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1[/
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
10The elements of a type about which a template metaprogrammer might be interested in finding
11out 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 particular user-defined nested type (struct/class, union, or enumaeration) with a particular name ?
16* Does it have a particular user-defined nested type with a particular name which fulfills some other possibility for that nested type.
17* Does it have a nested class template with a particular name ?
18* Does it have a nested class template with a particular name and a particular signature ?
19* Does it have a nested member function template with a particular name and a particular instantiated signature ?
20* Does it have a nested static function template with a particular name and a particular instantiated signature ?
21* Does it have a member function with a particular name and a particular signature ?
22* Does it have member data with a particular name and of a particular type ?
23* Does it have a static member function with a particular name and a particular signature ?
24* Does it have static member data with a particular name and of a particular type ?
25* Does it have either member data or static member data with a particular name and of a particular type ?
26* Does it have either a member function or static member function with a particular name and of a particular type ?
27
28These are some of the compile-time questions which the TTI library answers. It
29does this by creating metafunctions, which can be used at compile-time, using
30C++ macros. Each of the metafunctions created returns a compile time constant
31bool value which answers one of the above questions at compile time. When the
32particular element above exists the value is 'true', or more precisely
33boost::mpl::true_, while if the element does not exist the value is 'false',
34or more precisely boost::mpl::false_. In either case the type of this value
35is boost::mpl::bool_.
36
37This constant bool value, in the terminology of the Boost MPL library, is called an 'integral
38constant wrapper' and the metafunction generated is called a 'numerical metafunction'. The
39results from calling the metafunction can be passed to other metafunctions for type selection,
40the most popular of these being the boolean-valued operators in the Boost MPL library.
41
42All of the questions above attempt to find an answer about an inner element with
43a particular name. In order to do this using template metaprogramming, macros are used
44so that the name of the inner element can be passed to the macro. The macro will then
45generate an appropriate metafunction, which the template metaprogrammer can then use to
46introspect the information that is needed. The name itself of the inner element is always passed
47to the macro as a macro parameter, but other macro parameters may also be needed in some cases.
48
49All of the macros start with the prefix `BOOST_TTI_`, create their metafunctions as class
50templates in whatever scope the user invokes the macro, and come in two forms:
51
52# In the simplest macro form, which I call the simple macro form, the 'name' of the inner element
53  is used directly to generate the name of the metafunction as well as serving as the 'name'
54  to introspect. In generating the name of the metafunction from the macro name, the
55  `BOOST_TTI_` prefix is removed, the rest of the macro name is changed to lower case, and an
56  underscore ( '_' ) followed by the 'name' is appended.  As an example, for the macro
57  `BOOST_TTI_HAS_TYPE(MyType)` the name of the metafunction is `has_type_MyType` and it will
58  look for an inner type called 'MyType'.
59# In a more complicated macro form, which I call the complex macro form, the macro starts with
60  `BOOST_TTI_TRAIT_` and a 'trait' name is passed as the first parameter, with the 'name' of the inner
61  element as the second parameter. The 'trait' name serves solely to completely name the metafunction in
62  whatever scope the macro is invoked. As an example, for the macro
63  `BOOST_TTI_TRAIT_HAS_TYPE(MyTrait,MyType)` the name of
64  the metafunction is `MyTrait` and it will look for an inner type called `MyType`.
65
66Every macro metafunction has a simple macro form and a corresponding complex macro form.
67Once either of these two macro forms are used for a particular type of inner element, the
68corresponding macro metafunction works exactly the same way and has the exact same functionality.
69
70In the succeeding documentation all macro metafunctions will be referred by their simple form
71name, but remember that the complex form can always be used instead. The complex form is useful
72whenever using the simple form could create a duplicate name in the same name space, thereby
73violating the C++ one definition rule.
74
75[heading Macro Metafunction Name Generation]
76
77For the simple macro form, even though it is fairly easy to remember the algorithm by which
78the generated metafunction is named, TTI also provides, for each macro metafunction,
79a corresponding 'naming' macro which the end-user can use and whose sole purpose
80is to expand to the metafunction name. The naming macro for each macro metafunction has the form:
81'corresponding-macro'_GEN(name).
82
83As an example, BOOST_TTI_HAS_TYPE(MyType) creates a metafunction
84which looks for a nested type called 'MyType' within some enclosing type. The name of the metafunction
85generated, given our rule above is 'has_type_MyType'. A corresponding macro called
86BOOST_TTI_HAS_TYPE_GEN, invoked as BOOST_TTI_HAS_TYPE_GEN(MyType) in our example, expands to the
87same 'has_type_MyType' name. These name generating macros, for each of the metafunction generating macros,
88are purely a convenience for end-users who find using them easier than remembering the name-generating
89rule given above.
90
91[section:tti_functionality_nm_gen Macro metafunction name generation considerations]
92
93Because having a double underscore ( __ ) in a name is reserved by the C++ implementation,
94creating C++ identifiers with double underscores should be avoided by the end-user. When using
95a TTI macro to generate a metafunction using the simple macro form, TTI appends a single
96underscore to the macro name preceding the name of the element that is being introspected.
97The reason for doing this is because Boost discourages as non-portable C++ identifiers with mixed
98case letters and the underscore then becomes the normal way to separate parts of an identifier
99name so that it looks understandable. Because of this decision to use the underscore to generate
100the metafunction name from the macro name, any inner element starting with an underscore will cause
101the identifier for the metafunction name being generated to contain a double underscore.
