From 7691bb2713de48cda2b4ce1e71d03b5e122b958f Mon Sep 17 00:00:00 2001
From: John Maddock <john@johnmaddock.co.uk>
Date: Mon, 30 Apr 2001 11:29:06 +0000
Subject: [PATCH] shortened name of integral constant guidelines docs
[SVN r9994]
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<p><a href="microsoft_vcpp.html"><b>Portability Hints:
Microsoft VC++ 6.0 SP4</b></a> describes Microsoft C++
portability issues, with suggested workarounds.</p>
- <p><a href="integral_constant_guidelines.htm"><strong>Coding
+ <p><a href="int_const_guidelines.htm"><strong>Coding
Guidelines for Integral Constant Expressions</strong></a>
describes how to work through the maze of compiler related
bugs surrounding this tricky topic.</p>
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<hr>
<p>Revised <!--webbot bot="Timestamp" startspan s-type="EDITED"
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+content="text/html; charset=iso-8859-1">
+<meta name="Template"
+content="C:\PROGRAM FILES\MICROSOFT OFFICE\OFFICE\html.dot">
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+<body bgcolor="#FFFFFF" link="#0000FF" vlink="#800080">
+
+<h2 align="center">Coding Guidelines for Integral Constant
+Expressions</h2>
+
+<p>Integral Constant Expressions are used in many places in C++;
+as array bounds, as bit-field lengths, as enumerator
+initialisers, and as arguments to non-type template parameters.
+However many compilers have problems handling integral constant
+expressions; as a result of this, programming using non-type
+template parameters in particular can be fraught with difficulty,
+often leading to the incorrect assumption that non-type template
+parameters are unsupported by a particular compiler. This short
+article is designed to provide a set of guidelines and
+workarounds that, if followed, will allow integral constant
+expressions to be used in a manner portable to all the compilers
+currently supported by boost. Although this article is mainly
+targeted at boost library authors, it may also be useful for
+users who want to understand why boost code is written in a
+particular way, or who want to write portable code themselves.</p>
+
+<h3>What is an Integral Constant Expression?</h3>
+
+<p>Integral constant expressions are described in section 5.19 of
+the standard, and are sometimes referred to as "compile time
+constants". An integral constant expression can be one of
+the following:</p>
+
+<ol>
+ <li>A literal integral value, for example 0u or 3L.</li>
+ <li>An enumerator value.</li>
+ <li>Global integral constants, for example: <font
+ face="Courier New"><code><br>
+ </code></font><code>const int my_INTEGRAL_CONSTANT = 3;</code></li>
+ <li>Static member constants, for example: <br>
+ <code>struct myclass<br>
+ { static const int value = 0; };</code></li>
+ <li>Member enumerator values, for example:<br>
+ <code>struct myclass<br>
+ { enum{ value = 0 }; };</code></li>
+ <li>Non-type template parameters of integral or enumerator
+ type.</li>
+ <li>The result of a <code>sizeof</code> expression, for
+ example:<br>
+ <code>sizeof(foo(a, b, c))</code></li>
+ <li>The result of a <code>static_cast</code>, where the
+ target type is an integral or enumerator type, and the
+ argument is either another integral constant expression,
+ or a floating-point literal.</li>
+ <li>The result of applying a binary operator to two integral
+ constant expressions: <br>
+ <code>INTEGRAL_CONSTANT1 op INTEGRAL_CONSTANT2 <br>
+ p</code>rovided that the operator is not an assignment
+ operator, or comma operator.</li>
+ <li>The result of applying a unary operator to an integral
+ constant expression: <br>
+ <code>op INTEGRAL_CONSTANT1<br>
+ </code>provided that the operator is not the increment or
+ decrement operator.</li>
+</ol>
+
+<p> </p>
+
+<h3>Coding Guidelines</h3>
+
+<p>The following guidelines are declared in no particular order (in
+other words you need to obey all of them - sorry!), and may also
+be incomplete, more guidelines may be added as compilers change
+and/or more problems are encountered.</p>
+
+<p><b><i>When declaring constants that are class members always
+use the macro BOOST_STATIC_CONSTANT.</i></b></p>
+
+<pre>template <class T>
+struct myclass
+{
+ BOOST_STATIC_CONSTANT(int, value = sizeof(T));
+};</pre>
+
+<p>Rationale: not all compilers support inline initialisation of
+member constants, others treat member enumerators in strange ways
+(they're not always treated as integral constant expressions).
