ceph/src/boost/libs/tti/doc/tti_functionality.qbk

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   2   (C) Copyright Edward Diener 2011,2012
   3   Distributed under the Boost Software License, Version 1.0.
   4   (See accompanying file LICENSE_1_0.txt or copy at
   5   http://www.boost.org/LICENSE_1_0.txt).
   6 ]
   7
   8 [section:tti_functionality General Functionality]
   9
  10 The elements of a type about which a template metaprogrammer might be interested in finding
  11 out at compile time are:
  12
  13 * Does it have a nested type with a particular name ?
  14 * Does it have a nested type with a particular name which fulfills some other possibility for that nested type.
  15 * Does it have a nested class template with a particular name ?
  16 * Does it have a nested class template with a particular name and a particular signature ?
  17 * Does it have a member function with a particular name and a particular signature ?
  18 * Does it have member data with a particular name and of a particular type ?
  19 * Does it have a static member function with a particular name and a particular signature ?
  20 * Does it have static member data with a particular name and of a particular type ?
  21 * Does it have either member data or static member data with a particular name and of a particular type ?
  22 * Does it have either a member function or static member function with a particular name and of a particular type ?
  23
  24 These are some of the compile-time questions which the TTI library answers. It
  25 does this by creating metafunctions, which can be used at compile-time, using
  26 C++ macros. Each of the metafunctions created returns a compile time constant
  27 bool value which answers one of the above questions at compile time. When the
  28 particular element above exists the value is 'true', or more precisely
  29 boost::mpl::true_, while if the element does not exist the value is 'false',
  30 or more precisely boost::mpl::false_. In either case the type of this value
  31 is boost::mpl::bool_.
  32
  33 This constant bool value, in the terminology of the Boost MPL library, is called an 'integral
  34 constant wrapper' and the metafunction generated is called a 'numerical metafunction'. The
  35 results from calling the metafunction can be passed to other metafunctions for type selection,
  36 the most popular of these being the boolean-valued operators in the Boost MPL library.
  37
  38 All of the questions above attempt to find an answer about an inner element with
  39 a particular name. In order to do this using template metaprogramming, macros are used
  40 so that the name of the inner element can be passed to the macro. The macro will then
  41 generate an appropriate metafunction, which the template metaprogrammer can then use to
  42 introspect the information that is needed. The name itself of the inner element is always passed
  43 to the macro as a macro parameter, but other macro parameters may also be needed in some cases.
  44
  45 All of the macros start with the prefix `BOOST_TTI_`, create their metafunctions as class
  46 templates in whatever scope the user invokes the macro, and come in two forms:
  47
  48 # In the simplest macro form, which I call the simple macro form, the 'name' of the inner element
  49   is used directly to generate the name of the metafunction as well as serving as the 'name'
  50   to introspect. In generating the name of the metafunction from the macro name, the
  51   `BOOST_TTI_` prefix is removed, the rest of the macro name is changed to lower case, and an
  52   underscore ( '_' ) followed by the 'name' is appended.  As an example, for the macro
  53   `BOOST_TTI_HAS_TYPE(MyType)` the name of the metafunction is `has_type_MyType` and it will
  54   look for an inner type called 'MyType'.
  55 # In a more complicated macro form, which I call the complex macro form, the macro starts with
  56   `BOOST_TTI_TRAIT_` and a 'trait' name is passed as the first parameter, with the 'name' of the inner
  57   element as the second parameter. The 'trait' name serves solely to completely name the metafunction in
  58   whatever scope the macro is invoked. As an example, for the macro
  59   `BOOST_TTI_TRAIT_HAS_TYPE(MyTrait,MyType)` the name of
  60   the metafunction is `MyTrait` and it will look for an inner type called `MyType`.
  61
  62 Every macro metafunction has a simple macro form and a corresponding complex macro form.
  63 Once either of these two macro forms are used for a particular type of inner element, the
  64 corresponding macro metafunction works exactly the same way and has the exact same functionality.
