Why use double indirection? or Why use pointers to pointers?

Question

When should a double indirection be used in C? Can anyone explain with a example?

What I know is that a double indirection is a pointer to a pointer. Why would I need a pointer to a pointer?

Answer 1

If you want to have a list of characters (a word), you can use char *word

If you want a list of words (a sentence), you can use char **sentence

If you want a list of sentences (a monologue), you can use char ***monologue

If you want a list of monologues (a biography), you can use char ****biography

If you want a list of biographies (a bio-library), you can use char *****biolibrary

If you want a list of bio-libraries (a ??lol), you can use char ******lol

... ...

^{yes, I know these might not be the best data structures}

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Usage example with a very very very boring lol

#include <stdio.h> #include <stdlib.h> #include <string.h>  int wordsinsentence(char **x) {     int w = 0;     while (*x) {         w += 1;         x++;     }     return w; }  int wordsinmono(char ***x) {     int w = 0;     while (*x) {         w += wordsinsentence(*x);         x++;     }     return w; }  int wordsinbio(char ****x) {     int w = 0;     while (*x) {         w += wordsinmono(*x);         x++;     }     return w; }  int wordsinlib(char *****x) {     int w = 0;     while (*x) {         w += wordsinbio(*x);         x++;     }     return w; }  int wordsinlol(char ******x) {     int w = 0;     while (*x) {         w += wordsinlib(*x);         x++;     }     return w; }  int main(void) {     char *word;     char **sentence;     char ***monologue;     char ****biography;     char *****biolibrary;     char ******lol;      //fill data structure     word = malloc(4 * sizeof *word); // assume it worked     strcpy(word, "foo");      sentence = malloc(4 * sizeof *sentence); // assume it worked     sentence[0] = word;     sentence[1] = word;     sentence[2] = word;     sentence[3] = NULL;      monologue = malloc(4 * sizeof *monologue); // assume it worked     monologue[0] = sentence;     monologue[1] = sentence;     monologue[2] = sentence;     monologue[3] = NULL;      biography = malloc(4 * sizeof *biography); // assume it worked     biography[0] = monologue;     biography[1] = monologue;     biography[2] = monologue;     biography[3] = NULL;      biolibrary = malloc(4 * sizeof *biolibrary); // assume it worked     biolibrary[0] = biography;     biolibrary[1] = biography;     biolibrary[2] = biography;     biolibrary[3] = NULL;      lol = malloc(4 * sizeof *lol); // assume it worked     lol[0] = biolibrary;     lol[1] = biolibrary;     lol[2] = biolibrary;     lol[3] = NULL;      printf("total words in my lol: %d\n", wordsinlol(lol));      free(lol);     free(biolibrary);     free(biography);     free(monologue);     free(sentence);     free(word); } 

Output:

total words in my lol: 243