Examples would be eg, `string_view` or `ArraySegment`. They hold some offset relative to a base allocation, and when we index the string_view or ArraySegment we're indexing relative to that offset.
It's not holding an "offset", it's a fat pointer, (ptr,len) or possibly (start,end)
You're imagining this as (string_ptr,offset,len) but that's 50% bigger for no practical benefit, you cannot unwind a std::string_view to get the string it's a view into, indeed there may never have been such a string.
But... This loses the reason people are using indices to begin with: because the borrow checker cannot track what they do.
It's awkward to do get right because you need an indirect pointer whose address remains fixed, but points to another pointer which can change (and is volatile).
While it might be possible to make something like this lockless - it's much simpler to stick a mutex in the array header. When we access the array_segment we can take a lock to prevent some other thread reallocating mid-way through accessing.
There's probably a few improvements that could be made. In particular it doesn't handle use-after-free, so it's not thread safe w.r.t cleanup.
Similarly, CPU architectures that use descriptors can (have to?) have languages with that notion.
`thread_local` is an example of a "relative pointer" though. Instructions to access the thread local are prefixed with `fs:` or `gs:`, and point relative to the address in the respective segment register.
thread_local is an example of something else unusual though - a struct linker as seen in https://yosefk.com/blog/i-want-a-struct-linker.html and it even supports dynamic loading.
A far pointer sounds like the global based pointer described in that article. The far pointer Wikipedia article says they are problematic but doesn't give much reasoning as to why.
GCC still supports `__seg_fs` and `__seg_gs`, which behave similar to `far` in the example on the wiki page, as the FS and GS segment registers are still valid in x86-64 and used for TLS. Clang uses attributes `address_space(257)` and `address_space(256)` for the same thing.
The `__based` pointer in MSVC exploits the addressing modes by pinning the base in eg: `[base+index*scale+displacement]`. It's unrelated to segmentation.
Project CHERI would like to disagree.
Segmentation isn't used. There's no separate registers to hold the bounds information in CHERI - the bounds are held in the pointer value, unlike for example, the now obsolete Intel MPX, which held bounds information in separate registers.
There's some similarity to segmentation because the CHERI pointer restricts which addresses can be accessed, but I wouldn't compare them to far pointers.
Most modern processors have a single linear virtual address space and don't use segmentation, and even where segment registers exist (eg, FS and GS on x86-64), they're only superficial "address spaces" - allocated sections of the process's linear virtual address space which could be accessed without segmentation registers if you knew the base address held in FS or GS.
There is a similar proposal for trait objects in rust.