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  (C) Copyright Edward Diener 2011,2012
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:tti_functionality General Functionality]

The elements of a type about which a template metaprogrammer might be interested in finding 
out at compile time are:

* Does it have a nested type with a particular name ?
* Does it have a nested type with a particular name which fulfills some other possibility for that nested type.
* Does it have a nested class template with a particular name ?
* Does it have a nested class template with a particular name and a particular signature ?
* Does it have a member function with a particular name and a particular signature ?
* Does it have member data with a particular name and of a particular type ?
* Does it have a static member function with a particular name and a particular signature ?
* Does it have static member data with a particular name and of a particular type ?
* Does it have either member data or static member data with a particular name and of a particular type ?
* Does it have either a member function or static member function with a particular name and of a particular type ?

These are some of the compile-time questions which the TTI library answers. It 
does this by creating metafunctions, which can be used at compile-time, using 
C++ macros. Each of the metafunctions created returns a compile time constant 
bool value which answers one of the above questions at compile time. When the 
particular element above exists the value is 'true', or more precisely 
boost::mpl::true_, while if the element does not exist the value is 'false', 
or more precisely boost::mpl::false_. In either case the type of this value 
is boost::mpl::bool_.

This constant bool value, in the terminology of the Boost MPL library, is called an 'integral 
constant wrapper' and the metafunction generated is called a 'numerical metafunction'. The 
results from calling the metafunction can be passed to other metafunctions for type selection,
the most popular of these being the boolean-valued operators in the Boost MPL library.

All of the questions above attempt to find an answer about an inner element with 
a particular name. In order to do this using template metaprogramming, macros are used 
so that the name of the inner element can be passed to the macro. The macro will then 
generate an appropriate metafunction, which the template metaprogrammer can then use to 
introspect the information that is needed. The name itself of the inner element is always passed 
to the macro as a macro parameter, but other macro parameters may also be needed in some cases.

All of the macros start with the prefix `BOOST_TTI_`, create their metafunctions as class
templates in whatever scope the user invokes the macro, and come in two forms: 

# In the simplest macro form, which I call the simple macro form, the 'name' of the inner element 
  is used directly to generate the name of the metafunction as well as serving as the 'name' 
  to introspect. In generating the name of the metafunction from the macro name, the 
  `BOOST_TTI_` prefix is removed, the rest of the macro name is changed to lower case, and an 
  underscore ( '_' ) followed by the 'name' is appended.  As an example, for the macro 
  `BOOST_TTI_HAS_TYPE(MyType)` the name of the metafunction is `has_type_MyType` and it will 
  look for an inner type called 'MyType'.
# In a more complicated macro form, which I call the complex macro form, the macro starts with 
  `BOOST_TTI_TRAIT_` and a 'trait' name is passed as the first parameter, with the 'name' of the inner 
  element as the second parameter. The 'trait' name serves solely to completely name the metafunction in 
  whatever scope the macro is invoked. As an example, for the macro 
  `BOOST_TTI_TRAIT_HAS_TYPE(MyTrait,MyType)` the name of 
  the metafunction is `MyTrait` and it will look for an inner type called `MyType`. 
  
Every macro metafunction has a simple macro form and a corresponding complex macro form.  
Once either of these two macro forms are used for a particular type of inner element, the 
corresponding macro metafunction works exactly the same way and has the exact same functionality. 

In the succeeding documentation all macro metafunctions will be referred by their simple form 
name, but remember that the complex form can always be used instead. The complex form is useful
whenever using the simple form could create a duplicate name in the same name space, thereby
violating the C++ one definition rule.

[heading Macro Metafunction Name Generation]

For the simple macro form, even though it is fairly easy to remember the algorithm by which
the generated metafunction is named, TTI also provides, for each macro metafunction,
a corresponding 'naming' macro which the end-user can use and whose sole purpose 
is to expand to the metafunction name. The naming macro for each macro metafunction has the form:
'corresponding-macro'_GEN(name). 

As an example, BOOST_TTI_HAS_TYPE(MyType) creates a metafunction 
which looks for a nested type called 'MyType' within some enclosing type. The name of the metafunction
generated, given our rule above is 'has_type_MyType'. A corresponding macro called 
BOOST_TTI_HAS_TYPE_GEN, invoked as BOOST_TTI_HAS_TYPE_GEN(MyType) in our example, expands to the 
same 'has_type_MyType' name. These name generating macros, for each of the metafunction generating macros, 
are purely a convenience for end-users who find using them easier than remembering the name-generating 
rule given above.

[section:tti_functionality_nm_gen Macro metafunction name generation considerations]

Because having a double underscore ( __ ) in a name is reserved by the C++ implementation,
creating C++ identifiers with double underscores should be avoided by the end-user. When using 
a TTI macro to generate a metafunction using the simple macro form, TTI appends a single 
underscore to the macro name preceding the name of the element that is being introspected.
The reason for doing this is because Boost discourages as non-portable C++ identifiers with mixed
case letters and the underscore then becomes the normal way to separate parts of an identifier 
name so that it looks understandable. Because of this decision to use the underscore to generate
the metafunction name from the macro name, any inner element starting with an underscore will cause
the identifier for the metafunction name being generated to contain a double underscore. 

