new[] doesn't decrease available memory until populated

When your library allocates memory from the OS, the OS will just reserve an address range in the process's virtual address space. There's no reason for the OS to actually provide this memory until you use it - as you demonstrated.

If you look at e.g. /proc/self/maps you'll see the address range. If you look at top's memory use you won't see it - you're not using it yet.

Please look up for overcommit. Linux by default doesn't reserve memory until it is accessed. And if you end up by needing more memory than available, you don't get an error but a random process is killed. You can control this behavior with /proc/sys/vm/*.

IMO, overcommit should be a per process setting, not a global one. And the default should be no overcommit.

About the second half of your question:

The language standard doesn't allow any delays in throwing a bad_alloc. That must happen as an alternative to new[] returning a pointer. It cannot happen later!

Some OSs might try to overcommit memory allocations, and fail later. That is not conforming to the C++ language standard.