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 function

The TTI macro BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION introspects a static member function of a class.

BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION takes a single parameter which is the name of an inner static member function whose existence the programmer wants to check. The macro generates a metafunction called "has_static_member_function_'name_of_inner_static_member_function'".

The metafunction can be invoked in two different ways.

The first way is by passing it the enclosing type to introspect and a signature for the static member function as separate template arguments. The signature for the static member function consists of a return type, optional parameter types in the form of a boost::mpl forward sequence of types, and an optional Boost FunctionTypes tag type. A typical boost::mpl forward sequence of types is a boost::mpl::vector<>.

The second way is by passing it the enclosing type to introspect and a signature for the static member function as a function. The function has the form of:

Return_Type ( Parameter_Types )

where the Parameter_Types may be empty, or a comma-separated list of parameter types if there are more than one parameter 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 is true or false depending on whether the inner static member function, of the specified signature, exists or not.

Generating the metafunction

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


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

Invoking the metafunction

You invoke the metafunction by instantiating the template with an enclosing type to introspect and the signature of the static member function as a series of template parameters. Alternatively you can invoke the metafunction by passing it an enclosing type and the signature of the static member function as a single function type.

A return value called 'value' is a compile time bool constant.

  boost::mpl::vector<StaticMemberFunction_ParameterTypes>, // optional, can be any mpl forward sequence
  boost::function_types::SomeTagType                       // optional, can be any FunctionTypes tag type


  Return_Type ( Parameter_Types )


First we generate metafunctions for various inner static member function names:

#include <boost/tti/has_static_member_function.hpp>


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

struct AClass { };
struct Top
  static int function1();
  static AClass function2(double,short *);
struct Top2
  static long function2(Top &,int,bool,short,float);
  static Top * function3(long,int,AClass &);

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_static_member_function_function1<Top,int>::value; // true
has_static_member_function_function1<Top,int ()>::value; // true
has_static_member_function_function1<Top2,int>::value; // false

has_static_member_function_function2<Top,AClass,boost::mpl::vector<double,short *> >::value; // true
has_static_member_function_function2<Top2,AClass,boost::mpl::vector<double,short *> >::value; // false
has_static_member_function_function2<Top2,long (Top &,int,bool,short,float)>::value; // true

has_static_member_function_function3<Top2,int ()>::value; // false
has_static_member_function_function3<Top2,Top * (long,int,AClass &)>::value; // true;

Metafunction re-use

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

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