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## Introspecting member data

The TTI macro `BOOST_TTI_HAS_MEMBER_DATA` introspects member data of a class.

BOOST_TTI_HAS_MEMBER_DATA macro takes a single parameter which is the name of an inner member data whose existence the programmer wants to check. The macro generates a metafunction called 'has_member_data_'name_of_inner_member_data'.

The metafunction can be invoked in two different ways.

The first way is by passing it two parameters. The first parameter is the enclosing type to introspect and the second parameter is the type of the member data.

The second way is by passing it a single parameter, which is a pointer to member type. This type has the form of:

    MemberData_Type Enclosing_Type::*

The metafunction returns a single type called 'type', which is a boost::mpl::bool_. As a convenience the metafunction returns the value of this type directly as a compile time bool constant called 'value'. This value is true or false depending on whether the inner member data, of the specified type, exists or not.

#### Generating the metafunction

You generate the metafunction by invoking the macro with the name of an inner member data:

    BOOST_TTI_HAS_MEMBER_DATA(AMemberData)

generates a metafunction called 'has_member_data_AMemberData' in the current scope.

#### Invoking the metafunction

You invoke the metafunction by instantiating the template with an enclosing type to introspect and the type of the member data, or by instantiating the template with a pointer to member data type. The return value called 'value' is a compile time bool constant telling you whether or not the member data . exists.

    has_member_data_AMemberData<Enclosing_Type,MemberData_Type>::value

    OR

    has_member_data_AMemberData<MemberData_Type Enclosing_Type::*>::value

#### Examples

First we generate metafunctions for various inner member data names:

    #include <boost/tti/has_member_data.hpp>

    BOOST_TTI_HAS_MEMBER_DATA(data1)
    BOOST_TTI_HAS_MEMBER_DATA(data2)
    BOOST_TTI_HAS_MEMBER_DATA(data3)

Next let us create some user-defined types we want to introspect.

    struct AClass
      {
      };
    struct Top
      {
      int data1;
      AClass * data2;
      };
    struct Top2
      {
      long data1;
      Top data3;
      };

Finally we invoke our metafunction and return our value. This all happens at compile time, and can be used by programmers doing compile time template metaprogramming.

We will show both forms in the following examples. Both forms are completely interchangeable as to the result desired.

    has_member_data_data1<Top,int>::value; // true
    has_member_data_data1<Top,long>::value; // false

    has_member_data_data1<Top2,int>::value; // false
    has_member_data_data1<long Top2::*>::value; // true

    has_member_data_data2<AClass * Top::*>::value; // true
    has_member_data_data2<Top,int *>::value; // false

    has_member_data_data3<int Top2::*>::value; // false
    has_member_data_data3<Top Top2::*>::value; // true;

#### Metafunction re-use

The macro encodes only the name of the member data for which we are searching and the fact that we are introspecting for member data within an enclosing type.

Because of this, once we create our metafunction for introspecting an inner member data by name, we can reuse the metafunction for introspecting any enclosing type, having any inner member data type, for that name.