Imagine that you had a gun and one of the features of the gun was that if you had sunglasses on and something in your left pocket, holstering the gun would cause it to immediately fire. You could argue that the gun shouldn’t behave this way, but it’s also possible that others are dependent on this behavior and you can’t remove it.
This is a footgun - the way to avoid the holster firing is to simply not wear sunglasses, or keep something in your left pocket, and then it would never fire. But the problem is that both of those things are extremely common (for good reason). It’s a poorly thought out feature because it has severe consequences (potentially shooting your foot) for extremely common situations (wearing sunglasses and using your left pocket).
I basically don’t agree that anybody could depend on this holstering-causes-it-to-fire behavior. Or at least, their use-case requires design compromises that are so unthinkably ridiculous as to make the gun they want something that no reasonable person without that use-case would use.
It is possible that the entire field of programming is full of ridiculous people. But it seems more likely that C++ is like a gun with no safety, or something along those lines.
A lot of the footguns come from compiler authors wanting to make things UB because it allows them to perform certain optimizations, but then you end up with a lot of things that are formally UB even though in practice they usually do the intuitively expected thing. But then, because the widely done thing is actually UB, the compiler is allowed to do something counterintuitive which causes your program to blow up.
An obvious example is omitting NULL pointer checks. Passing a NULL pointer to certain system library functions is UB even if it would ordinarily be expected to be reasonable, e.g. memset(NULL, 0, 0), so some compilers will see that you passed a pointer to memset, and passing a NULL pointer to memset is UB, therefore it can omit a subsequent NULL pointer check guarding a call to something else when the something else isn't going to behave reasonably given a NULL pointer.
This is an insane footgun, but it also allows the compiler to omit a runtime NULL pointer check, which makes the program faster, so people who care most about performance lobby to keep it.
Another common way to shoot yourself in the foot is a gun which will go off if you drop it. An example of a gun where early models were especially susceptible is the Lanchester: https://en.wikipedia.org/wiki/Lanchester_submachine_gun
> I basically don’t agree that anybody could depend on this holstering-causes-it-to-fire behavior.
It's an idiom. It's not supposed to be entirely logically consistent. It means what it means because people have decided that it means what it means. Your objections aren't really relevant.
"Footgun" is I think a fairly recent addition to the English lexicon, but it's based on the centuries-old "to shoot yourself in the foot". It seems silly to argue with centuries of English idiomatic usage; no one, by definition, is going to win an argument against that.
Yeah I don't think that's a good analogy. Instead, you have guns that don't let you point at your feet. So you can never shoot yourself there. However, if you ever need to shoot straight down for a legitimate reason, you're out of luck. In C++, you can shoot everywhere without restrictions and sometimes that means shooting yourself in the foot or the head.
This is a footgun - the way to avoid the holster firing is to simply not wear sunglasses, or keep something in your left pocket, and then it would never fire. But the problem is that both of those things are extremely common (for good reason). It’s a poorly thought out feature because it has severe consequences (potentially shooting your foot) for extremely common situations (wearing sunglasses and using your left pocket).