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Dispatch TableIn 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)
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