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<div><h2 class="title">
<a name="boost_staticassert"></a>Chapter 29. 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>
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<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<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 class="toc">
<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="keyword">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="keyword">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 will 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="keyword">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="keyword">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="keyword">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="keyword">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="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>
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