GEOS Programming: 1.1 Basic Data Types and Structures: Records and Enumerated Types

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GEOS objects and routines make extensive use of flag records and enumerated types. A flag record is a byte, word, or dword in which each bit represents the state (on or off) of a particular attribute or function. An enumerated type is a byte or word in which each enumeration has a unique constant value.

To define a flag record, you should use one of these types and then define the flags to be bits within the record. To set flags, OR them (bitwise OR) with the record; to clear them, AND their bitwise inverses (bitwise AND) with the record. Creating and working with flag records is shown in Flag Records and ByteEnums .

There are two basic enumerated types: The standard enumerated type supported by your C compiler uses word-sized values. GEOS also allows byte-sized enumerated types with the ByteEnum type. Use of this type is shown in Flag Records and ByteEnums .

Code Display 5-1 Flag Records and ByteEnums

/* Define flag records to be the optimized length for the number of flags. For
 * example, the sample type MyFlag has six flags and therefore should be a byte. 
 * Flag values should be constants equivalent to having a single bit set in the
 * flag record. */
typedef ByteFlags MyFlag;
#define MF_FIRST_FLAG			0x01
#define MF_SECOND_FLAG			0x02
#define MF_THIRD_FLAG			0x04
#define MF_FOURTH_FLAG			0x08
#define MF_FIFTH_FLAG			0x10
#define MF_SIXTH_FLAG			0x20
/* In a section of code, to set a flag, bitwise OR it with the record. To clear the
 * flag, bitwise AND its inverse with the record. You can set any number of flags
 * at a time as shown in the following examples. */
    ...
    MyFlag		myFlagsRecord;		/* Set up a variable of the flag record type */
	/* Set the second and fourth flag. */
    myFlagsRecord = MF_SECOND_FLAG | MF_FOURTH_FLAG;
	/* Set the first flag and then clear the fifth and sixth flags. */
    myFlagsRecord = (myFlagsRecord | MF_FIRST_FLAG) & ~(MF_FIFTH_FLAG |
						    MF_SIXTH_FLAG);
/* The ByteEnum type can be used instead of the standard enumerated type, which
 * is implemented by most compilers as a word type. To define a ByteEnum, define
 * the type and then a unique constant value for each enumeration as in the
 * following example. */
typedef ByteEnum USCity;
#define USC_HARTFORD			0x00
#define USC_CHARLOTTE			0x01
#define USC_WICHITA			0x02
#define USC_PIERRE			0x03
#define USC_ORLANDO			0x04
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