A rule to avoid this problem is:

* When the name of the inner element to be introspected begins with an underscore, use
the complex macro form, where the name of the metafunction is specifically given.

Furthermore because TTI often generates not only a metafunction for the end-user to use but some
supporting detail metafunctions whose identifier, for reasons of programming clarity, is the same 
as the metafunction with further letters appended to it separated by an underscore, another rule is:

* When using the complex macro form, which fully gives the name of the generated 
macro metafunction, that name should not end with an underscore.

Following these two simple rules will avoid names with double underscores being 
generated by TTI.

[heading Reusing the named metafunction]

When the end-user uses the TTI macros to generate a metafunction for introspecting an inner 
element of a particular type, that metafunction can be re-used with any combination of valid 
template parameters which involve the same type of inner element of a particular name. 

As one example of this let's consider two different types called 
'AType' and 'BType', for each of which we want to determine whether an inner type called 
'InnerType' exists. For both these types we need only generate a single macro metafunction in 
the current scope by using:

 BOOST_TTI_HAS_TYPE(InnerType)

We now have a metafunction, which is a C++ class template, in the current scope whose C++ identifier
is 'has_type_InnerType'. We can use this same metafunction to introspect the existence of the nested type
'InnerType' in either 'AType' or 'BType' at compile time. Although the syntax for doing this has no yet 
been explained, I will give it here so that you can see how 'has_type_InnerType' is reused:

# 'has_type_InnerType<AType>::value' is a compile time constant telling us whether 'InnerType' is a 
type which is nested within 'AType'.

# 'has_type_InnerType<BType>::value' is a compile time constant telling us whether 'InnerType' is a 
type which is nested within 'BType'.

As another example of metafunction reuse let's consider a single type, called 'CType', for which we want to 
determine if it has either of two overloaded member functions with the same name of 'AMemberFunction' 
but with the different function signatures of 'int (int)' and 'double (long)'. For both these 
member functions we need only generate a single macro metafunction in the current scope by using:

 BOOST_TTI_HAS_MEMBER_FUNCTION(AMemberFunction)

We now have a metafunction, which is a C++ class template, in the current scope whose C++ identifier
is 'has_member_function_AMemberFunction'. We can use this same metafunction to introspect the 
existence of the member function 'AMemberFunction' with either the function signature of 
'int (int)' or 'double (long)' in 'CType' at compile time. Although the syntax for doing this has no yet 
been explained, I will give it here so that you can see how 'has_type_InnerType' is reused:

# 'has_member_function_AMemberFunction<CType,int,boost::mpl::vector<int> >::value' is a 
compile time constant telling us whether 'AMemberFunction' is a member function of type 'CType' 
whose function signature is 'int (int)'.

# 'has_member_function_AMemberFunction<CType,double,boost::mpl::vector<long> >::value' is a 
compile time constant telling us whether 'AMemberFunction' is a member function of type 'CType' 
whose function signature is 'double (long)'.

These are just two examples of the ways a particular macro metafunction can be reused. The two 
'constants' when generating a macro metafunction are the 'name' and 'type of inner element'. Once 
the macro metafunction for a particular name and inner element type has been generated, it can be reused
for introspecting the inner element(s) of any enclosing type which correspond to that name and 
inner element type.

[heading Avoiding ODR violations]

The TTI macro metafunctions are generating directly in the enclosing scope in which the 
corresponding macro is invoked. This can be any C++ scope in which a class template can 
be specified. Within this enclosing scope the name of the metafunction 
being generated must be unique or else a C++ ODR ( One Definition Rule ) violation will occur.
This is extremely important to remember, especially when the enclosing scope can occur in 
more than one translation unit, which is the case for namespaces and the global scope.

Because of ODR, and the way that the simple macro form metafunction name is directly dependent 
on the inner element and name of the element being introspected, it is the responsibility of the 
programmer using the TTI macros to generate metafunctions to avoid ODR within a module 
( application or library ). There are a few general methods for doing this:

# Create unique namespace names in which to generate the macro metafunctions and/or generate 
the macro metafunctions in C++ scopes which can not extend across translation units. The most
obvious example of this latter is within C++ classes.

# Use the complex macro form to specifically name the metafunction generated in order to 
provide a unique name.

# Avoid using the TTI macros in the global scope.

For anyone using TTI in a library which others will eventually use, it is important 
to generate metafunction names which are unique to that library.

TTI also reserves not only the name generated by the macro metafunction for its use 
but also any C++ identifier sequence which begins with that name. In other words 
if the metafunction being generated by TTI is named 'MyMetafunction' using the complex 
macro form, do not create any C++ construct with an identifier starting with 'MyMetaFunction',
such as 'MyMetaFunction_Enumeration' or 'MyMetaFunctionHelper' in the same scope. 
All names starting with the metafunction name in the current scope should be considered 
out of bounds for the programmer using TTI.

[endsect]

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