从迭代器返回对象的引用

Question

I want to return a reference of an object from a vector, and the object is in an iterator object. How can I do that?

I tried the following:

Customer& CustomerDB::getCustomerById (const string& id) {
    vector<Customer>::iterator i;
    for (i = customerList.begin(); i != customerList.end() && !(i->getId() == id); ++i);

    if (i != customerList.end())
        return *i; // is this correct?
    else
        return 0; // getting error here, cant return 0 as reference they say
}

In the code, customerList is a vector of customers, and the function getId returns the id of the customer.

Is the *i correct? And how can I return 0 or null as a reference?

Answer 1

return *i; is correct, however you can't return 0, or any other such value. Consider throwing an exception if the Customer is not found in the vector.

Also be careful when returning references to elements in vector. Inserting new elements in vector can invalidate your reference if vector needs to re-allocate its memory and move the contents.

Answer 2

There is no such thing as a "null" reference: if your method gets an id that's not in the vector, it will be unable to return any meaningful value. And as @reko_t points out, even a valid reference may become invalid when the vector reallocates its internals.

You should only use reference return types when you can always return a reference to an existing object that will stay valid for some time. In your case, neither is guaranteed.