Common CNCL Member Functions

CNCL requires that all classes have a common set of member functions available. These functions provide runtime type checking and type information, creation of objects via class descriptions, and safe type casts.

The common member functions are:

CNClassDesc CLASS::class_desc() const;

This function returns a pointer to the class description object, which must be available for every class in the CNCL hierarchy. This pointer is used for runtime type information.

bool CLASS::is_a(CNClassDesc desc) const;

This function allows runtime type checking. It returns TRUE if the queried object is type compatible with class desc.

void CLASS::print(ostream &strm = cout) const;

void CLASS::dump(ostream &strm = cout) const;

These functions output the object to the given stream strm. The function print() is defined in greater detail in the derived classes. The function dump() is intended for debug purposes.

The functions above are defined as pure virtual functions in the top-level class CNObject. They are required in every derived class.

Furthermore the following static member functions are required to allow object creation via the class description and safe type casts:

CLASS* CLASS::cast_from_object(CNObject *obj);

This function does a safe (CLASS *) type cast. It checks on type compatibility between the object passed with the obj pointer and CLASS. If this is a not true, an error message is printed and the program terminates. The type checking may be disabled by defining the preprocessor macro NO_TYPE_CHECK, e.g. by supplying -DNO_TYPE_CHECK on the compiler's command line. Example:

XYZ *px;               // CNClass XYZ derived from CNObject
ABC *pa;               // CNClass ABC derived from CNObject
DEF *pd;               // CNClass DEF derived from ABC
CNObject *po;

po = new DEF;          // Type compatible (C++ standard)
pd = (DEF *)po;        // The traditional way
pd = DEF::cast_from_object(po); // The CNCL way

pa = new DEF;
pd = DEF::cast_from_object(pa); // o.k.
px = XYZ::cast_from_object(pa); // Error

CNObject *CLASS::new_object(CNParam *param = NIL);

This function creates an object of type CLASS, optionally passing a pointer to a parameter object to the constructor. It is used by the class description CNClass to create new objects.

Every class in CNCL requires a class description object and a pointer constant pointing to this class description. Following the CNCL convention, the description object is named CLASS_desc and the pointer is named CN_CLASS.

Example:

// Describing object for class XYZ
static CNClass XYZ_desc("XYZ", "$Revision: 0.45 $", XYZ::new_object);

// "Type" for type checking functions
CNClassDesc CN_XYZ = &XYZ_desc;

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