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  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="boost_functiontypes.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
  28 </h2></div></div></div>
  29 <a name="boost_functiontypes.rationale.error_handling_rationale"></a><h4>
  30 <a name="boost_functiontypes.rationale.error_handling_rationale-heading"></a>
  31       <a class="link" href="rationale.html#boost_functiontypes.rationale.error_handling_rationale">Error
  32       handling rationale</a>
  33     </h4>
  34 <p>
  35       The library does not define the required members of class templates in case
  36       of an error. This technique causes the compiler to stop displaying diagnostics
  37       in client code, at the point where the error actually is, instead of tracing
  38       template instantiations into the implementation of the library.
  39     </p>
  40 <p>
  41       The library's components have limited error conditions, so problematic input
  42       can be spotted easily.
  43     </p>
  44 <a name="boost_functiontypes.rationale.why_mpl_sequences_"></a><h4>
  45 <a name="boost_functiontypes.rationale.why_mpl_sequences_-heading"></a>
  46       <a class="link" href="rationale.html#boost_functiontypes.rationale.why_mpl_sequences_">Why MPL Sequences?</a>
  47     </h4>
  48 <p>
  49       MPL provides algorithms on Sequences, so transformations (such as turning by-value
  50       parameter types into const references for optimized forwarding or computing
  51       a signature to specialize <a href="../../../../function/index.html" target="_top"><code class="literal">boost::function</code></a>
  52       after applying <a href="../../../../bind/index.html" target="_top"><code class="literal">boost::bind</code></a>)
  53       can be expressed more easily. The MPL Sequence concept is compatible with several
  54       other Boost libraries (most importantly <a href="../../../../fusion/index.html" target="_top">Fusion</a>),
  55       so another reason is interoperability.
  56     </p>
  57 <a name="boost_functiontypes.rationale.pointer_to_member_object_types"></a><h4>
  58 <a name="boost_functiontypes.rationale.pointer_to_member_object_types-heading"></a>
  59       <a class="link" href="rationale.html#boost_functiontypes.rationale.pointer_to_member_object_types">Pointer
  60       to member object types</a>
  61     </h4>
  62 <p>
  63       Despite their syntax, pointer to member object types can be seen as dereferencing
  64       functionals.
  65     </p>
  66 <a name="boost_functiontypes.rationale.the_classtransform_template_parameter"></a><h4>
  67 <a name="boost_functiontypes.rationale.the_classtransform_template_parameter-heading"></a>
  68       <a class="link" href="rationale.html#boost_functiontypes.rationale.the_classtransform_template_parameter">The
  69       ClassTransform template parameter</a>
  70     </h4>
  71 <p>
  72       <code class="literal">This</code>-pointer, <code class="literal">this</code>-reference or just
  73       the object (or maybe even a smart pointer to the object) plus adjustments of
  74       cv-qualification - all these cases have their place, somewhere and there is
  75       no single best answer.
  76     </p>
  77 <p>
  78       Special treatment of the class type within the sequence can significantly complicate
  79       client code. A custom <code class="literal">ClassTransform</code> argument allows the
  80       client to adjust the class type before the sequence is formed and then treat
  81       all parameters uniformly.
  82     </p>
  83 <a name="boost_functiontypes.rationale.why_tag_types_"></a><h4>
  84 <a name="boost_functiontypes.rationale.why_tag_types_-heading"></a>
  85       <a class="link" href="rationale.html#boost_functiontypes.rationale.why_tag_types_">Why tag types?</a>
  86     </h4>
  87 <p>
  88       Let's consider the alternatives.
  89     </p>
  90 <p>
  91       The first one is just using more templates so every property has to be asked
  92       for explicitly. This approach results in more complicated client code if more
  93       than one propery has to be checked and in a exponentially larger library interface.
  94     </p>
  95 <p>
  96       The second alternative is having the client pass in bit patterns via non-type
  97       template parameters. The logic has to be performed by the client and there
  98       are much more error conditions. Further, class templates with non-type template
  99       parameters do not work within MPL lambda expressions and can cause problems
 100       with older compilers.
 101     </p>
 102 <a name="boost_functiontypes.rationale.is_it_safe_to_have_the_synthesis_templates_take_a_callable__builtin_type_or_an_mpl_sequence_as_the_first_template_argument_"></a><h4>
 103 <a name="boost_functiontypes.rationale.is_it_safe_to_have_the_synthesis_templates_take_a_callable__builtin_type_or_an_mpl_sequence_as_the_first_template_argument_-heading"></a>
 104       <a class="link" href="rationale.html#boost_functiontypes.rationale.is_it_safe_to_have_the_synthesis_templates_take_a_callable__builtin_type_or_an_mpl_sequence_as_the_first_template_argument_">Is
 105       it safe to have the synthesis templates take a callable builtin type or an
 106       MPL sequence as the first template argument?</a>
 107     </h4>
 108 <p>
 109       Yes, but it isn't immediately obvious as the set of possible MPL sequences
 110       isn't inherently disjoint from the set of callable builtin types.
 111     </p>
 112 <p>
 113       However, any attempt to make a builtin type work as an MPL sequence is a bad
 114       idea, because builtin types are accessible before the headers that make the
 115       type a sequence have been included, which can easily violate the ODR.
 116     </p>
 117 <a name="boost_functiontypes.rationale.why_does_the_hidden__literal_this__literal__parameter_count_for_the__function_arity_of_member_functions_"></a><h4>
 118 <a name="boost_functiontypes.rationale.why_does_the_hidden__literal_this__literal__parameter_count_for_the__function_arity_of_member_functions_-heading"></a>
 119       <a class="link" href="rationale.html#boost_functiontypes.rationale.why_does_the_hidden__literal_this__literal__parameter_count_for_the__function_arity_of_member_functions_">Why
 120       does the hidden <code class="literal">this</code> parameter count for the function arity
 121       of member functions?</a>
 122     </h4>
 123 <p>
 124       It was found preferable that the following condition holds:
 125     </p>
 126 <pre class="programlisting"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">size</span><span class="special">&lt;</span> <a class="link" href="reference/decomposition.html#boost_functiontypes.reference.decomposition.parameter_types" title="parameter_types">parameter_types</a><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span class="special">==</span> <a class="link" href="reference/decomposition.html#boost_functiontypes.reference.decomposition.function_arity" title="function_arity">function_arity</a><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span>
 127 </pre>
 128 <a name="boost_functiontypes.rationale.why_ignore_top_level_cv_qualifiers_on_pointers_"></a><h4>
 129 <a name="boost_functiontypes.rationale.why_ignore_top_level_cv_qualifiers_on_pointers_-heading"></a>
 130       <a class="link" href="rationale.html#boost_functiontypes.rationale.why_ignore_top_level_cv_qualifiers_on_pointers_">Why
 131       ignore top-level cv-qualifiers on pointers?</a>
 132     </h4>
 133 <p>
 134       A cv-qualified pointer is still a pointer. It usually doesn't matter and even
 135       if it does, it's a job for <a href="../../../../type_traits/index.html" target="_top">Boost.TypeTraits</a>.
 136     </p>
 137 </div>
 138 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 139 <td align="left"></td>
 140 <td align="right"><div class="copyright-footer">Copyright &#169; 2004-2007 Tobias
 141       Schwinger<p>
 142         Distributed under the Boost Software License, Version 1.0. (See accompanying
 143         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>)
 144       </p>
 145 </div></td>
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