ceph/src/boost/libs/utility/identity_type/doc/identity_type.qbk

   1
   2 [/ Copyright (C) 2009-2012 Lorenzo Caminiti ]
   3 [/ Distributed under the Boost Software License, Version 1.0 ]
   4 [/ (see accompanying file LICENSE_1_0.txt or a copy at ]
   5 [/ http://www.boost.org/LICENSE_1_0.txt) ]
   6 [/ Home at http://www.boost.org/libs/utility/identity_type ]
   7
   8 [library Boost.Utility/IdentityType
   9     [quickbook 1.5]
  10     [version 1.0.0]
  11     [copyright 2009-2012 Lorenzo Caminiti]
  12     [purpose wraps types with round parenthesis]
  13     [license
  14         Distributed under the Boost Software License, Version 1.0
  15         (see accompanying file LICENSE_1_0.txt or a copy at
  16         [@http://www.boost.org/LICENSE_1_0.txt])
  17     ]
  18     [authors [Caminiti <email>lorcaminiti@gmail.com</email>, Lorenzo]]
  19     [category Utilities]
  20 ]
  21
  22 This library allows to wrap types within round parenthesis so they can always be passed as macro parameters.
  23
  24 [import ../test/var_error.cpp]
  25 [import ../test/var.cpp]
  26 [import ../test/template.cpp]
  27 [import ../test/abstract.cpp]
  28 [import ../test/paren.cpp]
  29
  30 [section Motivation]
  31
  32 Consider the following macro which declares a variable named `var`[^['n]] with the specified [^['type]] (see also [@../../test/var_error.cpp =var_error.cpp=]):
  33
  34 [var_error]
  35
  36 The first macro invocation works correctly declaring a variable named `var1` of type `int`.
  37 However, the second macro invocation fails generating a preprocessor error similar to the following:
  38
  39 [pre
  40     error: macro "VAR" passed 3 arguments, but takes just 2
  41 ]
  42
  43 That is because the `std::map` type passed as the first macro parameter contains a comma `,` not wrapped by round parenthesis `()`.
  44 The preprocessor interprets that unwrapped comma as a separation between macro parameters concluding that a total of three (and not two) parameters are passed to the macro in the following order:
  45
  46 # `std::map<int`
  47 # `char>`
  48 # `2`
  49
  50 Note that, differently from the compiler, the preprocessor only recognizes round parenthesis `()`.
  51 Angular `<>` and squared `[]` parenthesis are not recognized by the preprocessor when parsing macro parameters.
  52
  53 [endsect]
  54
  55 [section Solution]
  56
  57 In some cases, it might be possible to workaround this issue by avoiding to pass the type expression to the macro all together.
  58 For example, in the case above a `typedef` could have been used to specify the type expression with the commas outside the macro (see also [@../../test/var.cpp =var.cpp=]):
  59
  60 [var_typedef]
  61
  62 When this is neither possible nor desired (e.g., see the function template `f` in the section below), this library header [headerref boost/utility/identity_type.hpp] defines a macro [macroref BOOST_IDENTITY_TYPE] which can be used to workaround the issue while keeping the type expression as one of the macro parameters (see also [@../../test/var.cpp =var.cpp=]).
  63
  64 [var_ok]
  65
  66 The [macroref BOOST_IDENTITY_TYPE] macro expands to an expression that evaluates (at compile-time) to the specified type.
  67 The specified type is never split into multiple macro parameters because it is always wrapped by a set of extra round parenthesis `()`.
  68 In fact, a total of two sets of round parenthesis must be used: The parenthesis to invoke the macro `BOOST_IDENTITY_TYPE(...)` plus the inner parenthesis to wrap the type passed to the macro `BOOST_IDENTITY_TYPE((...))`.
  69
  70 This macro works on any [@http://www.open-std.org/JTC1/SC22/WG21/docs/standards C++03] compiler (and it does not use [@http://en.wikipedia.org/wiki/Variadic_macro variadic macros]).
  71 [footnote
  72 Using variadic macros, it would be possible to require a single set of extra parenthesis `BOOST_IDENTITY_TYPE(`[^['type]]`)` instead of two `BOOST_IDENTITY_TYPE((`[^['type]]`))` but variadic macros are not part of C++03 (even if nowadays they are supported by most modern compilers and they are also part of C++11).
  73 ]
  74 The authors originally developed and tested this library using GNU Compiler Collection (GCC) C++ 4.5.3 (with and without C++11 features enabled `-std=c++0x`) on Cygwin and Miscrosoft Visual C++ (MSVC) 8.0 on Windows 7.
  75 See the library [@http://www.boost.org/development/tests/release/developer/utility-identity_type.html regressions test results] for more information on supported compilers and platforms.
  76
  77 [endsect]
  78
  79 [section Templates]
  80
  81 This macro must be prefixed by `typename` when used within templates.
  82 For example, let's program a macro that declares a function parameter named `arg`[^['n]] with the specified [^['type]] (see also [@../../test/template.cpp =template.cpp=]):
  83
  84 [template_f_decl]
  85 [template_f_call]
  86
  87 However, note that the template parameter `char` must be manually specified when invoking the function as in `f<char>(a)`.
