Defining an Object for the GDS Library

If you don't know about the basics of the GDS library, be sure to read this first:

The Generic Data Structure Library

In order to work with the data structures defined in the GDS library, your structure (which we shall call an object from now on) must be defined in a certain way. In this article I will provide you with a simple example on how to define such an object.

The first thing that you must do is the structure declaration:

typedef struct
{
   int value;
}Integer;

The structure above is a simple wrapper for a integer, containing only one member who shall represent the value of the integer.

The object must implement the following operations defined by the Interface data structure:

Operation	Description
Copy	Creates a hard copy of the object
Compare	Compares an object with another object of the same type.
Destroy	Frees the memory allocated for the current object

Our example implementation is:

Integer* Integer_Copy(const Integer* integer)
{
   Integer *i = NULL;
   i = malloc(sizeof(Integer));
   i->value = integer->value;
   return i;
}

Integer* Integer_Destroy(Integer* integer)
{
   free(integer);
   return NULL;
}

int Integer_Compare(const Integer* i1, const Integer* i2)
{
   return (*(int*)(i1->value) - *(int*)(i2->value));
}

As you probably observed in the int Integer_Compare(const Integer *i1, const Integer *i2) the member value of the Integer object uses a double cast. This must be done because the interface functions have void* parameters.

Also, I consider good practice to define an "constructor"-like function for the structure:

Integer* Integer_Create(int value)
{
   Integer *i = NULL;
   i = malloc(sizeof(Integer));
   i->value = value;
   return i;
}

To see the full source check the following links:

SampleObject.h

SampleObject.c