Question

I’m working on a project of which I should store constant multi dimensional arrays.I would like to do it with a pointer which points to multi-dimensional arrays.However, I could’nt succeeded.I write this code but it didn’t compiled.

int darray[1][2];
    int darray2[2][3];
    int (*p)[1][2];
    p= new int[2];
    p[0] = darray;
    p[1] = darray2;

Answer 1

Well, let’s go through your code line by line (OK, I’ll lump the first two lines together)

int darray[1][2];
int darray2[2][3];

OK, this defines two two-dimensional arrays in exactly the way you (probably) expect. Note that formally, those are actually arrays of arrays of ints. More exactly, darray is of type “array of 1 array of 2 ints each”, and darray2 is of type “array of 2 arrays of 3 ints each”

int (*p)[1][2];

This defines a pointer to a two-dimensional array of int, or to the first element of an array of such arrays. That is, the type of p is “pointer to array of 1 array of 2 ints each”.

p= new int[2];

Here you are allocating space for two ints (i.e. an one-dimensional array of ints, of length two), and new returns a pointer to the first element of that array, i.e. a pointer to int. You try to assign that pointer to int to p, which is a pointer to array of 1 array of 2 ints each, as mentioned above. This gives a type mismatch and should not compile.

It’s not entirely clear what you want at that point, but given that you do assignments to p[0] and p[1] afterward, and given that the return value is assigned to p which is of type int (*)[1][2] the obvious choice would be p = new int[2][1][2];

p[0] = darray;

This again doesn’t work, because arrays in C++ are not first-class objects. That is, instead of assigning the value of darray to p[0] as the line would suggest (and p[0] indeed would have the right type for that) the rules of C++ say that the array darray is promoted to a pointer to its first element. That is, what this code actually tries to do is to assign a pointer to the first element of darray (of type int (*)[2]) to p[0] (of type int[1][2]), which of course fails. In C++, arrays are simply not assignable.

p[1] = darray2;

This suffers from the same problem, however note that even if arrays were first-class, assignable objects in C++, this still would be a type mismatch because p[1] is of type int[1][2] while darray2 is of type int[2][3].

Note that you get around most of those limitations (apart from the last one) by just wrapping you array into a class C++11 actually provides a standard class template called std::array for this purpose):

struct array12 { int data[1][2]; };
array12 darray;
array12* p;
p = new array12[2];
p[0] = darray;

With a bit more of programming you could also handle the assignment from larger to smaller arrays (your darray2 case) by copying only part of the data by hand.