7.4 Pointers to Pointers - C Programming Foundations

Changing a pointer variable itself

Passing an int ** lets a function change the caller's pointer variable, not merely the final integer. Pass the pointer's address, as in redirect(&p, &other).

void redirect(int **target, int *destination) {
    *target = destination;
}

QUESTION

Which object is changed by **q = 9?

QUESTION

What should a function receive to change caller pointer p?

Implement choose_larger(int **selected, int *a, int *b) so selected points to the larger integer. Test with 4 and 9, then print 9 through **selected.

Initially point p at first in main.
Accept int **target and a replacement int *.
Assign *target = replacement and verify the caller's *p.

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A pointer has its own address, so another pointer can store that address. int **q means q points to an int *; *q reaches p, and **q follows p to data.

#include <stdio.h>

int main(void) {
    int data = 5;
    int *p = &data;
    int **q = &p;

    printf("%d\n", data);
    printf("%d\n", *p);
    printf("%d\n", **q);

    return 0;
}

QUESTION

What is the type of *q?

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Changing a pointer variable itself

Passing an int ** lets a function change the caller's pointer variable, not merely the final integer. Pass the pointer's address, as in redirect(&p, &other).

void redirect(int **target, int *destination) {
    *target = destination;
}

QUESTION

Which object is changed by **q = 9?

QUESTION

What should a function receive to change caller pointer p?

Implement choose_larger(int **selected, int *a, int *b) so selected points to the larger integer. Test with 4 and 9, then print 9 through **selected.

Initially point p at first in main.
Accept int **target and a replacement int *.
Assign *target = replacement and verify the caller's *p.

Pointers to Pointers

KNOWLEDGE QUIZ

Read and write through an address chain

Changing a pointer variable itself

KNOWLEDGE QUIZ

KNOWLEDGE QUIZ

Redirect a pointer

Pointers to Pointers

KNOWLEDGE QUIZ

Read and write through an address chain

Changing a pointer variable itself

KNOWLEDGE QUIZ

KNOWLEDGE QUIZ

Redirect a pointer