  65
  66 In the succeeding documentation all macro metafunctions will be referred by their simple form
  67 name, but remember that the complex form can always be used instead. The complex form is useful
  68 whenever using the simple form could create a duplicate name in the same name space, thereby
  69 violating the C++ one definition rule.
  70
  71 [heading Macro Metafunction Name Generation]
  72
  73 For the simple macro form, even though it is fairly easy to remember the algorithm by which
  74 the generated metafunction is named, TTI also provides, for each macro metafunction,
  75 a corresponding 'naming' macro which the end-user can use and whose sole purpose
  76 is to expand to the metafunction name. The naming macro for each macro metafunction has the form:
  77 'corresponding-macro'_GEN(name).
  78
  79 As an example, BOOST_TTI_HAS_TYPE(MyType) creates a metafunction
  80 which looks for a nested type called 'MyType' within some enclosing type. The name of the metafunction
  81 generated, given our rule above is 'has_type_MyType'. A corresponding macro called
  82 BOOST_TTI_HAS_TYPE_GEN, invoked as BOOST_TTI_HAS_TYPE_GEN(MyType) in our example, expands to the
  83 same 'has_type_MyType' name. These name generating macros, for each of the metafunction generating macros,
  84 are purely a convenience for end-users who find using them easier than remembering the name-generating
  85 rule given above.
  86
  87 [section:tti_functionality_nm_gen Macro metafunction name generation considerations]
  88
  89 Because having a double underscore ( __ ) in a name is reserved by the C++ implementation,
  90 creating C++ identifiers with double underscores should be avoided by the end-user. When using
  91 a TTI macro to generate a metafunction using the simple macro form, TTI appends a single
  92 underscore to the macro name preceding the name of the element that is being introspected.
  93 The reason for doing this is because Boost discourages as non-portable C++ identifiers with mixed
  94 case letters and the underscore then becomes the normal way to separate parts of an identifier
  95 name so that it looks understandable. Because of this decision to use the underscore to generate
  96 the metafunction name from the macro name, any inner element starting with an underscore will cause
  97 the identifier for the metafunction name being generated to contain a double underscore.
  98
  99 A rule to avoid this problem is:
 100
 101 * When the name of the inner element to be introspected begins with an underscore, use
 102 the complex macro form, where the name of the metafunction is specifically given.
 103
 104 Furthermore because TTI often generates not only a metafunction for the end-user to use but some
 105 supporting detail metafunctions whose identifier, for reasons of programming clarity, is the same
 106 as the metafunction with further letters appended to it separated by an underscore, another rule is:
 107
 108 * When using the complex macro form, which fully gives the name of the generated
 109 macro metafunction, that name should not end with an underscore.
 110
 111 Following these two simple rules will avoid names with double underscores being
 112 generated by TTI.
 113
 114 [heading Reusing the named metafunction]
 115
 116 When the end-user uses the TTI macros to generate a metafunction for introspecting an inner
 117 element of a particular type, that metafunction can be re-used with any combination of valid
 118 template parameters which involve the same type of inner element of a particular name.
 119
 120 As one example of this let's consider two different types called
 121 'AType' and 'BType', for each of which we want to determine whether an inner type called
 122 'InnerType' exists. For both these types we need only generate a single macro metafunction in
 123 the current scope by using:
 124
 125  BOOST_TTI_HAS_TYPE(InnerType)
 126
 127 We now have a metafunction, which is a C++ class template, in the current scope whose C++ identifier
 128 is 'has_type_InnerType'. We can use this same metafunction to introspect the existence of the nested type
 129 'InnerType' in either 'AType' or 'BType' at compile time. Although the syntax for doing this has no yet
 130 been explained, I will give it here so that you can see how 'has_type_InnerType' is reused:
 131
 132 # 'has_type_InnerType<AType>::value' is a compile time constant telling us whether 'InnerType' is a
 133 type which is nested within 'AType'.
