Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards


Introspecting static member data

The TTI macro BOOST_TTI_HAS_STATIC_MEMBER_DATA introspects static member data of a class.

BOOST_TTI_HAS_STATIC_MEMBER_DATA macro takes a single parameter which is the name of an inner static member data whose existence the programmer wants to check. The macro generates a metafunction called "has_static_member_data_'name_of_inner_static_member_data'".

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


generates a metafunction called 'has_static_member_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 static member data. A return value called 'value' is a compile time bool constant.



First we generate metafunctions for various inner member data names:

#include <boost/tti/has_static_member_data.hpp>


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

struct AClass
struct Top
  static int data1;
  static AClass * data2;
struct Top2
  static long data1;
  static 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_static_member_data_data1<Top,int>::value; // true
has_static_member_data_data1<Top,long>::value; // false
has_static_member_data_data1<Top2,int>::value; // false
has_static_member_data_data1<Top2,long>::value; // true

has_static_member_data_data2<Top,AClass *>::value; // true
has_static_member_data_data2<Top,int *>::value; // false

has_static_member_data_data3<Top2,int>::value; // false
has_static_member_data_data3<Top2,Top>::value; // true;

Metafunction re-use

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

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