Q:

Will Endianness be an issue for this type of binary IO operation?

For space efficiency, I have decided to code my save files using binary. Every byte represents an id for the type of tile. Would this cause an issue with different Endian computing?
Also, out of curiosity, is it the CPU or the operating system which sets the Endian type?
Additional information: I am using C++ and building an x-platform game. I do not want to use an additional API such as Boost.

A:

Yes, it will cause issues - if the saved file from BE gets loaded on LE or vice versa. This is why some Unicode encodings such as UTF-16 and UTF-32 have a so-called byte-order marker.
If your code is usually compiled on BE you will still have to make sure that the LE code will swap the byte order before making use of the data.
The CPU sets the Endianess and some chips (e.g. some MIPS CPUs) allow that to be switched when bootstrapping the system.

A:

We could use a little more info. Cross platform is one thing, but what platforms? If you mean cross platform like x86 Mac, x86 Linux and x86 Windows, then no you won't need to worry about it (although struct packing may still be an issue if you try to just fwrite structs to disk and compile with different compilers on different platforms). Even if you have a couple of different OS/CPU combos you can make a list of everything you want to support and if they all have the same endianess, don't worry about it.
If you have no expectation that save data will be moved from platform to platform, you also don't need to worry about it. Endianess is only an issue when you want create data on a big-endian machine and then read it on a little-endian machine or vice versa. If these are just local data files, no big deal, although it's probably safe to assume that if your users can copy their saves from one platform to another, they will, as they will do pretty much anything you don't want them to do and don't support.
Additionally, since you only mention bytes, if a byte array is as complex as your data is going to get, you actually don't need to worry about endianess. It's only an issue for multi-byte data types. So if you are just saving byte arrays, and the rest of your bookkeeping data also fits in bytes, there's nothing to worry about, but as soon as you save a short, int or float you'll have potential endian issues.
My personal opinion is whenever you serialize take endianess into account, but I have an extremely multplatform background (i.e. shipping the same product on 5 game systems). It's pretty easy, the swap macros are already there, and when you inevitably decide to move to another endianess you won't have to rewrite stuff. If the data is more complex or structured, maybe consider a library like Protocol Buffers or BSON.
Both the CPU and operating system may be responsible for endianess. Historically it was baked into the CPU, and while x86 is still hardwired as little-endian, most modern RISC derivatives can operate in either mode, making it the choice of the hardware and OS developers.

