What do I get from front() of empty std container?

Question

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

Answer 1

Yes, you can use 'at' like Graham mentioned instead of using front.

But, at(0) is only available for some containers - vectors, deque and not for others - list, queue, stack. In these cases you've to fall back on the safety of the 'empty' check.

Answer 2

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().

Answer 3

Undefined Behaviour

Answer 4

You've always have to be sure your container is not empty before calling front() on this instance. Calling empty() as a safe guard is good.

Of course, depending on your programm design, always having a non-empty container could be an invariant statement allowing you to prevent and save the call to empty() each time you call front(). (or at least in some part of your code?)

But as stated above, if you want to avoid undefinied behavior in your program, make it a strong invariant.

Answer 5

You get undefined behaviour.

To get range checking use at(0). If this fails you get a out_of_range exception.