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