函数指针数组应用实例

从概念上分析，函数指针数组首先是一个数组，数组的元素都是函数指针，而函数指针又是一个指针，但它不是指向普通数据类型或者结构体的指针，而是一个指向函数的指针。

 

以下代码的背景是为两组字符编码转换函数做单元测试，使用函数指针数组可以使程序可读性更强，否则，就像末尾注释的那段那样，需要冗长的代码代替。

 

#include <stdio.h>#include <stdlib.h>#include <string.h>#define ENCODINGS_CNT 7struct cdr_node {int length;unsigned char *content;};typedef struct cdr_node Cdr;/* * convert the content of cdr to ascii */char *cdr2ascii(char *out, const Cdr *cdr) {int i;for ( i = 0; i < cdr->length; i++ ) {*(out + i) = *(cdr->content + i);}return out;}/* * convert ascii to the content of cdr */Cdr *ascii2cdr(Cdr *cdr, const char *in) { cdr->length = strlen(in); cdr->content = (unsigned char *) malloc(sizeof(unsigned char) * (cdr->length + 1)); memset(cdr->content, 0, sizeof(cdr->content)); int i; for ( i = 0; i < cdr->length; i++ ) { *(cdr->content + i) = *(in + i); } return cdr;}/* * convert the content of cdr to bcds */char *cdr2bcds(char *out, const Cdr *cdr) {int i;for ( i = 0; i < cdr->length; i++ ) {int num_low = *(cdr->content + i) & 0x0f;if ( num_low == 0x0f ) break;sprintf(out + i * 2, "%x", num_low);int num_high = (*(cdr->content + i) & 0xf0) >> 4;if ( num_high == 0x0f ) break;sprintf(out + i * 2 + 1, "%x", num_high);}return out;}/* * convert bcds to the content of cdr */Cdr *bcds2cdr(Cdr *cdr, const char *in) { cdr->length = (strlen(in) + 1 ) / 2; cdr->content = (unsigned char *) malloc(sizeof(unsigned char) * (cdr->length + 1)); memset(cdr->content, 0, sizeof(cdr->content)); int i; for ( i = 0; i < cdr->length; i++ ) { int num_low = *(in + i * 2) - '0'; int num_high = *(in + i * 2 + 1) - '0'; num_high = ( num_high < 0 ) ? 0x0f : num_high; *(cdr->content + i) = (num_high << 4) + num_low; } return cdr;}/* * convert the content of cdr to bin */char *cdr2bin(char *out, const Cdr *cdr) {int i;for ( i = 0; i < cdr->length; i++ ) {sprintf(out + i * 2, "%02x", *(cdr->content + i));}return out;}/* * convert bin to the content of cdr */Cdr *bin2cdr(Cdr *cdr, const char *in) { cdr->length = strlen(in) / 2; cdr->content = (unsigned char *) malloc(sizeof(unsigned char) * (cdr->length + 1)); memset(cdr->content, 0, sizeof(cdr->content)); int i; for ( i = 0; i < cdr->length; i++ ) { int num_high = (*(in + i * 2) > '9') ? (*(in + i * 2) - 'a' + 10) : (*(in + i * 2) - '0'); int num_low = (*(in + i * 2 + 1) > '9') ? (*(in + i * 2 + 1) - 'a' + 10) : (*(in + i * 2 + 1) - '0'); *(cdr->content + i) = (num_high << 4) + num_low; } return cdr;}/* * convert the content of cdr to num */char *cdr2num(char *out, const Cdr *cdr) {int i;for ( i = 0; i < cdr->length; i++ ) {sprintf(out + i * 3, "%03d", *(cdr->content + i));}return out;}/* * convert num to the content of cdr */Cdr *num2cdr(Cdr *cdr, const char *in) { cdr->length = strlen(in) / 3; cdr->content = (unsigned char *) malloc(sizeof(unsigned char) * (cdr->length + 1)); memset(cdr->content, 0, sizeof(cdr->content)); int i; for ( i = 0; i < cdr->length; i++ ) { *(cdr->content + i) = (*(in + i * 3) - '0') * 100 + (*(in + i * 3 + 1) - '0') * 10 + (*(in + i * 3 + 2) - '0'); } return cdr;}/* * convert the content of cdr to oct */char *cdr2oct(char *out, const Cdr *cdr) {int i;for ( i = 0; i < cdr->length; i++ ) {sprintf(out + i * 2, "%02x", *(cdr->content + i));}return out;}/* * convert oct to the content of cdr */Cdr *oct2cdr(Cdr *cdr, const char *in) { cdr->length = strlen(in) / 2; cdr->content = (unsigned char *) malloc(sizeof(unsigned char) * (cdr->length + 1)); memset(cdr->content, 0, sizeof(cdr->content)); int i; for ( i = 0; i < cdr->length; i++ ) { int num_high = (*(in + i * 2) > '9') ? (*(in + i * 2) - 'a' + 10) : (*(in + i * 2) - '0'); int num_low = (*(in + i * 2 + 1) > '9') ? (*(in + i * 2 + 1) - 'a' + 10) : (*(in + i * 2 + 1) - '0'); *(cdr->content + i) = (num_high << 4) + num_low; } return cdr;}/* * convert the content of cdr to octs */char *cdr2octs(char *out, const Cdr *cdr) {int i;for ( i = 0; i < cdr->length; i++ ) {int num_low = *(cdr->content + i) & 0x0f;int num_high = (*(cdr->content + i) & 0xf0) >> 4;sprintf(out + i * 2, "%x", num_low);sprintf(out + i * 2 + 1, "%x", num_high);}return out;}/* * convert octs to the content of cdr */Cdr *octs2cdr(Cdr *cdr, const char *in) { cdr->length = strlen(in) / 2; cdr->content = (unsigned char *) malloc(sizeof(unsigned char) * (cdr->length + 1)); memset(cdr->content, 0, sizeof(cdr->content)); int i; for ( i = 0; i < cdr->length; i++ ) { int num_low = (*(in + i * 2) > '9') ? (*(in + i * 2) - 'a' + 10) : (*(in + i * 2) - '0'); int num_high = (*(in + i * 2 + 1) > '9') ? (*(in + i * 2 + 1) - 'a' + 10) : (*(in + i * 2 + 1) - '0'); *(cdr->content + i) = (num_high << 4) + num_low; } return cdr;}/* * print the encodings unit test usage */void print_encodings_usage(const char *program_name) {printf("Usage: %s ascii bcds bin num oct octs/n", program_name);}/* * unit test */int main(int argc, char *argv[]) { if ( argc < ENCODINGS_CNT ) { print_encodings_usage(argv[0]); exit(1); } Cdr cdr; char str[128]; char *(*cdr2txt[ENCODINGS_CNT])(char *, const Cdr *) = { NULL, cdr2ascii, cdr2bcds, cdr2bin, cdr2num, cdr2oct, cdr2octs }; Cdr *(*txt2cdr[ENCODINGS_CNT])(Cdr *, const char *) = { NULL, ascii2cdr, bcds2cdr, bin2cdr, num2cdr, oct2cdr, octs2cdr }; int i; for ( i = 1; i < ENCODINGS_CNT; i++ ) { printf("convert string %s into cdr/n", argv[i]); txt2cdr[i](&cdr, argv[i]); memset(str, 0, sizeof(str)); printf("convert cdr into string %s/n/n", cdr2txt[i](str, &cdr)); free(cdr.content); }/* printf("convert ascii string %s into cdr/n", argv[1]); ascii2cdr(&cdr, argv[1]); memset(str, 0, sizeof(str)); printf("convert cdr into ascii: %s/n/n", cdr2ascii(str, &cdr)); free(cdr.content); printf("convert bcds string %s into cdr/n", argv[2]); bcds2cdr(&cdr, argv[2]); memset(str, 0, sizeof(str)); printf("convert cdr into bcds : %s/n/n", cdr2bcds(str, &cdr)); free(cdr.content); printf("convert bin string %s into cdr/n", argv[3]); bin2cdr(&cdr, argv[3]); memset(str, 0, sizeof(str)); printf("convert cdr into bin : %s/n/n", cdr2bin(str, &cdr)); free(cdr.content); printf("convert num string %s into cdr/n", argv[4]); num2cdr(&cdr, argv[4]); memset(str, 0, sizeof(str)); printf("convert cdr into num : %s/n/n", cdr2num(str, &cdr)); free(cdr.content); printf("convert oct string %s into cdr/n", argv[5]); oct2cdr(&cdr, argv[5]); memset(str, 0, sizeof(str)); printf("convert cdr into oct : %s/n/n", cdr2oct(str, &cdr)); free(cdr.content); printf("convert octs string %s into cdr/n", argv[6]); octs2cdr(&cdr, argv[6]); memset(str, 0, sizeof(str)); printf("convert cdr into octs : %s/n/n", cdr2octs(str, &cdr)); free(cdr.content);*/ return 0;}