How to assign a multi-dimensional array to a temporary variable?

Question

I want to assign a statically allocated, multi-dimensional array to a temporary variable. Consider the following example:

void foo(int b[3][2])
{
    b[1][1] = 1; // no segmentation fault
}

int main()
{
    int a[3][2] = {{1, 2}, {11, 12}, {21, 22}};

    foo(a);

    int** c;
    c = (int**)&a;
    c[1][1] = 1; // segmentation fault on execution

    int* d[3];
    d[0] = (int*)&(a[0]);
    d[1] = (int*)&(a[1]);
    d[2] = (int*)&(a[2]);
    d[1][1] = 1; // no segmentation fault

    return 0;
}

Basically I want to do what the compiler does with the parameter b of foo(). But the only working solution I could come up with is d. Is there no less complicated way?

Answer 1

cdecl (man page) is your friend:

cdecl> explain int b[3][2]
declare b as array 3 of array 2 of int
cdecl> declare b as pointer to array 2 of int
int (*b)[2]

So, try this:

void foo(int b[3][2])
{
    b[1][1] = 1; // no segmentation fault
}

int main()
{
    int a[3][2] = {{1, 2}, {11, 12}, {21, 22}};

    foo(a);

    int (*b)[2] = a;

    b[1][1] = 1;

    return 0;
}

Answer 2

int[3][2] and int** are incompatible types. You cannot cast one to another.

Try this:

int (*c)[2];
c = a; //no need to cast 
c[1][1] = 1; //ok

Or you could do this (declaration as well as initialization):

int (*c)[2] = a; //no need to cast 
c[1][1] = 1; //ok

---

Thumb of rule:

• Don't use c-style cast in C++. Use C++-style cast. Had you used C++-style cast, the compiler would have told you the problem much before (ideone) (no need to run the code to see the problem):

  prog.cpp:5: error: invalid static_cast from type ‘int (*)[3][2]’ to type ‘int**’
  

  But C-style cast compiles it fine (ideone), as you already know.

• And whenever you use cast, even C++-style cast, your first doubt should be the cast itself if the program doesn't work as expected.