From OPeNDAP Documentation

return to Libdap Overview

1 Using the Libdap Classes

In order to be used by real client-libraries and data servers, many of the classes in the libdap toolkit must be subclassed. For example, the type hierarchy classes, which represent the data types in the OPeNDAP data model, are all abstract classes. In order to use them in a program they must be subclassed. The Connect class also must be subclassed if it to hold additional information about the connection.

1.1 Sub-classing the Type Hierarchy

In order to link a program with libdap, the DAP's abstract classes must be subclassed and those subclasses must ensure that all of the member functions of those classes have valid definitions. The next three sections cover sub-classing the simple, vector and compound classes, respectively. In addition, a sample set of classes (called the Test classes because they are used by the DAP tests) is included with the DAP distribution. You can read the source code for those classes to find out how they were created.

Each of the sub-classed types must supply:

  • A constructor that takes a String argument and returns an object with that name.
  • A virtual destructor.
  • A copy function called ptr_duplicate.
  • The read function, to read data from a disk and load it into the type class object.

The ptr_duplicate member function returns a pointer to a new instance of the type of object from which it was invoked. This member function exists so that objects that are referenced through pointers to BaseType can correctly copy themselves. (If you were to use the operator new to copy an object referenced through BaseType , you would get a BaseType, not a new instance of the type of the referenced object.) Note that ptr_duplicate is a virtual function so an object which is a descendent of BaseType will get the most specific definition of that function.

The read function is much more complicated to write than ptr_duplicate, and the difficulty varies depending on the data type to be read. However, this function is only used on the server side of the system and not by the client. That is, it can be implemented with a null function body if all you are building is a client library. The read function takes two arguments, the dataset name, and an error flag. It must read from that dataset the values specified by the current constraint expression. The error flag is passed by reference so that read can set its value and callers can test it. The class BaseType contains a number of member functions to facilitate writing a read function. The return value of read is TRUE is there is more data to be read (by additional calls to read) or FALSE otherwise.

1.1.1 Sub-classing the Simple Types

Creating a read function for the simple type classes is a fairly straightforward operation. Simply read the function using the data access API's standard protocol, and use the type class's val2buf function to load the data into the type object.

Here's an example, from the OPeNDAP implementation of the netCDF client library.

NCByte::read(const String &dataset, int &error) 
   int varid;                  /* variable Id */
   nc_type datatype;           /* variable data type */
   long cor[MAX_NC_DIMS];      /* corner coordinates */
   int num_dim;                /* number of dim. in variable */
   long nels = -1;             /* number of elements in buffer */
   int id;

   if (read_p()) // already done
     return true;

   int ncid = lncopen(dataset, NC_NOWRITE); /* netCDF id */

   if (ncid == -1) { 
     cerr << "ncopen failed on " << dataset<< endl;
     return false;
   varid = lncvarid( ncid, name());
   (void)lncvarinq( ncid, varid, (char *)0, &datatype, 
                    &num_dim, (int *)0, (int *)0);

   if(nels == -1){  
     for (id = 0; id < num_dim; id++) 
       cor[id] = 0;

   if (datatype == NC_BYTE){
     dods_byte Dbyte;

     (void) lncvarget1 (ncid, varid, cor, &Dbyte);
     val2buf( &Dbyte );

     (void) lncclose(ncid);  
     return true;
   return false;

The following points are worth consideration about the above example.

line 10 
Check to see if this variable has already been read.
line 13 
The lncopen() function call is simply the ncopen() function from the NetCDF library. Also, the lncvarid function is renamed ncvarid and so on. The names have been changed to avoid link-time problems. (Remember that the whole point of this exercise is to create a new ncopen() function and its friends.
line 33 
This sets the flag tested with the read_p function.
line 35 
This command transfers values from the dataset's variable to the OPeNDAP instance. This is the point where the read actually happens.

Note the use of dods_byte in the code example. The OPeNDAP configuration process creates definitions for the simple data types like this one. They are stored in config_dap.h.

The read member function is used by the constraint expression evaluator to extract data from a dataset during evaluation of the constraint expression. This is particularly important to remember because the read member function for a simple data type will be called when reading an aggregate type such as Structure.

1.1.2 Sub-classing the Vector Types

The vector data types require the same abstract member functions be defined as the simple types. The definition for ptr_duplicate is also the same for vector as for simple types. However, the read member function for the vector types (classes Array and List) is more complicated than for the simple types because vectors of values are represented in two ways in OPeNDAP, depending on the type of variable in the vector. Arrays are stored as C would store them for the simple types such as Byte, Int32 and Float64. However, compound types are stored as arrays of the OPeNDAP objects (with the exception of arrays themselves, but more on that later).

When reading an array of Byte values, the val2buf member function should be passed a pointer to values stored in a contiguous piece of memory. For example, when read is called to read a variable byte-array, it must determine how much memory to allocate to hold that much information, use new to allocate an adequate amount of memory, use the dataset's API calls to read byte-array into the newly allocated memory and then pass that memory to val2buf. This same procedure can be followed for all the simple types.

However, when reading an array of Structure, for example, the values must be stored in the OPeNDAP Array object one at a time using the Array member function set_vec. An Array object containing a 3x4 array of Structure objects will actually point to 12 different instances of that class. An Array object containing Byte objects contains only one instance of the Byte class, as a template for the array elements.

Arrays in OPeNDAP are unlike arrays in C in that an array object may have more than one dimension. In terms of the way a value is stored, however, an Array is a single dimensional object. When an Array is declared as having two or more dimensions, those are mapped onto a single vector. To access the element A_ij of array A , you must know the size of the first dimension, I , and use the expression i * I + j to compute the offset into the vector.

Since the class List is a single dimension array without declared size (the size of each value of the List object is stored in the object) the rules for Array's read member function apply.

1.1.3 Sub-classing the Compound Types

The read member function of the compound data types simply iterates over the contained variables calling their read member functions. In the future, this definition will move into the supplied classes (That is, read will no longer be a abstract member function for the compound types.).

The two constructor types Sequence and Function are different from all the other types in the DAP in that they have state . That is, the value of a sequence depends on how many values have been read previously. This is very different from an array where the i^{th} element has the same value regardless of what has happened before. When you write implementations for read in the Sequence and Function classes, you must be sure to write those member functions so that they can be called repeatedly and that each call to read returns the next value of the corresponding data Sequence or Function.

This is true because the constraint expression evaluator must be able to apply certain constraints to values of individual sequence elements and is actually implemented in the DDS class by first calling the read member function, evaluating the constraint expression based on the values and, if the constraint expression is satisfied, calling the serialize member function. See the member function DDS::send.

If, for some reason, it is not possible to write read so that it gets called once for each sequence value, then you must re-implement DDS::send so that its functions are performed. For example, you could implement Sequence::read so that the entire sequence is read in and overload Sequence::serialize and Sequence::deserialize so that the next set of values are sent/received. You would then build a send that called the Sequence::read member function once and extracted each successive value, evaluated the constraint expression using on that value and used the result of that evaluation to determine whether to send the value or not.