This is a function within my doubly linked list class, but every time I compile, I get this message: "Invalid initialization of non-const reference of type 'int&' from a temporary of type 'int'. I just can't get how else to do it.
int& LinkedList::operator[](int index)
{
Node* current = head_;
for(int i = 0; i < index; i++){
current = current->getNextNode();
}
return(current->getValue()); // getValue() returns an int
}
Thanks in advance!
Simple answer: getValue has to return an int& as well.
If you want to be able to use this to modify the value stored in the list, then it will have to return a reference to that value. That means that you'll need to either make Node::getValue() return a reference, or add another way to get a reference to the value stored in the node.
If you don't want to use this to modify the list contents, then you could change the return type to int.
The problem is that you cannot bind a non-const reference to an rvalue (in this case the temporary returned by the getValue() function). If you want to provide a reference to the value stored in the list so that it can be modified by the caller you will need to modify getValue() to return a reference.
As of the general idea, you might want to consider offering a random access operation to the list. It might give the wrong idea that it is a cheap. Users might, for example try to iterate over the list like:
for (int i = 0; i < list.size(); ++i)
std::cout << list[i] << std::endl;
But that iteration is actually O(N^2) rather than O(N)
(You should, you know, just use std::list... which doesn't offer this operation for a reason...)
The reason you are returning int& is so that someone who writes mylist[i] = 42 will actually modify the contents of the list.
However, you are getting at the value by using the getValue helper which, based on the error message, returns a copy. The compiler has, by analyzing the data types, found your logical error. If you want to be able to modify the actual list data, then you have to return a reference to the actual list data, not a copy.
Thus, getValue needs to return int& as well, as Philipp suggests.
list[i] = 42wouldn't work as expected.