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<div><h2 class="title">
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<a name="boost_staticassert"></a>Chapter 38. Boost.StaticAssert</h2></div>
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<div><div class="author"><h3 class="author">
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<span class="firstname">John</span> <span class="surname">Maddock</span>
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</h3></div></div>
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<div><div class="author"><h3 class="author">
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<span class="firstname">Steve</span> <span class="surname">Cleary</span>
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<div><p class="copyright">Copyright © 2000, 2005 Steve Cleary and John Maddock</p></div>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">
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http://www.boost.org/LICENSE_1_0.txt </a>)
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<p><b>Table of Contents</b></p>
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<dl class="toc">
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<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.intro">Overview and Tutorial</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.namespace">Use at namespace scope.</a></span></dt>
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<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.function">Use at function scope</a></span></dt>
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<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.class">Use at class scope</a></span></dt>
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<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.templates">Use in templates</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="boost_staticassert/how.html">How it works</a></span></dt>
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<dt><span class="section"><a href="boost_staticassert/test.html">Test Programs</a></span></dt>
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</dl>
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<p>
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This manual is also available in <a href="http://sourceforge.net/projects/boost/files/boost-docs/" target="_top">printer
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friendly PDF format</a>.
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</p>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_staticassert.intro"></a>Overview and Tutorial</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.namespace">Use at namespace scope.</a></span></dt>
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<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.function">Use at function scope</a></span></dt>
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<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.class">Use at class scope</a></span></dt>
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<dt><span class="section"><a href="boost_staticassert.html#boost_staticassert.templates">Use in templates</a></span></dt>
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</dl></div>
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<p>
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The header <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="keyword">static_assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
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supplies two macros:
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</p>
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<pre class="programlisting"><span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span>
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<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>
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</pre>
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<p>
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Both generate a compile time error message if the integral-constant-expression
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<code class="computeroutput"><span class="identifier">x</span></code> is not true. In other words,
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they are the compile time equivalent of the assert macro; this is sometimes
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known as a "compile-time-assertion", but will be called a "static
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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
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code nor data - and the macros can also be used at either namespace, class
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or function scope. When used in a template, the static assertion will be evaluated
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at the time the template is instantiated; this is particularly useful for validating
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template parameters.
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</p>
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<p>
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If the C++0x <code class="computeroutput"><span class="keyword">static_assert</span></code> feature
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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>,
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the error message will be a stringized version of <code class="computeroutput"><span class="identifier">x</span></code>.
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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>
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<span class="identifier">msg</span><span class="special">)</span></code>,
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the error message will be the <code class="computeroutput"><span class="identifier">msg</span></code>
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string.
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</p>
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<p>
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If the C++0x <code class="computeroutput"><span class="keyword">static_assert</span></code> feature
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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>
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<span class="identifier">msg</span><span class="special">)</span></code>
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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>.
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</p>
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<p>
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The material that follows assumes the C++0x <code class="computeroutput"><span class="keyword">static_assert</span></code>
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feature is not available.
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</p>
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<p>
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One of the aims of <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code>
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is to generate readable error messages. These immediately tell the user that
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a library is being used in a manner that is not supported. While error messages
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obviously differ from compiler to compiler, but you should see something like:
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</p>
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<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>
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</pre>
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<p>
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Which is intended to at least catch the eye!
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</p>
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<p>
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You can use <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code>
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at any place where you can place a declaration, that is at class, function
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or namespace scope, this is illustrated by the following examples:
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</p>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_staticassert.namespace"></a>Use at namespace scope.</h3></div></div></div>
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<p>
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The macro can be used at namespace scope, if there is some requirement must
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always be true; generally this means some platform specific requirement.
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Suppose we require that <code class="computeroutput"><span class="keyword">int</span></code>
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be at least a 32-bit integral type, and that <code class="computeroutput"><span class="keyword">wchar_t</span></code>
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be an unsigned type. We can verify this at compile time as follows:
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">climits</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cwchar</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">limits</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="keyword">static_assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="keyword">namespace</span> <span class="identifier">my_conditions</span> <span class="special">{</span>
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<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>
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<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>
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<span class="special">}</span> <span class="comment">// namespace my_conditions</span>
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||
</pre>
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<p>
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The use of the namespace my_conditions here requires some comment. The macro
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<code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code> works
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by generating an typedef declaration, and since the typedef must have a name,
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the macro generates one automatically by mangling a stub name with the value
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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
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||
or function scope then each use of <code class="computeroutput"><span class="identifier">BOOST_STATIC_ASSERT</span></code>
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||
is guaranteed to produce a name unique to that scope (provided you only use
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the macro once on each line). However when used in a header at namespace
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||
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>
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||
in a header and at namespace scope, then enclose them in a namespace unique
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||
to that header.
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||
</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="keyword">static_assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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||
<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>
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|
||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">RandomAccessIterator</span> <span class="special">></span>
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||
<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>
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||
<span class="special">{</span>
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<span class="comment">// this template can only be used with</span>
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<span class="comment">// random access iterators...</span>
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||
<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>
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||
<span class="identifier">RandomAccessIterator</span> <span class="special">>::</span><span class="identifier">iterator_category</span> <span class="identifier">cat</span><span class="special">;</span>
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||
<span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span>
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||
<span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_convertible</span><span class="special"><</span>
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||
<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>
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||
<span class="comment">// detail goes here...</span>
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||
<span class="keyword">return</span> <span class="identifier">from</span><span class="special">;</span>
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||
<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="keyword">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_MSG</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="string">"myclass can only be specialized for types with numeric_limits support."</span><span class="special">);</span>
|
||
<span class="identifier">BOOST_STATIC_ASSERT_MSG</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="string">"Template argument UnsignedInt must have at least 16 bits precision."</span><span class="special">)</span>
|
||
<span class="identifier">BOOST_STATIC_ASSERT_MSG</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="string">"Template argument UnsignedInt must be an integer."</span><span class="special">);</span>
|
||
<span class="identifier">BOOST_STATIC_ASSERT_MSG</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="string">"Template argument UnsignedInt must not be 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>
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