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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_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]
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