Big integer calculator c++ - c++

Question

So far I've got an add, subtract and print function as well as a constructor that initializes to zero the array. For some reason the operations(+ and -) made alot of sense to me(i think) so I kind of got ahead of my self and am not too sure how to initialize a big integer, could I get some help with a function such as
void assign(const bigint & A) or something like that? Also if there is something already wrong with my code please tell me. Thanks
const int size=30; //just to make things easier, will change to something big later
class bigint
{
int digits[size];
public:
// initializes to zero
bigint()
{
for (int i = 0; i < size; i++)
digits[i] = 0;
}
// prints a big int
void print()
{
bigint B;
for (int i = size - 1; i >= 0; i--)
{
int dig = B.digits[i];
if (dig != 0)
std::cout << dig;
}
}
// subtracts a bigint(B) from another(A)
void subtract(bigint & A, bigint & B)
{
for (int i = 0, borrow = 0; i < size; i++)
{
if (borrow = ((A.digits[i] -= B.digits[i] + borrow) < 0))
{
A.digits[i] += 10;
}
}
}
// adds a bigint(A) to another(B)
void add(bigint & A, bigint & B)
{
for (int i = 0, carry = 0; i < size; i++)
{
if (carry = ((A.digits[i] += B.digits[i] + carry) < 9))
{
A.digits[i] -= 10;
}
}
}
};