Question

I have some problems using two dimensional array.

static const int PATTERNS[20][4];

static void init_PATTERN()
{
   // problem #1
   int (&patterns)[20][4] = const_cast<int[20][4]>(PATTERNS);  
   ...
}

extern void UsePattern(int a, const int** patterns, int patterns_size);

// problem #2
UsePattern(10, PATTERNS, sizeof(PATTERNS)/sizeof(PATTERNS[0]));

In the first statement, I need to cast the const off the two dimensional array PATTERNS. The reason for this is that the init function is called only once, and in the remaining code, PATTERNS is strictly read-only.

In the second statement, I need to pass PATTERNS array to the int** argument. Direct passing resulted a compile error.

---

I have solved the problem, just about the same time that @Andrey posted the answer. Yes int[][] can’t be casted to int**.

It can be casted to int* through &(PATTERNS[0][0]), and the function prototype must be modified with row size (the number of elements in a row). The array can be const_cast away with reference syntax.

Answer 1

Firstly, there’s no such thing as cast to array type (or to function type) in C++. Yet this is what you are trying to do. If you want to cast away constness from something, you have to cast to either pointer or reference type. In your case you have a reference on the receiving end of the cast, so the cast itself has to be to reference type as well

int (&patterns)[20][4] = const_cast<int (&)[20][4]>(PATTERNS); 

Of course, as Bill already noted, casting away constness from a constant object (and then attempting to modify the object) leads to undefined behavior.

Secondly, a two-dimensional array cannot be passed anywhere as an int ** pointer. If you want to pass your PATTERNS somewhere, you can pass it as const int (&)[20][4], const int (*)[20][4], const int [][4], const int (*)[4] or something similar to that, but not as int **. Do a search on SO and/or read some FAQ on arrays to understand why. This has been explained too many times to repeat it again.