运算符<<不返回对象内容

Question

我有一个任务，我要负责创建一个程序来构造双向链表以容纳非常长的整数。创建用于operator +重载的方法以添加两个非常长的int对象的任务之一。当我尝试使用operator <<方法来打印单个对象时，它的效果很好，并且可以打印出我期望的结果。但是，当我添加两个对象时，什么也没打印出来……好像我的对象丢失了。谁能帮我调试一下吗？为什么在调用operator +方法之后，我的LongInt对象没有被打印到控制台？

问题是a + b返回空白： https://laracasts.com/discuss/channels/laravel/running-python-in-laravel

// Arithmetic Binary Operator +
// Adds two LongInt objects
// Input: a LongInt object
// Output: a new LongInt which is the sum of lhs (current) and rhs (arguement)
LongInt LongInt::operator+( const LongInt &rhs ){
    Deque<char> lhs = digits;
    Deque<char> rhs2 = rhs.digits;
    Deque<char> newLongInt;

    int carry = 0;
    while (!lhs.isEmpty() && !rhs2.isEmpty()){
        newLongInt.addFront((lhs.getBack()+rhs2.getBack() + carry) % 10);
        carry = (lhs.removeBack() + rhs2.removeBack()) / 10;
    }
    LongInt test;
    test.digits = newLongInt;
    return test;
}

// Prints LongInt to console
// Input: Reference to LongInt
// Output: string of LongInt printed out
ostream &operator<<( ostream &out, const LongInt &rhs ) {
    // check if LongInt is negative and if so, add '-'
    rhs.negative == true ? out << "-" : out << "";

    // temp deque<char>
    Deque<char> temp = rhs.digits;

    // check if it is empty
    if (temp.isEmpty()) {
        out << "0";
    } else {
        // loop through linked list then output LongInt
        while(!temp.isEmpty()){
            out << temp.removeFront();
        }
    }
    return out;
}