div class="titlepage"><div><div><h3 class="title"> +<a name="boost_staticassert.namespace"></a>Use at namespace scope.</h3></div></div></div> +<p> + The macro can be used at namespace scope, if there is some requirement must + always be true; generally this means some platform specific requirement. + Suppose we require that <code class="computeroutput"><span class="keyword">int</span></code> + be at least a 32-bit integral type, and that <code class="computeroutput"><span class="keyword">wchar_t</span></code> + be an unsigned type. We can verify this at compile time as follows: + </p> +<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">climits</span><span class="special">></span> +<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cwchar</span><span class="special">></span> +<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">limits</span><span class="special">></span> +<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">static_assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> + +<span class="keyword">namespace</span> <span class="identifier">my_conditions</span> <span class="special">{</span> + + <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">digits</span> <span class="special">>=</span> <span class="number">32</span><span class="special">);</span> + <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="identifier">WCHAR_MIN</span> <span class="special">>=</span> <span class="number">0</span><span class="special">);</span> + +<span class="special">}</span> <span class="comment">// namespace my_conditions</span> +</pre> +<p> + The use of the namespace my_conditions here requires some comment. The macro + <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> works + by generating an typedef declaration, and since the typedef must have a name, + the macro generates one automatically by mangling a stub name with the value + of <code class="computeroutput"><span class="identifier">__LINE__</span></code>. When <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> is used at either class + or function scope then each use of <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> + is guaranteed to produce a name unique to that scope (provided you only use + the macro once on each line). However when used in a header at namespace + scope, that namespace can be continued over multiple headers, each of which + may have their own static assertions, and on the "same" lines, + thereby generating duplicate declarations. In theory the compiler should + silently ignore duplicate typedef declarations, however many do not do so + (and even if they do they are entitled to emit warnings in such cases). To + avoid potential problems, if you use <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> + in a header and at namespace scope, then enclose them in a namespace unique + to that header. + </p> +</div> +<div class="section"> +<div class="titlepage"><div><div><h3 class="title"> +<a name="boost_staticassert.function"></a>Use at function scope</h3></div></div></div> +<p> + The macro is typically used at function scope inside template functions, + when the template arguments need checking. Imagine that we have an iterator-based + algorithm that requires random access iterators. If the algorithm is instantiated + with iterators that do not meet our requirements then an error will be generated + eventually, but this may be nested deep inside several templates, making + it hard for the user to determine what went wrong. One option is to add a + static assertion at the top level of the template, in that case if the condition + is not met, then an error will be generated in a way that makes it reasonably + obvious to the user that the template is being misused. + </p> +<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iterator</span><span class="special">></span> +<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">static_assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> +<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> + +<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">RandomAccessIterator</span> <span class="special">></span> +<span class="identifier">RandomAccessIterator</span> <span class="identifier">foo</span><span class="special">(</span><span class="identifier">RandomAccessIterator</span> <span class="identifier">from</span><span class="special">,</span> + <span class="identifier">RandomAccessIterator</span> <span class="identifier">to</span><span class="special">)</span> +<span class="special">{</span> + <span class="comment">// this template can only be used with</span> + <span class="comment">// random access iterators...</span> + <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">iterator_traits</span><span class="special"><</span> + <span class="identifier">RandomAccessIterator</span> <span class="special">>::</span><span class="identifier">iterator_category</span> <span class="identifier">cat</span><span class="special">;</span> + <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span> + <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_convertible</span><span class="special"><</span> + <span class="identifier">cat</span><span class="special">,</span> + <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">random_access_iterator_tag</span><span class="special">&>::</span><span class="identifier">value</span><span class="special">));</span> + <span class="comment">//</span> + <span class="comment">// detail goes here...</span> + <span class="keyword">return</span> <span class="identifier">from</span><span class="special">;</span> +<span class="special">}</span> +</pre> +<p> + A couple of footnotes are in order here: the extra set of parenthesis around + the assert, is to prevent the comma inside the <code class="computeroutput"><span class="identifier">is_convertible</span></code> + template being interpreted by the preprocessor as a macro argument separator; + the target type for <code class="computeroutput"><span class="identifier">is_convertible</span></code> + is a reference type, as some compilers have problems using <code class="computeroutput"><span class="identifier">is_convertible</span></code> when the conversion is via + a user defined constructor (in any case there is no guarantee that the iterator + tag classes are copy-constructible). + </p> +</div> +<div class="section"> +<div class="titlepage"><div><div><h3 class="title"> +<a name="boost_staticassert.class"></a>Use at class scope</h3></div></div></div> +<p> + The macro is typically used inside classes that are templates. Suppose we + have a template-class that requires an unsigned integral type with at least + 16-bits of precision as a template argument, we can achieve this using something + like this: + </p> +<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">limits</span><span class="special">></span> +<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">static_assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> + +<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">UnsignedInt</span><span class="special">></span> +<span class="keyword">class</span> <span class="identifier">myclass</span> +<span class="special">{</span> +<span class="keyword">private</span><span class="special">:</span> + <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">UnsignedInt</span><span class="special">>::</span><span class="identifier">digits</span> <span class="special">>=</span> <span class="number">16</span><span class="special">)</span> + <span class="special">&&</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">UnsignedInt</span><span class="special">>::</span><span class="identifier">is_specialized</span> + <span class="special">&&</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">UnsignedInt</span><span class="special">>::</span><span class="identifier">is_integer</span> + <span class="special">&&</span> <span class="special">!</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">UnsignedInt</span><span class="special">>::</span><span class="identifier">is_signed</span><span class="special">);</span> +<span class="keyword">public</span><span class="special">:</span> + <span class="comment">/* details here */</span> +<span class="special">};</span> +</pre> +</div> +<div class="section"> +<div class="titlepage"><div><div><h3 class="title"> +<a name="boost_staticassert.templates"></a>Use in templates</h3></div></div></div> +<p> + Normally static assertions when used inside a class or function template, + will not be instantiated until the template in which it is used is instantiated. + However, there is one potential problem to watch out for: if the static assertion + is not dependent upon one or more template parameters, then the compiler + is permitted to evaluate the static assertion at the point it is first seen, + irrespective of whether the template is ever instantiated, for example: + </p> +<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> +<span class="keyword">struct</span> <span class="identifier">must_not_be_instantiated</span> +<span class="special">{</span> + <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span> +<span class="special">};</span> +</pre> +<p> + Will produce a compiler error with some compilers (for example Intel 8.1 + or gcc 3.4), regardless of whether the template is ever instantiated. A workaround + in cases like this is to force the assertion to be dependent upon a template + parameter: + </p> +<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> +<span class="keyword">struct</span> <span class="identifier">must_not_be_instantiated</span> +<span class="special">{</span> + <span class="comment">// this will be triggered if this type is instantiated</span> + <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">T</span><span class="special">)</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span> +<span class="special">};</span> +</pre> +</div> +</div> +</div> +<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> +<td align="left"><p><small>Last revised: April 25, 2011 at 18:27:05 +0100</small></p></td> +<td align="right"><div class="copyright-footer"></div></td> +</tr></table> +<hr> +<div class="spirit-nav"> +<a accesskey="p" href="signals2/tests.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.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="boost_staticassert/how.html"><img src="../../doc/src/images/next.png" alt="Next"></a> +</div> +</body> +</html> |