102
103A rule to avoid this problem is:
104
105* When the name of the inner element to be introspected begins with an underscore, use
106the complex macro form, where the name of the metafunction is specifically given.
107
108Furthermore because TTI often generates not only a metafunction for the end-user to use but some
109supporting detail metafunctions whose identifier, for reasons of programming clarity, is the same
110as the metafunction with further letters appended to it separated by an underscore, another rule is:
111
112* When using the complex macro form, which fully gives the name of the generated
113macro metafunction, that name should not end with an underscore.
114
115Following these two simple rules will avoid names with double underscores being
116generated by TTI.
117
118[heading Reusing the named metafunction]
119
120When the end-user uses the TTI macros to generate a metafunction for introspecting an inner
121element of a particular type, that metafunction can be re-used with any combination of valid
122template parameters which involve the same type of inner element of a particular name.
123
124As one example of this let's consider two different user-defined types called
125'AType' and 'BType', for each of which we want to determine whether an inner type called
126'InnerType' exists. For both these types we need only generate a single macro metafunction in
127the current scope by using:
128
129 BOOST_TTI_HAS_TYPE(InnerType)
130
131We now have a metafunction, which is a C++ class template, in the current scope whose C++ identifier
132is 'has_type_InnerType'. We can use this same metafunction to introspect the existence of the nested type
133'InnerType' in either 'AType' or 'BType' at compile time. Although the syntax for doing this has no yet
134been explained, I will give it here so that you can see how 'has_type_InnerType' is reused:
135
136# 'has_type_InnerType<AType>::value' is a compile time constant telling us whether 'InnerType' is a
137type which is nested within 'AType'.
138
139# 'has_type_InnerType<BType>::value' is a compile time constant telling us whether 'InnerType' is a
140type which is nested within 'BType'.
141
142As another example of metafunction reuse let's consider a single user-defined type, called 'CType',
143for which we want to determine if it has either of two overloaded member functions with the same name
144of 'AMemberFunction' but with the different function signatures of 'int (int)' and 'double (long)'.
145For both these member functions we need only generate a single macro metafunction in the current scope
146by using:
147
148 BOOST_TTI_HAS_MEMBER_FUNCTION(AMemberFunction)
149
150We now have a metafunction, which is a C++ class template, in the current scope whose C++ identifier
151is 'has_member_function_AMemberFunction'. We can use this same metafunction to introspect the
152existence of the member function 'AMemberFunction' with either the function signature of
153'int (int)' or 'double (long)' in 'CType' at compile time. Although the syntax for doing this has no yet
154been explained, I will give it here so that you can see how 'has_type_InnerType' is reused:
155
156# 'has_member_function_AMemberFunction<CType,int,boost::mpl::vector<int> >::value' is a
157compile time constant telling us whether 'AMemberFunction' is a member function of type 'CType'
158whose function signature is 'int (int)'.
159
160# 'has_member_function_AMemberFunction<CType,double,boost::mpl::vector<long> >::value' is a
161compile time constant telling us whether 'AMemberFunction' is a member function of type 'CType'
162whose function signature is 'double (long)'.
163
164These are just two examples of the ways a particular macro metafunction can be reused. The two
165'constants' when generating a macro metafunction are the 'name' and 'type of inner element'. Once
166the macro metafunction for a particular name and inner element type has been generated, it can be reused
167for introspecting the inner element(s) of any enclosing type which correspond to that name and
168inner element type.
169
170[heading Avoiding ODR violations]
171
172The TTI macro metafunctions are generating directly in the enclosing scope in which the
173corresponding macro is invoked. This can be any C++ scope in which a class template can
174be specified. Within this enclosing scope the name of the metafunction
175being generated must be unique or else a C++ ODR ( One Definition Rule ) violation will occur.
176This is extremely important to remember, especially when the enclosing scope can occur in
177more than one translation unit, which is the case for namespaces and the global scope.
178
179Because of ODR, and the way that the simple macro form metafunction name is directly dependent
180on the inner element and name of the element being introspected, it is the responsibility of the
181programmer, using the TTI macros to generate metafunctions, to avoid ODR within a module
182( application or library ). There are a few general methods for doing this:
183
184# Create unique namespace names in which to generate the macro metafunctions and/or generate
185the macro metafunctions in C++ scopes which can not extend across translation units. The most
186obvious example of this latter is within C++ classes.
187
188# Use the complex macro form to specifically name the metafunction generated in order to
189provide a unique name.
190
191# Avoid using the TTI macros in the global scope.
192
193For anyone using TTI in a library which others will eventually use, it is important
194to generate metafunction names which are unique to that library.
195
196TTI also reserves not only the name generated by the macro metafunction for its use
197but also any C++ identifier sequence which begins with that name. In other words
198if the metafunction being generated by TTI is named 'MyMetafunction' using the complex
199macro form, do not create any C++ construct with an identifier starting with 'MyMetaFunction',
200such as 'MyMetaFunction_Enumeration' or 'MyMetaFunctionHelper' in the same scope.
201All names starting with the metafunction name in the current scope should be considered
202out of bounds for the programmer using TTI.
203
204[endsect]
205
206[endsect]
207