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Fixed-size characters can be helpful, but even if there is a fixed byte count per code point (as in UTF-32), there is not a fixed byte count per displayed character due to [[combining character]]s. Considering these incompatibilities and other quirks among different encoding schemes, handling unicode data with the same (or compatible) protocol throughout and across the interfaces (e.g. using an API/library, handling unicode characters in client/server model, etc.) can in general simplify the whole pipeline while eliminating a potential source of bugs at the same time.
UTF-16 is popular because many APIs date to the time when Unicode was 16-bit fixed width (referred as UCS-2). However, using UTF-16 makes characters outside the [[Mapping of Unicode character planes|Basic Multilingual Plane]] a special case which increases the risk of oversights related to their handling. That said, programs that mishandle surrogate pairs probably also have problems with combining sequences, so using UTF-32 is unlikely to solve the more general problem of poor handling of multi-code-unit characters.
If any stored data is in UTF-8 (such as file contents or names), it is very difficult to write a system that uses UTF-16 or UTF-32 as an API. This is due to the oft-overlooked fact that the byte array used by UTF-8 can physically contain invalid sequences. For instance, it is impossible to fix an invalid UTF-8 filename using a UTF-16 API, as no possible UTF-16 string will translate to that invalid filename. The opposite is not true: it is trivial to translate invalid UTF-16 to a unique (though technically invalid) UTF-8 string, so a UTF-8 API can control both UTF-8 and UTF-16 files and names, making UTF-8 preferred in any such mixed environment. An unfortunate but far more common workaround used by UTF-16 systems is to interpret the UTF-8 as some other encoding such as [[CP-1252]] and ignore the [[mojibake]] for any non-ASCII data.
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