How to explain undefined behavior to know-it-all newbies?
Two possibilities come to my mind:
You could ask them "just because you can drive on the motorway the opposite direction at midnight and survive, would you do it regularly?"
The more involved solution might be to set up a different compiler / run environment to show them how it fails spectacularly under different circumstances.
Undefined means explicitly unreliable. Software should be reliable. You shouldn't have to say much else.
A frozen pond is a good example of an undefined walking surface. Just because you make it across once doesn't mean you should add the shortcut to your paper route, especially if you're planning for the four seasons.
"Congratulations, you've defined the behavior that compiler has for that operation. I'll expect the report on the behavior that the other 200 compilers that exist in the world exhibit to be on my desk by 10 AM tomorrow. Don't disappoint me now, your future looks promising!"
Simply quote from the standard. If they can't accept that, they aren't C++ programmers. Would Christians deny the bible? ;-)
1.9 Program execution
The semantic descriptions in this International Standard define a parameterized nondeterministic abstract machine. [...]
Certain aspects and operations of the abstract machine are described in this International Standard as implementation-defined (for example,
sizeof(int)). These constitute the parameters of the abstract machine. Each implementation shall include documentation describing its characteristics and behavior in these respects. [...]Certain other aspects and operations of the abstract machine are described in this International Standard as unspecified (for example, order of evaluation of arguments to a function). Where possible, this International Standard defines a set of allowable behaviors. These define the nondeterministic aspects of the abstract machine. [...]
Certain other operations are described in this International Standard as undefined (for example, the effect of dereferencing the null pointer). [ Note: this International Standard imposes no requirements on the behavior of programs that contain undefined behavior. —end note ]
You can't get any clearer than that.