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## Introspecting inner data

The TTI macro `BOOST_TTI_HAS_DATA` introspects the data of a class. The data can be member data or static member data.

BOOST_TTI_HAS_DATA macro takes a single parameter, which is the name of an inner member data or static member data, whose existence the programmer wants to check. The macro generates a metafunction called "has_data_'name_of_inner_data'".

The metafunction can be invoked by passing it the enclosing type to introspect and the type of the data.

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 is true or false depending on whether the inner 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 data member:

    BOOST_TTI_HAS_DATA(AData)

generates a metafunction called 'has_data_AStaticMemberData' 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 data. A return value called 'value' is a compile time bool constant.

    has_data_AData<Enclosing_Type,Data_Type>::value

#### Examples

First we generate metafunctions for various inner data names:

    #include <boost/tti/has_data.hpp>

    BOOST_TTI_HAS_DATA(data1)
    BOOST_TTI_HAS_DATA(data2)
    BOOST_TTI_HAS_DATA(data3)

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

    struct AClass
      {
      };
    struct Top
      {
      int data1;
      static AClass * data2;
      };
    struct Top2
      {
      static 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.

    has_data_data1<Top,int>::value; // true
    has_data_data1<Top,long>::value; // false
    has_data_data1<Top2,int>::value; // false
    has_data_data1<Top2,long>::value; // true

    has_data_data2<Top,AClass *>::value; // true
    has_data_data2<Top,int *>::value; // false

    has_data_data3<Top2,int>::value; // false
    has_data_data3<Top2,Top>::value; // true;

#### Metafunction re-use

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

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