free the memory of: A** mat = new A*[2];

Question

I defined:

A** mat = new A*[2];

but how can I delete it? With delete[] mat; or delete[] *mat;?

Answer 1

It's delete[] mat; only when you do not do additional allocations. However, if you allocated the arrays inside the array of arrays, you need to delete them as well:

A** mat = new A*[2];
for (int i = 0 ; i != 2 ; i++) {
    mat[i] = new A[5*(i+3)];
}
...
for (int i = 0 ; i != 2 ; i++) {
    delete[] mat[i];
}
delete[] mat;

Answer 2

the first one, delete[] mat

the second one would delete what the first element in the array was pointing to (which would be nothing if that is really all the code you have) it is equivalent to delete [] mat[0]

also, if the pointers in the array did end up pointing to allocated memory that you also wanted freed, you would have to delete each element manually. eg:

A** mat = new A*[2];
mat[0] = new A;
mat[1] = new A[3];

then you would need to do:

delete mat[0];
delete[] mat[1];
delete[] mat;