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A collection of Unicode utilities for C++ using Range-V3

This header-only library contains facilities for transforming between UTF-8, UTF-16 and UTF-32 encoded strings (eagerly and lazily), as well as dealing with byte-order marks and transforming line endings.


A quick overview is best supplied by an example. The following reads a UTF-8 encoded input stream and outputs a UTF-16BE byte stream with byte-order mark:

    namespace rng = ::ranges::v3;
    namespace utf = ::tcb::utf_ranges;

    std::ifstream in_file{"input_file.utf8.txt", std::ios::binary};
    std::ofstream out_file{"output_file.utf16be.txt", std::ios::binary};

    auto view = utf::istreambuf(in_file) // Read range from input stream
            | utf::view::consume_bom     // Remove UTF-8 "BOM" if present
            | utf::view::utf16           // Convert to UTF-16
            | utf::view::add_bom         // Prepend UTF-16 BOM to start of range
            | utf::view::endian_convert<boost::endian::order::big> // Convert to big-endian
            | utf::view::bytes;          // Write to disk as bytes

    rng::copy(view, utf::ostreambuf_iterator<char>{out_file}); // Do the copy

(see example/utf8_to_utf16be.cpp for the full code).


For "eager" encoding conversions, the library broadly follows the API specified in Beman Dawes' proposed Unicode conversion library, albeit (currently) with simplified error handling (invalid Unicode characters are simply replaced by the Unicode replacement character U+FFFD). The actual conversion uses code taken from Boost.Locale.

To convert a range of characters between UTF-8, UTF-16 or UTF-32, use the tcb::utf_ranges::utf_convert() function. This takes an InputRange with a value type that is an arithmetic type of size 1, 2 or 4 bytes (for UTF-8, UTF-16 and UTF-32 respectively), and an OutputIterator with a value type similarly defined. For example:

std::string in = u8"Hello world";
std::u16string out;
// Note that the output type cannot be determined automatically, so must be specified
tcb::utf_ranges::utf_convert<char16_t>(in, std::back_inserter(out));

To tranform directly to a new string, the to_utf_string() function is supplied:

std::u16string in = u"Hello world";
std::string out = tcb::utf_ranges::to_utf_string<char>(in);

Convenience functions to_u8string(), to_u16string(), to_u32string() and to_wstring() are also provided (but please don't use the last one):

std::u32string in = U"Hello world";
std::u16string out = tcb::utf_ranges::to_u16string(in);


If you're familiar with Range-V3, you'll know that views perform lazy transformations on a given range -- that is, conversion is done one element at a time when the view is iterated over.

Encoding conversions

This library provides views which lazily perform the same transformations as above. For consistency with Range-V3, these are in the view:: sub-namespace.

std::u16string in = u"Hello world";

auto view = tcb::utf_ranges::view::utf8(in);

ranges::v3::copy(view, std::ostream_iterator<char>(std::cout));

There are similar utf16 and utf32 views

Endian transformations

For UTF-16 and UTF-32, the library provides views which perform byte-swapping between native-, big- and little-ending representations, using code from Boost. The output endianness is specifed by a template parameter, and the input endianness is passed as an argument to the constructor. Both default to boost::endian::native. For example:

std::u16string in = u"Hello world"; // native endian
auto view = tcb::utf_ranges::view::endian_convert<boost::endian::order::big>(in);
std::vector<std::int16_t> out = view; // Copy byte-swapped values to vector

Byte order mark handling

The library provides two views for dealing with "byte order marks", that is, the Unicode non-breaking space character U+FEFF which is often placed at the start of files to allow the endianness to be detected.

To detect a byte-order mark, using the consume_bom view:

std::u16string in = u"\uFEFFHello world"; // native-endian UTF-16 with BOM
auto view = tcb::utf_ranges::view::consume_bom(in);
std::u16string out = view; // copy to new string with BOM removed

As suggested by the name, the byte order mark is removed if present. If a BOM is found an has non-native endianness, endian conversion is automatically performed -- that is, the output of the view will always be native-endian. For UTF-8, if a BOM is detected it is simply removed. If no BOM is present, the string is assumed to be native-endian (for UTF-16 and -32), and is passed through unchanged.

To place a byte-order mark at the start of a string, use the add_bom view:

std::u16string in = u"Hello world";
auto view = tcb::utf_ranges::view::add_bom(in);
std::u16string out = view; // copy to new string, with BOM prepended

Line ending transformation

Unicode specifies eight possible line endings, and recommends that these are converted to the machine native line ending representation on input. In C++, the native representation is "\n". The line_end_transform view performs such a conversion. For example:

std::string in = u8"Hello world\r\n"; // Windows-style
std::string out = tcb::utf_ranges::view::line_end_transform(in);
assert(out == "Hello world\n");

Chaining views

As with Range-V3, operator| is overloaded for views, allowing them to be easily concatenated together, as in the example above.


This library is provided under the Boost licence. See LICENCE_1_0.txt for details.


A collection of Unicode utilities for C++ using Range-V3







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