+The BOOST_STATIC_CONSTANT macro uses the most appropriate method
+for the compiler in question.</p>
+
+<p><b><i>Don't declare integral constant expressions whose type
+is wider than int.</i></b></p>
+
+<p>Rationale: while in theory all integral types are usable in
+integral constant expressions, in practice many compilers limit
+integral constant expressions to types no wider than <b>int</b>.</p>
+
+<p><b><i>Don't use logical operators in integral constant
+expressions; use template meta-programming instead.</i></b></p>
+
+<p>The header <boost/type_traits/ice.hpp> contains a number
+of workaround templates, that fulfil the role of logical
+operators, for example instead of:</p>
+
+<p><code>INTEGRAL_CONSTANT1 | INTEGRAL_CONSTANT2</code></p>
+
+<p>Use:</p>
+
+<p><code>::boost::type_traits::ice_or<INTEGRAL_CONSTANT1,INTEGRAL_CONSTANT2>::value</code></p>
+
+<p>Rationale: A number of compilers (particularly the Borland and
+Microsoft compilers), tend to not to recognise integral constant
+expressions involving logical operators as genuine integral
+constant expressions. The problem generally only shows up when
+the integral constant expression is nested deep inside template
+code, and is hard to reproduce and diagnose.</p>
+
+<p><b><i>Don't use any operators in an integral constant
+expression used as a non-type template parameter</i></b></p>
+
+<p>Rather than:</p>
+
+<p><code>typedef myclass<INTEGRAL_CONSTANT1 ==
+INTEGRAL_CONSTANT2> mytypedef;</code></p>
+
+<p>Use:</p>
+
+<p><code>typedef myclass< some_symbol> mytypedef;</code></p>
+
+<p>Where <code>some_symbol</code> is the symbolic name of a an
+integral constant expression whose value is <code>(INTEGRAL_CONSTANT1
+== INTEGRAL_CONSTANT2).</code></p>
+
+<p>Rationale: the older EDG based compilers (some of which are
+used in the most recent version of that platform's compiler),
+don't recognise expressions containing operators as non-type
+template parameters, even though such expressions can be used as
+integral constant expressions elsewhere.</p>
+
+<p><b><i>Always use a fully qualified name to refer to an
+integral constant expression.</i></b></p>
+
+<p>For example:</p>
+
+<pre><code>typedef</code> myclass< ::boost::is_integral<some_type>::value> mytypedef;</pre>
+
+<p>Rationale: at least one compiler (Borland's), doesn't
+recognise the name of a constant as an integral constant
+expression unless the name is fully qualified (which is to say it
+starts with ::).</p>
+
+<p><b><i>Always leave a space after a '<' and before '::'</i></b></p>
+
+<p>For example:</p>
+
+<pre><code>typedef</code> myclass< ::boost::is_integral<some_type>::value> mytypedef;
+ ^
+ ensure there is space here!</pre>
+
+<p>Rationale: <: is a legal digraph in it's own right, so <::
+is interpreted as the same as [:.</p>
+
+<p><b><i>Don't use local names as integral constant expressions</i></b></p>
+
+<p>Example:</p>
+
+<pre>template <class T>
+struct foobar
+{
+ BOOST_STATIC_CONSTANT(int, temp = computed_value);
+ typedef myclass<temp> mytypedef; // error
+};</pre>
+
+<p>Rationale: At least one compiler (Borland's) doesn't accept
+this.</p>
+
+<p>Although it is possible to fix this by using:</p>
+
+<pre>template <class T>
+struct foobar
+{
+ BOOST_STATIC_CONSTANT(int, temp = computed_value);
+ typedef foobar self_type;
+ typedef myclass<(self_type::temp)> mytypedef; // OK
+};</pre>
+
+<p>This breaks at least one other compiler (VC6), it is better to
+move the integral constant expression computation out into a
+separate traits class:</p>
+
+<pre>template <class T>
+struct foobar_helper
+{
+ BOOST_STATIC_CONSTANT(int, temp = computed_value);
+};
+
+template <class T>
+struct foobar
+{
+ typedef myclass< ::foobar_helper<T>::value> mytypedef; // OK
+};</pre>
+
+<p><b><i>Don't use dependent default parameters for non-type
+template parameters.</i></b></p>
+
+<p>For example:</p>
+
+<pre>template <class T, int I = ::boost::is_integral<T>::value> // Error can't deduce value of I in some cases.
