What do I get from front() of empty std container?
If front() returns a reference and the container is empty what do I get, an undefined reference? Does it mean I need to check empty() before each front()?
You get undefined behaviour - you need to check that the container contains something using empty() (which checks if the container is empty) before calling front().
You get undefined behaviour.
To get range checking use at(0). If this fails you get a out_of_range exception.