c++ - Difference between double pointer and array of pointers

Question

In a normal c/c++ program we write main function as either

int main(int c, char **argv)

or

int main(int c, char *argv[])

Here argv represents an array of pointers but we even represent double pointer(pointer to pointer) using **.

ex:

char p,*q,**r;
q=&p;
r=&q;

Here r is a double pointer and not array of pointers.

Can any one explain the difference?

Answer 1

When used as a function parameter

char a[]  // compiler interpret it as pointer to char

is equivalent to

char *a

and similarly, in main's signature, char *argv[] is equivalent to char **argv. Note that in both of the cases char *argv[] and char **argv, argv is of type char ** (not an array of pointers!).

The same is not true for the declaration

char **r;
char *a[10];

In this case, r is of type pointer to pointer to char while a is of type array of pointers to char.
The assignment

r = a;   // equivalent to r = &a[0] => r = &*(a + 0) => r = a

is valid because in this expression again array type a will be converted to pointer to its first element and hence of the type char **.

Always remember that arrays and pointers are two different types. The pointers and arrays equivalence means pointer arithmetic and array indexing are equivalent.

Suggested reading:

• But I heard that char a[] was identical to char *a.
• Why are array and pointer declarations interchangeable as function formal parameters?