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| <div class="chapter"> |
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| <div><h2 class="title"> |
| <a name="boost_staticassert"></a>Chapter 20. 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="id2567629"></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://svn.boost.org/svn/boost/sandbox/pdf/static_assert/release/static_assert.pdf" 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 a single macro <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code>, |
| which generates 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 |
| it is 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 macro will generate neither code |
| nor data - and the macro 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> |
| 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: February 23, 2009 at 18:39:32 +0000</small></p></td> |
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