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c++pointersdereference
Rohith Yeravothula
Rohith Yeravothula

Reputation: 435

cpp double pointer vs 2 dimensional array pointer

#include <iostream>

int main() {
    int a[2][2] = {{1,2}, {3,4}};
    int *c = *a;
    int **b = &c;
    std::cout << **(a+1);  // outputs 3
    std::cout << **(b+1);  // segmentation fault
}

Why does one cout results in segmentation fault and other doesn't? Shouldn't they be referring to the same value?

Upvotes: 2

Views: 130

Answers (3)

C. R. Ward
C. R. Ward

Reputation: 93

I rewrote the code to highlight what is happening, as below:

#include <iostream>

int main() {
  int a[2][2] = {{1,2}, {3,4}};
  int *c[2] = {a[0], a[1]};
  int **b = c;

  std::cout << **(a  ) << ','; // outputs 1
  std::cout << **(b  ) << ";\n"; // outputs 1
  std::cout << **(a+1) << ','; // outputs 3
  std::cout << **(b+1) << ";\n"; // outputs 3
}

LINK: https://ideone.com/ixj3NV

UPDATED LINK: https://ideone.com/g7jjVN (Clarified the original source to extend the program)

Upvotes: 0

Vlad from Moscow
Vlad from Moscow

Reputation: 310970

In this statement

cout << **(b+1);

the expression b+1 points outside the array (that is more precisely outside the object c). You should write

cout << *( *b + 2 );

The dereferenced pointer b points to the pointer to the first element of the two-dimensional array. When adding to it the number of elements in the array of the type int[2] you will get the pointer to the first element of the second "row" of the two-dimensional array. Now you need again to dereference it to output the pointed value.

Upvotes: 3

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122450

Lets start with 

int *c;

Actually what comes before is not that relevant, because c is just a pointer and then here

int **b = &c;

you store the address of c in b. The address of c has nothing to do with what value is stored in c. c is just a pointer, taking its adress doesn't let you magically access a 2d array. 

cout << **(b+1); // segmentation fault

Already b+1 is undefined behaviour. Dereferencing that pointer cannot give you something meaningful.

PS: tbh I cannot tell you how to use the double pointers correctly here. Once I started to learn c++ I unlearned anything I knew about working with arrays via pointers. Use std::vector or std::array and save yourself some headaces.

Upvotes: 4

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