Detect encoding of a string in C/C++

It's not an easy problem to solve, and generally relies on heuristics to take a best guess at what the input encoding is, which can be tripped up by relatively innocuous inputs - for example, take a look at this Wikipedia article and The Notepad file encoding Redux for more details.

If you're looking for a Windows-only solution with minimal dependencies, you can look at using a combination of IsTextUnicode and MLang's DetectInputCodePage to attempt character set detection.

If you are looking for portability, but don't mind taking on a fairly large dependency in the form of ICU then you can make use of it's character set detection routines to achieve the same thing in a portable manner.

Assuming you know the length of the input array, you can make the following guesses:

1. First, check to see if the first few bytes match any well know byte order marks (BOM) for Unicode. If they do, you're done!
2. Next, search for '\0' before the last byte. If you find one, you might be dealing with UTF-16 or UTF-32. If you find multiple consecutive '\0's, it's probably UTF-32.
3. If any character is from 0x80 to 0xff, it's certainly not ASCII or UTF-7. If you are restricting your input to some variant of Unicode, you can assume it's UTF-8. Otherwise, you have to do some guessing to determine which multi-byte character set it is. That will not be fun.
4. At this point it is either: ASCII, UTF-7, Base64, or ranges of UTF-16 or UTF-32 that just happen to not use the top bit and do not have any null characters.

I have written a small C++ library for detecting text file encoding. It uses Qt, but it can be just as easily implemented using just the standard library.

It operates by measuring symbol occurrence statistics and comparing it to pre-computed reference values in different encodings and languages. As a result, it not only detects encoding but also the language of the text. The downside is that pre-computed statistics must be provided for the target language to detect this language properly.

https://github.com/VioletGiraffe/text-encoding-detector