  88 In fact, when the [macroref BOOST_IDENTITY_TYPE] macro is used to wrap a function template parameter, the template parameter can no longer be automatically deduced by the compiler form the function call as `f(a)` would have done.
  89 [footnote
  90 This is because the implementation of [macroref BOOST_IDENTITY_TYPE] wraps the specified type within a meta-function.
  91 ]
  92 (This limitation does not apply to class templates because class template parameters must always be explicitly specified.)
  93 In other words, without using the [macroref BOOST_IDENTITY_TYPE] macro, C++ would normally be able to automatically deduce the function template parameter as shown below:
  94
  95 [template_g_decl]
  96 [template_g_call]
  97
  98 [endsect]
  99
 100 [section Abstract Types]
 101
 102 On some compilers (e.g., GCC), using this macro on abstract types (i.e., classes with one or more pure virtual functions) generates a compiler error.
 103 This can be avoided by manipulating the type adding and removing a reference to it.
 104
 105 Let's program a macro that performs a static assertion on a [@http://en.wikipedia.org/wiki/Template_metaprogramming Template Meta-Programming] (TMP) meta-function (similarly to Boost.MPL [@http://www.boost.org/doc/libs/1_36_0/libs/mpl/doc/refmanual/assert.html `BOOST_MPL_ASSERT`]).
 106 The [macroref BOOST_IDENTITY_TYPE] macro can be used to pass a meta-function with multiple template parameters to the assert macro (so to handle the commas separating the template parameters).
 107 In this case, if the meta-function is an abstract type, it needs to be manipulated adding and removing a reference to it (see also [@../../test/abstract.cpp =abstract.cpp=]):
 108
 109 [abstract]
 110
 111 [endsect]
 112
 113 [section Annex: Usage]
 114
 115 The [macroref BOOST_IDENTITY_TYPE] macro can be used either when calling a user-defined macro (as shown by the examples so far), or internally when implementing a user-defined macro (as shown below).
 116 When [macroref BOOST_IDENTITY_TYPE] is used in the implementation of the user-defined macro, the caller of the user macro will have to specify the extra parenthesis (see also [@../../test/paren.cpp =paren.cpp=]):
 117
 118 [paren]
 119
 120 However, note that the caller will /always/ have to specify the extra parenthesis even when the macro parameters contain no comma:
 121
 122 [paren_always]
 123
 124 In some cases, using [macroref BOOST_IDENTITY_TYPE] in the implementation of the user-defined macro might provide the best syntax for the caller.
 125 For example, this is the case for `BOOST_MPL_ASSERT` because the majority of template meta-programming expressions contain unwrapped commas so it is less confusing for the user to always specify the extra parenthesis `((...))` instead of using [macroref BOOST_IDENTITY_TYPE]:
 126
 127     BOOST_MPL_ASSERT(( // Natural syntax.
 128         boost::mpl::and_<
 129               boost::is_const<T>
 130             , boost::is_reference<T>
 131         >
 132     ));
 133
 134 However, in other situations it might be preferable to not require the extra parenthesis in the common cases and handle commas as special cases using [macroref BOOST_IDENTITY_TYPE].
 135 For example, this is the case for [@http://www.boost.org/libs/local_function `BOOST_LOCAL_FUNCTION`] for which always requiring the extra parenthesis `((...))` around the types would lead to an unnatural syntax for the local function signature:
 136
 137     int BOOST_LOCAL_FUNCTION( ((int&)) x, ((int&)) y ) { // Unnatural syntax.
 138         return x + y;
 139     } BOOST_LOCAL_FUNCTION_NAME(add)
 140
 141 Instead requiring the user to specify [macroref BOOST_IDENTITY_TYPE] only when needed allows for the more natural syntax `BOOST_LOCAL_FUNCTION(int& x, int& y)` in the common cases when the parameter types contain no comma (while still allowing to specify parameter types with commas as special cases using `BOOST_LOCAL_FUNCTION(BOOST_IDENTITY_TYPE((std::map<int, char>))& x, int& y)`).
 142
 143 [endsect]
 144
 145 [section Annex: Implementation]
 146
 147 The implementation of this library macro is equivalent to the following:
 148 [footnote
 149 There is absolutely no guarantee that the macro is actually implemented using the code listed in this documentation.
 150 The listed code is for explanatory purposes only.
 151 ]
 152
 153     #include <boost/type_traits/function_traits.hpp>
 154
 155     #define BOOST_IDENTITY_TYPE(parenthesized_type) \
 156         boost::function_traits<void parenthesized_type>::arg1_type
 157
 158 Essentially, the type is wrapped between round parenthesis `(std::map<int, char>)` so it can be passed as a single macro parameter even if it contains commas.
 159 Then the parenthesized type is transformed into the type of a function returning `void` and with the specified type as the type of the first and only argument `void (std::map<int, char>)`.
 160 Finally, the type of the first argument `arg1_type` is extracted at compile-time using the `function_traits` meta-function therefore obtaining the original type from the parenthesized type (effectively stripping the extra parenthesis from around the specified type).
 161
 162 [endsect]
 163
 164 [xinclude reference.xml]
 165