They asked for evidence of bounds checks being expensive, please try to keep the context in mind.
Note that this is not something C++ is able to do
Or you could just not mutate the container.
Note that this is not something C++ is able to do
Or you could just not mutate the container.
They said "bounds-checking penalties are small" not that it was free. You've described a situation where bounds checking happens and declared that it "should be" expensive because it happens. You haven't given evidence that it's expensive.
You can profile it yourself, but it is the difference between direct access to cached memory (from prefetching) and two comparisons plus two branches then the memory access.
To someone who doesn't care about performance having something trivial become multiple times the cost might be acceptable (people use scripting languages for a reason), but if your time is spent looping through large amounts of data, that extra cost is a problem.