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Class Structure

The FITS classes fall naturally into two groups, the classes that directly model the header-data-units themselves and the classes that form the underlying infrastructure that manipulates the FITS records, parses the header records, enforces the FITS formatting rules, and relates the header-data-units to these records.

The structure of the header-data-units is depicted in figure one. The class HeaderDataUnit contains what is common to all header-data-units, including the collection of keywords. This keyword collection is in the form of a linked list of objects of the Keyword class, which contains the name, value and any comment. On input, this linked list is formed from the FITS header; on output, it is used to form the FITS header. There are an extensive number of functions for manipulating this linked list of keywords. Classes derived from the HeaderDataUnit are specific types of FITS header-data-units: PrimaryArray, PrimaryGroup, ImageExtension, ExtensionHeaderDataUnit, BinaryTableExtension, and AsciiTableExtension. Each one of these has a rich assortment of functions for accessing and manipulating data of specific types.

The classes that form the underlying infrastructure are depicted in figure two. The highest level class, and the most important to the user, is FitsIO. The HeaderDataUnit class, from figure one, uses the FitsIO class for its I/O. FitsIO is the class that contains the knowledge of how to translate FITS fixed length records into keyword lists and blocks of data. It uses a class, FitsKeyCardTranslator, to translate between Keyword lists and the fixed FITS ``card'' format. The FITS parser is contained within the FitsKeyCardTranslator. In addition to providing this translation service, FITSIO keeps track of vital I/O information, such as the current type of header-data-unit, the current record type, size of remaining data, etc. The classes FitsInput and FitsOutput, derived from FitsIO, are the classes that users will instantiate to identify the FITS file being read or written. FitsInput also has a function allowing a reader to skip an entire header-data-unit.

FitsInput and FitsOutput use classes that implement fixed-length blocked I/O, the BlockIO, BlockInput, and BlockOutput classes. Specific physical devices supported by these classes are derived from BlockInput and BlockOutput. They include disk I/O, standard I/O, and 9-track tape I/O. By isolating physical devices in this manner, other types of media, such as optical disks or helical scan tapes, are easy to add to the list of supported devices.

One final class must be mentioned in the overview, the FITS class. This class consists of a series of static functions and enumerations. Many of the static functions are utility functions used internally in the implementation of the member functions of the FITS classes. They are placed in a single class to encapsulate them and to avoid adding many names to the global namespace. More important, from the user's perspective, is the enumerations. They form the basic vocabulary of a FITS application. The names in the enumerations may be used by prepending FITS:: to the names, e. g., to refer to the FITS NAXIS keyword FITS::NAXIS should be used. They are used in the following manner. Suppose we have a FITS input file, as in the following declaration.

    FitsInput fin("myfile.fit",FITS::Disk);

Then, a test for the primary FITS array might be the following.

    if (fin.rectype() == FITS::HDURecord && 
        fin.hdutype() == FITS::PrimaryArrayHDU)

The enumerations in the FITS class are listed below.

             Table of Enumerations in the FITS Class

    ValueType // Basic FITS Data Types for keywords and data 
        NOVALUE   LOGICAL   BIT       CHAR      BYTE 
        SHORT     LONG      FLOAT     DOUBLE    COMPLEX 
        ICOMPLEX  DCOMPLEX  VADESC    STRING
 
    ReservedName // FITS Reserved Names 
        USER_DEF  AUTHOR    BITPIX    BLANK     BLOCKED   BSCALE  
        BUNIT     BZERO     CDELT     COMMENT   CROTA     CRPIX  
        CRVAL     CTYPE     DATAMAX   DATAMIN   DATE      DATE_OBS  
        END       EPOCH     EQUINOX   EXTEND    EXTLEVEL  EXTNAME  
        EXTVER    GCOUNT    GROUPS    HISTORY   INSTRUME  NAXIS  
        OBJECT    OBSERVER  ORIGIN    PCOUNT    PSCAL     PTYPE  
        PZERO     REFERENC  SIMPLE    SPACES    TBCOL     TDIM  
        TDISP     TELESCOP  TFIELDS   TFORM     THEAP     TNULL  
        TSCAL     TTYPE     TUNIT     TZERO     XTENSION  ERRWORD

    FitsRecType // Types of FITS Records 
        InitialState          BadBeginningRecord  HDURecord  
        UnrecognizableRecord  SpecialRecord       EndOfFile 

    FitsDevice // Supported FITS Physical Devices 
        Disk  Std  Tape9

    HDUType // Types of FITS Header-Data Units 
        NotAHDU         PrimaryArrayHDU    PrimaryGroupHDU     
        AsciiTableHDU   BinaryTableHDU     ImageExtensionHDU  
        UnknownExtensionHDU
 
    FitsArrayOption // Options on FITS array manipulations 
        NoOpt   CtoF   FtoC

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