Bare bones SinglyLinkedList in C++11
It seems to me that the ListNode type is part of the SinglyLinkedList. So there's no reason for that to be visible outside the SLL. I suggest making it a nested/private/hidden/secret class. (There are many ways to hide it, pick the one you like.)
Your method names are confusing. The std namespace has any number of collections defined. Pick one of them, and model your naming on that. For example: std::vector has a size method that returns the size. It also defines push_back and pop_back. Alternatively, std::list also defines push_front and pop_front. You need to pick a usage model for your SLL, and then choose method names compatible with it.
What, no iterators?