boost/libs/tti/doc/tti_nested_type_and_signatures.qbk

[/ 
  (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_func_sig Nested Types and Function Signatures]

The strength of `BOOST_TTI_MEMBER_TYPE` to represent a type which may or may not exist, and which 
then can be subsequently used in other macro metafunctions whenever a type is needed as a template 
parameter without producing a compiler error, should not be underestimated. It is one of the  
reasons why we have two different ways of using our generated metafunction when introspecting 
for member data, a member function, or a static member function of an enclosing type.

In the cases where we specify a composite syntax when using `BOOST_TTI_HAS_MEMBER_DATA`,
`BOOST_TTI_HAS_MEMBER_FUNCTION`, or `BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION`, the signature 
for the member data, member function, or static member function is a single type. For 
`BOOST_TTI_HAS_MEMBER_DATA` the signature is a pointer to member data, for  
`BOOST_TTI_HAS_MEMBER_FUNCTION` the signature is a pointer to a member function, and for
`BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION` the signature  is divided between an enclosing type
and a function in composite format. This makes for a syntactical notation which is natural 
to specify, but because of the notation we can not use the nested type functionality in 
`BOOST_TTI_MEMBER_TYPE` for potential parts of these composite types. If any part of this 
signature, which specifies a composite of various types, is invalid, a compiler time error 
will occur.

But in the more specific cases, when we use `BOOST_TTI_HAS_MEMBER_DATA`, 
`BOOST_TTI_HAS_MEMBER_FUNCTION`, and `BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION`, our composite 
type in our signatures is broken down into their individual types so that using 
`BOOST_TTI_MEMBER_TYPE` for any one of the individual types will not lead to a compile time 
error if the type specified does not actually exist.

A few examples will suffice. 

Given known types T and U, and the supposed type Ntype as a 
nested type of U, we want to find out if type T has a member function whose signature is 
`void aMemberFunction(U::Ntype)`.

First using `BOOST_TTI_HAS_MEMBER_FUNCTION` using our composite form we would code:

 #include <boost/tti/has_member_function.hpp>
 
 BOOST_TTI_HAS_MEMBER_FUNCTION(aMemberFunction)
  
 has_member_function_aMemberFunction<void (T::*)(U::Ntype)>::value;
 
If the nested type U::Ntype does not exist, this leads to a compiler error. 
We really want to avoid this situation, so let's try our alternative.

Second using `BOOST_TTI_HAS_MEMBER_FUNCTION` using our specific form we would code:
  
 #include <boost/tti/member_type.hpp>
 #include <boost/tti/has_member_function.hpp>
 
 BOOST_TTI_HAS_MEMBER_TYPE(Ntype)
 BOOST_TTI_HAS_MEMBER_FUNCTION(aMemberFunction)
  
 typedef typename has_member_type_Ntype<U>::type OurType;
 has_member_function_aMemberFunction<T,void,boost::mpl::vector<OurType> >::value;
 
If the nested type U::Ntype does exist and T does have a member function 
whose signature is `void aMemberFunction(U::Ntype)` our 'value' is true, 
otherwise it is false. We will never get a compiler error in this case.

As a second example we will once again use the suppositions of our first
example; given known types T and U, and the supposed type Ntype as a 
nested type of U. But this time let us look for a static member function
whose signature is `void aStaticMemberFunction(U::Ntype)`.

First using `BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION` using our composite form we would code:

 #include <boost/tti/has_static_member_function.hpp>
 
 BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION(aStaticMemberFunction)
  
 has_static_member_function_aStaticMemberFunction<T,void (U::Ntype)>::value;
 
Once again if the nested type U::Ntype does not exist, this leads to a compiler error, 
so let's try our alternative.

Second using `BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION` using our specific form we would code:
  
 #include <boost/tti/member_type.hpp>
 #include <boost/tti/has_static_member_function.hpp>
 
 BOOST_TTI_HAS_MEMBER_TYPE(Ntype)
 BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION(aStaticMemberFunction)
  
 typedef typename has_member_type_Ntype<U>::type OurType;
 has_static_member_function_aStaticMemberFunction<T,void,boost::mpl::vector<OurType> >::value;
 
If the nested type U::Ntype does exist and T does have a member function 
whose signature is `void aMemberFunction(U::Ntype)` our 'value' is true, 
otherwise it is false. We will never get a compiler error in this case.

[endsect]