multiply bigints c++

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C++

This is my multiply algorithm that is part of my big num class. The arr is a pointer to a class that I created for a Safe resizable array. The main() is obviously incomplete(just put it there to show how my bigint class works in relation to my multiply function) What I really need here is a fix for my algorithm here or give my advice on how to fix it. Thanks

         void multiply(const bigint &A)
{
    bigint temp1; // bigint with value 0

    int carry = 0;
    int shift = 0;
    bigint temp2;
    for(int j = size-1; j >= 0; j--) //no member size in bigint
    {
        for(int i=size-1; i>=0; i--)
        {
            // bigint with value 0 and size: size + A.size
            int result = (arr->get(i)*A.arr->get(j)+carry);

            //if(size - shift - (i - size-1) >= 0)
                temp2.arr->set(size-i, result%10);


            carry=result/10;
        }
        shift++;
        temp1.add_pos(temp2);
    }
    this->assign(temp1);
}


                int main()
                {
                   a.assign (12345);
                   b.assign (12342357);
                   a.multiply (b);
                }