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<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><i>© Copyright Dr John Maddock 2001</i></p>
<p><i>Use, modification and distribution are subject to the Boost Software License,
Version 1.0. (See accompanying file <a href="../LICENSE_1_0.txt">LICENSE_1_0.txt</a>
or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
<p> </p>
<p> </p>
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