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 function

The TTI macro BOOST_TTI_HAS_FUNCTION introspects an inner function of a class. The function can be either a member function or a static member function.

BOOST_TTI_HAS_FUNCTION takes a single parameter which is the name of an inner function whose existence the programmer wants to check. The macro generates a metafunction called "has_function_'name_of_inner_function'".

The metafunction can be invoked by passing it the enclosing type to introspect and a signature for the function as separate template arguments. The signature for the 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 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 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 function:


generates a metafunction called 'has_function_AnInnerFunction' 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 function as a series of template parameters.

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

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


First we generate metafunctions for various inner function names:

#include <boost/tti/has_function.hpp>


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

struct AClass { };
struct Top
  static int function1();
  AClass function2(double,short *);
struct Top2
  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.

has_function_function1<Top,int>::value; // true
has_function_function1<Top2,int>::value; // false

has_function_function2<Top,AClass,boost::mpl::vector<double,short *> >::value; // true
has_function_function2<Top2,AClass,boost::mpl::vector<double,short *> >::value; // false

has_function_function3<Top2,int>::value; // false
has_function_function3<Top2,Top *,boost::mpl::vector<long,int,AClass &> >::value; // true;

Metafunction re-use

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

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