Boost C++ Libraries

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Introspecting member function

The TTI macro BOOST_TTI_HAS_MEMBER_FUNCTION introspects a member function of a class.

BOOST_TTI_HAS_MEMBER_FUNCTION takes a single parameter which is the name of an inner member function whose existence the programmer wants to check. The macro generates a metafunction called 'has_member_function_'name_of_inner_member_function'.

The metafunction can be invoked in two different ways.

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

The optional Boost FunctionTypes tag type may be used to specify cv-qualification. This means you can add 'const', 'volatile', or both by specifying an appropriate tag type. An alternate to using the tag type is to specify the enclosing type as 'const', 'volatile', or both. As an example if you specify the tag type as 'boost::function_types::const_qualified' or if you specify the enclosing type as 'const T', the member function which you are introspecting must be a const function.

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

Return_Type Enclosing_Type::* ( Parameter_Types ) cv_qualifier(s)

where the Parameter_Types may be empty, or a comma-separated list of parameter types if there are more than one parameter type. The cv-qualifier may be 'const', 'volatile', or 'const volatile'.

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 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 member function:

BOOST_TTI_HAS_MEMBER_FUNCTION(AMemberFunction)

generates a metafunction called 'has_member_function_AMemberFunction' 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 member function as a series of template parameters. Alternatively you can invoke the metafunction by passing it a single type which is a pointer to member function.

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

has_member_function_AMemberFunction
  <
  Enclosing_Type,
  MemberFunction_ReturnType,
  boost::mpl::vector<MemberFunction_ParameterTypes>, // optional, can be any mpl forward sequence
  boost::function_types::SomeTagType                 // optional, can be any FunctionTypes tag type
  >::value

OR

has_member_function_AMemberFunction
  <
  MemberFunction_ReturnType Enclosing_Type::* (MemberFunction_ParameterTypes) optional_cv_qualification
  >::value

Examples

First we generate metafunctions for various inner member function names:

#include <boost/tti/has_member_function.hpp>

BOOST_TTI_HAS_MEMBER_FUNCTION(function1)
BOOST_TTI_HAS_MEMBER_FUNCTION(function2)
BOOST_TTI_HAS_MEMBER_FUNCTION(function3)

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

struct AClass
  {
  };
struct Top
  {
  int function1();
  AClass function2(double,short *);
  };
struct Top2
  {
  long function2(Top &,int,bool,short,float);
  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_member_function_function1<Top,int>::value; // true
has_member_function_function1<Top,int,boost::mpl::vector<> >::value; // true
has_member_function_function1<Top2,int>::value; // false

has_member_function_function2<AClass Top::* (double,short *)>::value; // true
has_member_function_function2<AClass Top2::* (double,short *)>::value; // false
has_member_function_function2<long Top2::* (Top &,int,bool,short,float)>::value; // true

has_member_function_function3<int Top2::* ()>::value; // false
has_member_function_function3<Top2,Top *,boost::mpl::vector<long,int,AClass &> >::value; // true;

Metafunction re-use

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

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


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