This could be a part of a meta-programming framework in C++ that uses macros and templates to generate metadata about classes.

gistfile1.txt

#define BEGIN_META_MAP(classname)                                               \
        template<int id, int> friend struct CDescriptor;                        \
    private:                                                                    \
        enum { ID_REFERENCE_POINT = __COUNTER__ };                              \
        typedef classname TheClass;                                             \
        template<int id, int=id> struct CDescriptor                             \
        {                                                                       \
        };                                                                      \
        template<int id> struct CDescriptor<id, 0>                              \
        {                                                                       \
        };

#define END_META_MAP                                                            \
    private:                                                                    \
        typedef CDescriptor<__COUNTER__ - ID_REFERENCE_POINT - 1> CFinalDescr;
				
#define META_PROP_INTERNAL(name)                                                \
    private:                                                                    \
        template<int id> struct CDescriptor<id, __COUNTER__-ID_REFERENCE_POINT> \
        {                                                                       \
        };

• BEGIN_META_MAP(classname): This macro starts the definition of metadata for a class named classname. It does several things:

• Declares a template structure CDescriptor as a friend, allowing it to access private members of the class.

• Defines an enumeration ID_REFERENCE_POINT which is set to the value of COUNTER, a preprocessor variable that increments each time it's used (presumably a custom or compiler-specific feature).

• Defines a type alias TheClass to refer to the classname.

• Declares two template specializations of CDescriptor. The first is a general template, and the second is a specialization for when the second template parameter is zero.

• END_META_MAP: This macro defines a type alias CFinalDescr that refers to a CDescriptor with a specific ID. This ID is calculated based on the difference between the current value of COUNTER and ID_REFERENCE_POINT, minus one. This likely marks the end of the metadata definitions for the class.

• META_PROP_INTERNAL(name): This macro is used to define metadata for a property within the class. It declares a specialized version of CDescriptor with an ID that is the difference between the current COUNTER value and ID_REFERENCE_POINT.

• The id is a compile-time constant expression that uniquely identifies each instance of CDescriptor. It's used to create a sequence of descriptors, each corresponding to a different class member. Please see https://stackoverflow.com/questions/1162401/is-it-possible-to-access-values-of-non-type-template-parameters-in-specialized-t/52699027#52699027

Here's how the code might be used in practice:

class MyClass {
BEGIN_META_MAP(MyClass)
// Metadata definitions for MyClass properties would go here.
END_META_MAP
};

// Later, you could use the metadata to do things like serialization, reflection, etc.
// Note that metadata definitions do not have to strictly follow each other and may be interspersed with other declarations, statements, etc.