What does double **a mean in C++?

One * means a pointer. Two means a pointer to a pointer, and so on. In this case, a points to not just one pointer, but an array of m pointers. And each pointer in that array points to a double. In this case, not just one double but an array of n1 of them.

When the for loop is done, you'll have a dynamic two-dimensional arrays where a[i] is a pointer to the the (i+1)th array of doubles and then a[i][j] is the (j+1)th double in that array. (0 <= i < m and 0 <= j < n1)

Understand this with the below example:

int i = 200; // Creating a variable i, where we are storing 200 and also lets assume that the memory location allocated to store i is location address 400 to 403, as int is 4 bytes.

int* p = &i; //Here we are creating a pointer variable whose name is p, which can store an address of a integer variable. hence from the above, the value stored in p is 400. And here lets assume that the address where p is stored is 800 to 803, as pointer will take 4 bytes of memory space.

// Now if you need to store the address of a pointer variable, you will create a double pointer, as:

int** p1 = &p; // Here p1 stores the value 800.

Lets not use a as the variable name, because it makes it confusing to explain in words (since a is also a word).

Assume it was double ** pd;

Then:

pd is a pointer to a pointer. The pointer that pd points to is a pointer to a double.

pd ---> somepointer ---> somedouble

What is allocated by new is an array of pointers to double. pd is a pointer to this array, so it points to the first element in the array, which is a pointer to a double. So pd points to a pointer to a double.

That denotes that a is a pointer to a pointer. Its just another level of referencing.

This program is creating a pointer (a) to a pointer array of pointers to double. Most likely you will return a reference to 'a' later in your program assuming this is a function that returns a pointer to double.

double 'a' represents a pointer to the array 'a'. Array 'a' contains pointers to double.