 134
 135 # 'has_type_InnerType<BType>::value' is a compile time constant telling us whether 'InnerType' is a
 136 type which is nested within 'BType'.
 137
 138 As another example of metafunction reuse let's consider a single type, called 'CType', for which we want to
 139 determine if it has either of two overloaded member functions with the same name of 'AMemberFunction'
 140 but with the different function signatures of 'int (int)' and 'double (long)'. For both these
 141 member functions we need only generate a single macro metafunction in the current scope by using:
 142
 143  BOOST_TTI_HAS_MEMBER_FUNCTION(AMemberFunction)
 144
 145 We now have a metafunction, which is a C++ class template, in the current scope whose C++ identifier
 146 is 'has_member_function_AMemberFunction'. We can use this same metafunction to introspect the
 147 existence of the member function 'AMemberFunction' with either the function signature of
 148 'int (int)' or 'double (long)' in 'CType' at compile time. Although the syntax for doing this has no yet
 149 been explained, I will give it here so that you can see how 'has_type_InnerType' is reused:
 150
 151 # 'has_member_function_AMemberFunction<CType,int,boost::mpl::vector<int> >::value' is a
 152 compile time constant telling us whether 'AMemberFunction' is a member function of type 'CType'
 153 whose function signature is 'int (int)'.
 154
 155 # 'has_member_function_AMemberFunction<CType,double,boost::mpl::vector<long> >::value' is a
 156 compile time constant telling us whether 'AMemberFunction' is a member function of type 'CType'
 157 whose function signature is 'double (long)'.
 158
 159 These are just two examples of the ways a particular macro metafunction can be reused. The two
 160 'constants' when generating a macro metafunction are the 'name' and 'type of inner element'. Once
 161 the macro metafunction for a particular name and inner element type has been generated, it can be reused
 162 for introspecting the inner element(s) of any enclosing type which correspond to that name and
 163 inner element type.
 164
 165 [heading Avoiding ODR violations]
 166
 167 The TTI macro metafunctions are generating directly in the enclosing scope in which the
 168 corresponding macro is invoked. This can be any C++ scope in which a class template can
 169 be specified. Within this enclosing scope the name of the metafunction
 170 being generated must be unique or else a C++ ODR ( One Definition Rule ) violation will occur.
 171 This is extremely important to remember, especially when the enclosing scope can occur in
 172 more than one translation unit, which is the case for namespaces and the global scope.
 173
 174 Because of ODR, and the way that the simple macro form metafunction name is directly dependent
 175 on the inner element and name of the element being introspected, it is the responsibility of the
 176 programmer using the TTI macros to generate metafunctions to avoid ODR within a module
 177 ( application or library ). There are a few general methods for doing this:
 178
 179 # Create unique namespace names in which to generate the macro metafunctions and/or generate
 180 the macro metafunctions in C++ scopes which can not extend across translation units. The most
 181 obvious example of this latter is within C++ classes.
 182
 183 # Use the complex macro form to specifically name the metafunction generated in order to
 184 provide a unique name.
 185
 186 # Avoid using the TTI macros in the global scope.
 187
 188 For anyone using TTI in a library which others will eventually use, it is important
 189 to generate metafunction names which are unique to that library.
 190
 191 TTI also reserves not only the name generated by the macro metafunction for its use
 192 but also any C++ identifier sequence which begins with that name. In other words
 193 if the metafunction being generated by TTI is named 'MyMetafunction' using the complex
 194 macro form, do not create any C++ construct with an identifier starting with 'MyMetaFunction',
 195 such as 'MyMetaFunction_Enumeration' or 'MyMetaFunctionHelper' in the same scope.
 196 All names starting with the metafunction name in the current scope should be considered
 197 out of bounds for the programmer using TTI.
 198
 199 [endsect]
 200
 201 [endsect]