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Added section describing problems with non-dependent static asserts in templates.

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Added table of contents.


[SVN r25725]
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John Maddock
2004-10-14 11:19:53 +00:00
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static_assert.htm
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@@ -7,7 +7,15 @@
</head> </head>
<body bgcolor="#ffffff" text="#000000" link="#0000ff" vlink="#800080"> <body bgcolor="#ffffff" text="#000000" link="#0000ff" vlink="#800080">
<h1><img src="../../boost.png" width="276" height="86">Header &lt;<a href="../../boost/static_assert.hpp">boost/static_assert.hpp</a>&gt;</h1> <h1><img src="../../boost.png" width="276" height="86">Header &lt;<a href="../../boost/static_assert.hpp">boost/static_assert.hpp</a>&gt;</h1>
<p>The header &lt;boost/static_assert.hpp&gt; supplies a single macro <H3>Contents</H3>
<dl class="index">
<dt><a href="#overview">Overview</a> <dt><a href="#namespace">Use at namespace scope</a>
<dt><a href="#function">Use at function scope</a> <dt><a href="#class">Use at class
scope</a> <dt><a href="#templates">Use in templates</a> <dt><a href="#how">How it
works</a> <dt><a href="#test">Test programs</a></dt>
</dl>
<H3><A name="overview"></A>Overview</H3>
<P>The header &lt;boost/static_assert.hpp&gt; supplies a single macro
BOOST_STATIC_ASSERT(x), which generates a compile time error message if the <a href="../../more/int_const_guidelines.htm"> BOOST_STATIC_ASSERT(x), which generates a compile time error message if the <a href="../../more/int_const_guidelines.htm">
integral-constant-expression</a> <i>x</i> is not true. In other words it is integral-constant-expression</a> <i>x</i> is not true. In other words it is
the compile time equivalent of the assert macro; this is sometimes known as a the compile time equivalent of the assert macro; this is sometimes known as a
@@ -17,7 +25,7 @@
class or function scope. When used in a template, the static assertion will be 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 evaluated at the time the template is instantiated; this is particularly useful
for validating template parameters. for validating template parameters.
</p> </P>
<p>One of the aims of BOOST_STATIC_ASSERT is to generate readable error messages. <p>One of the aims of BOOST_STATIC_ASSERT is to generate readable error messages.
These immediately tell the user that a library is being used in a manner that 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 is not supported. While error messages obviously differ from compiler to
@@ -28,7 +36,7 @@
<p>You can use BOOST_STATIC_ASSERT at any place where you can place a declaration, <p>You can use BOOST_STATIC_ASSERT at any place where you can place a declaration,
that is at <a href="#class">class</a>, <a href="#function">function</a> or <a href="#namespace"> that is at <a href="#class">class</a>, <a href="#function">function</a> or <a href="#namespace">
namespace</a> scope, this is illustrated by the following examples:</p> namespace</a> scope, this is illustrated by the following examples:</p>
<h3><a name="namespace"></a>Use at namespace scope.</h3> <h3><a name="namespace"></a><A name="namespace"></A>Use at namespace scope.</h3>
<p>The macro can be used at namespace scope, if there is some requirement must <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. always be true; generally this means some platform specific requirement.
Suppose we require that <b>int</b> be at least a 32-bit integral type, and that <b> Suppose we require that <b>int</b> be at least a 32-bit integral type, and that <b>
@@ -59,7 +67,7 @@ BOOST_STATIC_ASSERT(WCHAR_MIN &gt;= 0);
entitled to emit warnings in such cases). To avoid potential problems, if you entitled to emit warnings in such cases). To avoid potential problems, if you
use BOOST_STATIC_ASSERT in a header and at namespace scope, then enclose them use BOOST_STATIC_ASSERT in a header and at namespace scope, then enclose them
in a namespace unique to that header.</p> in a namespace unique to that header.</p>
<h3><a name="function"></a>Use at function scope</h3> <h3><a name="function"></a><A name="function"></A>Use at function scope</h3>
<p>The macro is typically used at function scope inside template functions, when <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 the template arguments need checking. Imagine that we have an iterator-based
algorithm that requires random access iterators. If the algorithm is algorithm that requires random access iterators. If the algorithm is
@@ -91,7 +99,7 @@ RandomAccessIterator foo(RandomAccessIterator from, RandomAccessIterator to)
is_convertible when the conversion is via a user defined constructor (in any is_convertible when the conversion is via a user defined constructor (in any
case there is no guarantee that the iterator tag classes are case there is no guarantee that the iterator tag classes are
copy-constructible).</p> copy-constructible).</p>
<h3><a name="class"></a>Use at class scope</h3> <h3><a name="class"></a><A name="class"></A>Use at class scope</h3>
<p>The macro is typically used inside classes that are templates. Suppose we have <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 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 of precision as a template argument, we can achieve this using something like
@@ -111,7 +119,21 @@ public:
/* details here */ /* details here */
}; };
</pre> </pre>
<h3>How it works</h3> <h3><A name="templates"></A>Use in templates</h3>
<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.&nbsp; However, there is one potential problem to watch out for:
if the static assertion is <EM>not</EM> dependent upon&nbsp;one or
more&nbsp;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>template &lt;class T&gt;<BR>struct must_not_be_instantiated<BR>{<BR> BOOST_STATIC_ASSERT(false);<BR><BR><BR><BR>};</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.&nbsp; A
workaround in cases like this is to force the assertion to be dependent upon a
template parameter:</P>
<PRE>template &lt;class T&gt;<BR>struct must_not_be_instantiated<BR>{<BR> BOOST_STATIC_ASSERT(sizeof(T) == 0); // will be triggered if this type is instantiated<BR><BR><BR><BR>};</PRE>
<H3><A name="how"></A>How it works</H3>
<p>BOOST_STATIC_ASSERT works as follows. There is class STATIC_ASSERTION_FAILURE <p>BOOST_STATIC_ASSERT works as follows. There is class STATIC_ASSERTION_FAILURE
which is defined as:</p> which is defined as:</p>
<pre>namespace boost{ <pre>namespace boost{
@@ -129,7 +151,7 @@ template &lt;&gt; struct STATIC_ASSERTION_FAILURE&lt;true&gt;{};
invent a static assert that avoided macros, all to no avail. The general invent a static assert that avoided macros, all to no avail. The general
conclusion was that the good of a static assert working at namespace, function, conclusion was that the good of a static assert working at namespace, function,
and class scope outweighed the ugliness of a macro.</p> and class scope outweighed the ugliness of a macro.</p>
<h3>Test Programs</h3> <h3><A name="test"></A>Test Programs</h3>
<p>The following test programs are provided with this library:</p> <p>The following test programs are provided with this library:</p>
<table border="0" width="100%"> <table border="0" width="100%">
<tr> <tr>