+struct foobar;</pre>
+
+<p>Rationale: this kind of usage fails for Borland C++. Note that
+this is only an issue where the default value is dependent upon a
+previous template parameter, for example the following is fine:</p>
+
+<pre>template <class T, int I = 3> // OK, default value is not dependent
+struct foobar;</pre>
+
+<p> </p>
+
+<h3>Unresolved Issues</h3>
+
+<p>The following issues are either unresolved or have fixes that
+are compiler specific, and/or break one or more of the coding
+guidelines.</p>
+
+<p><b><i>Be careful of numeric_limits</i></b></p>
+
+<p>There are three issues here:</p>
+
+<ol>
+ <li>The header <limits> may be absent - it is
+ recommended that you never include <limits>
+ directly but use <boost/pending/limits.hpp> instead.
+ This header includes the "real" <limits>
+ header if it is available, otherwise it supplies it's own
+ std::numeric_limits definition. Boost also defines the
+ macro BOOST_NO_LIMITS if <limits> is absent.</li>
+ <li>The implementation of std::numeric_limits may be defined
+ in such a way that its static-const members may not be
+ usable as integral constant expressions. This contradicts
+ the standard but seems to be a bug that affects at least
+ two standard library vendors; boost defines
+ BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS in <boost/config.hpp>
+ when this is the case.</li>
+ <li>There is a strange bug in VC6, where the members of std::numeric_limits
+ can be "prematurely evaluated" in template
+ code, for example:</li>
+</ol>
+
+<pre>template <class T>
+struct limits_test
+{
+ BOOST_STATIC_ASSERT(::std::numeric_limits<T>::is_specialized);
+};</pre>
+
+<p>This code fails to compile with VC6 even though no instances
+of the template are ever created; for some bizarre reason <code>::std::numeric_limits<T>::is_specialized
+</code>always evaluates to false, irrespective of what the
+template parameter T is. The problem seems to be confined to
+expressions which depend on std::numeric_limts: for example if
+you replace <code>::std::numeric_limits<T>::is_specialized</code>
+with <code>::boost::is_arithmetic<T>::value</code>, then
+everything is fine. The following workaround also works but
+conflicts with the coding guidelines:</p>
+
+<pre>template <class T>
+struct limits_test
+{
+ BOOST_STATIC_CONSTANT(bool, check = ::std::numeric_limits<T>::is_specialized);
+ BOOST_STATIC_ASSERT(check);
+};</pre>
+
+<p>So it is probably best to resort to something like this:</p>
+
+<pre>template <class T>
+struct limits_test
+{
+#ifdef BOOST_MSVC
+ BOOST_STATIC_CONSTANT(bool, check = ::std::numeric_limits<T>::is_specialized);
+ BOOST_STATIC_ASSERT(check);
+#else
+ BOOST_STATIC_ASSERT(::std::numeric_limits<T>::is_specialized);
+#endif
+};</pre>
+
+<p><b><i>Be careful how you use the sizeof operator</i></b></p>
+
+<p>As far as I can tell, all compilers treat sizeof expressions
+correctly when the argument is the name of a type (or a template-id),
+however problems can occur if:</p>
+
+<ol>
+ <li>The argument is the name of a member-variable, or a local
+ variable (code may not compile with VC6).</li>
+ <li>The argument is an expression which involves the creation
+ of a temporary (code will not compile with Borland C++).</li>
+ <li>The argument is an expression involving an overloaded
+ function call (code compiles but the result is a garbage
+ value with Metroworks C++).</li>
+</ol>
+
+<p><b><i>Don't use boost::is_convertible unless you have to</i></b></p>
+
+<p>Since is_convertible is implemented in terms of the sizeof
+operator, it consistently gives the wrong value when used with
+the Metroworks compiler, and may not compile with the Borland's
+compiler (depending upon the template arguments used).</p>
+
+<hr>
+
+<p>Copyright Dr John Maddock 2001, all rights reserved.</p>
+
+<p> </p>
+
+<p> </p>
+</body>
+</html>