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## 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.