Let us start with my decision: I vote against inclusion of Boost.Locale into Boost in its current state, but strongly encourage the author to reconsider resubmitting it after some design issues have been reconsidered. In which case, I would be in favour of putting it on the fast-track for a mini-review. Since I am the author of another Unicode library that I'd like to see included in Boost, I hope that my review will not be considered biased. I believe Boost.Locale is a valuable and useful library, and I believe both libraries can coexist. The main reason for my decision is the interface that the library provides: albeit it aims at abstracting the backend used to implement the facilities, it exposes too much of how those facilities are implemented, which prevents benefiting from future backends that may not have the same limitations as the current backends do. Indeed, the Boost.Locale rationale clearly says that it only takes std::basic_string and const char* as input because that's what the backend libraries expect anyway. Yet there are various scenarios where this is useful: - I might want to apply some operation to data in a file without putting it all to memory first; Boost.Locale could copy that memory by chunks to apply that operation for backends that have no native support for this. Copying the whole file to memory is a good enough approximation for a first version. - The input I have may not be directly available in the form of a contiguous buffer, but I wouldn't mind if Boost.Locale copied it. If we consider the functions from_utf, to_utf and between, they can either take a const char*, two const char*, or a std::basic_string. Clearly this would be better if those functions took a range, which covers all three of these cases. It's not just a matter of being generic, it makes the library easier to use since the range is defined by a single argument which can be anything I want, I do not need to convert to what the library needs. If the library needs to do conversion behind the scenes, it can do it just like that, behind the scenes. Taking a range causes the problem however of what the return type should be: I think there should be two variants, one which returns a string, and one which appends to a container (like Boost.StringAlgo) or write to an output iterator (like STL algorithms). Obviously, the former would be defined in terms of the latter. This causes however a big problem: all backends would not be interchangeable with a stable unique ABI. Maybe some template specializations could be injected at link-time, but they would have to be full concrete specializations. Maybe the idea of backends being interchangeable at the ABI level needs to be rethought. After all, isn't being interchangeable at the API level enough, since you can easily mangle the configuration in a symbol to cause an error if linking with the non-matching backend? Still talking about conversion between encodings, I would like it if it was possible to statically choose the encodings we're converting between, as some backends could directly expose a specialized and optimized some_encoding_to_some_other_encoding function. Note the remarks about conversion between encodings also applies to case folding, collation etc. Being able to specify statically the normalization form you're normalizing to and things like that could be useful just as for the character encodings. Perhaps not made clear from the docs, but Boost.Locale embeds another mechanism than the functions in the conv namespace to convert between character encodings: using the classes derived from boost::locale::util::base_converter. This is the only interface that can be passed incomplete data, as obtained through buffering. That interface was designed to implement codecvt facets, and unfortunately, it's not much better. The library implements its own UTF-8/UTF-16/UTF-32 conversion logic there regardless of which backend is used; it is a shame it cannot reuse the to_utf or from_utf functions in all cases. This implementation is also quite tricky since the state management prevents the code from being simple and self-contained. That interface does not allow optimized conversion functions to be used. Compatibility with codecvt facets is a nice feature, but it shouldn't impair the interface of the component that could have other usages. Furthermore, I do not find the boundary analysis interface satisfying at all. It is not made clear in the docs that doing boundary analysis actually goes through the whole string and returns a vector of positions corresponding to the boundaries. You can use that vector to adapt any range into a range of strings. Each dereferencing copies the subrange defined by the vector into a new temporary string. This is pretty terrible. I want to be able to find the boundaries as I advance through a string, or be able to find the closest boundary from an arbitrary position without having to process the whole string. I do not want to have to copy my whole input range into little strings for no reason. Usage of this feature is also unnecessarily verbose. It should not be more than a function call to turn a range of characters into a range of tagged ranges of characters. This part of the library is the one that needs the more work. Ironically enough, it's also the only one that deals a bit with iterators and ranges, and I just asked for more iterators and ranges at the beginning of this review ;). Finally, I will not comment on the implementation too much, since I think other Boosters already did that (though I did not read all of it). There is some unnecessary or non-optimal code, some code more complicated than it should be, some compile-time values tested at runtime, some things that could have reused existing utilities from Boost, and some inappropriate use of reinterpret_cast. While most of the backends are separated, it is not the case for encoding/codepage.cpp, which is shared by all backends and contains ifdefs. This is not a very nice extension point. I'm also a bit concerned about the generalized usage of virtual functions anywhere; it seems unnecessary in quite a few places, please prefer using templates when dynamic binding is not required. A lot of new and vectors too, I'd prefer if ideally the library never allocated anything. I hope my review will be well received and that you will consider all of my "feature requests". Your library is practical and can get things done for sure, but maybe it's not quite as good as it could be in terms of interface yet. Also, if I want Boost.Unicode to ever be a possible backend, Boost.Locale needs to be able to make the most of it ;).