Dispatch table

In computer science, a dispatch table is a list of pointers to the actual implementation of each method. Use of such a table is a common technique when implementing late binding in object-oriented programming. Generally, OOP languages support the table as a built-in feature to provide seamless late binding.

Hand-coded implementation

Example implementation in C (Spinellis, 2003, pp. 312–313). The following structure declaration—excerpted from the FreeBSD/NetBSD implementation of the pax archive reading and writing program—contains pointers to functions used for performing archive processing operations:

typedef struct {
	char *name;		/* name of format, this is the name the user */
	/* [...] */
	int (*id)(char *, int); /* checks if a buffer is a valid header */
	int (*st_rd)(void);	/* initialize routine for read. */
	int (*rd)(ARCHD *, char *);/* read header routine. */
	off_t (*end_rd)(void);	/* read cleanup. */
	int (*st_wr)(void);	/* initialize routine for write operations */
	int (*wr)(ARCHD *);	/* write archive header. */
	int (*end_wr)(void);	/* end write. write the trailer and do any */
	int (*trail)(char *, int, int *);
				/* returns 0 if a valid trailer, -1 if not */
	int (*subtrail)(ARCHD *);
				/* read/process file data from the archive */
	int (*rd_data)(ARCHD *, int, off_t *);
				/* read/process file data from the archive */
	int (*wr_data)(ARCHD *, int, off_t *);
				/* write/process file data to the archive */
	int (*options)(void);	/* process format specific options (-o) */
} FSUB;

An array of these structures is initialized with functions appropriate for each different archive type:

FSUB fsub[] = {
/* 0: OLD BINARY CPIO */
	{ "bcpio", bcpio_id, cpio_strd,
	bcpio_rd, bcpio_endrd, cpio_stwr, bcpio_wr, cpio_endwr, NULL,
	cpio_subtrail, rd_wrfile, wr_rdfile, bad_opt },

/* 1: OLD OCTAL CHARACTER CPIO */
	{ "cpio", cpio_id, cpio_strd,
	cpio_rd, cpio_endrd, cpio_stwr, cpio_wr, cpio_endwr, NULL,
	cpio_subtrail, rd_wrfile, wr_rdfile, bad_opt },

/* 2: SVR4 HEX CPIO */
	{ "sv4cpio", vcpio_id, cpio_strd,
	vcpio_rd, vcpio_endrd, cpio_stwr, vcpio_wr, cpio_endwr, NULL,
	cpio_subtrail, rd_wrfile, wr_rdfile, bad_opt },

/* 3: SVR4 HEX CPIO WITH CRC */
	{ "sv4crc", crc_id, crc_strd,
	vcpio_rd, vcpio_endrd, crc_stwr, vcpio_wr, cpio_endwr, NULL,
	cpio_subtrail, rd_wrfile, wr_rdfile, bad_opt },

/* 4: OLD TAR */
	{ "tar", tar_id, no_op,
	tar_rd, tar_endrd, no_op, tar_wr, tar_endwr, tar_trail,
	NULL, rd_wrfile, wr_rdfile, tar_opt },

/* 5: POSIX USTAR */
	{ "ustar", ustar_id, ustar_strd,
	ustar_rd, tar_endrd, ustar_stwr, ustar_wr, tar_endwr, tar_trail,
	NULL, rd_wrfile, wr_rdfile, bad_opt }
};

At runtime, all operations are performed through a FSUB pointer—frmt in the following example—thus dispatching control to the appropriate method for the archive being processed:

	/*
	 * pass the format any options and start up format
	 */
	if (((*frmt->options)() < 0) || ((*frmt->st_rd)() < 0))
		return;

OOP compiler implementation

In object-oriented programming languages the compiler will automatically create a dispatch table for objects with dynamically bound methods. This table is called a virtual table or vtable.

References

• Diomidis Spinellis (2003). Code Reading: The Open Source Perspective. Boston, MA: Addison-Wesley. (ISBN 0-201-79940-5)