]>
Commit | Line | Data |
---|---|---|
7c673cae FG |
1 | <html> |
2 | <head> | |
3 | <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> | |
4 | <title>Rationale</title> | |
5 | <link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> | |
6 | <meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> | |
7 | <link rel="home" href="../index.html" title="Chapter 1. Boost.FunctionTypes 2.5"> | |
8 | <link rel="up" href="../index.html" title="Chapter 1. Boost.FunctionTypes 2.5"> | |
9 | <link rel="prev" href="reference/macros.html" title="Macros"> | |
10 | <link rel="next" href="acknowledgements.html" title="Acknowledgements"> | |
11 | </head> | |
12 | <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> | |
13 | <table cellpadding="2" width="100%"><tr> | |
14 | <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> | |
15 | <td align="center"><a href="../../../../../index.html">Home</a></td> | |
16 | <td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> | |
17 | <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> | |
18 | <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> | |
19 | <td align="center"><a href="../../../../../more/index.htm">More</a></td> | |
20 | </tr></table> | |
21 | <hr> | |
22 | <div class="spirit-nav"> | |
23 | <a accesskey="p" href="reference/macros.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="acknowledgements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> | |
24 | </div> | |
25 | <div class="section"> | |
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"><</span> <a class="link" href="reference/decomposition.html#boost_functiontypes.reference.decomposition.parameter_types" title="parameter_types">parameter_types</a><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">>::</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"><</span><span class="identifier">T</span><span class="special">>::</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 © 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> | |
146 | </tr></table> | |
147 | <hr> | |
148 | <div class="spirit-nav"> | |
149 | <a accesskey="p" href="reference/macros.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="acknowledgements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> | |
150 | </div> | |
151 | </body> | |
152 | </html> |