Table of Contents

Introduction

Many C++ developers miss an easy and portable way of handling Unicode encoded strings. C++ Standard is currently Unicode agnostic, and while some work is being done to introduce Unicode to the next incarnation called C++0x, for the moment nothing of the sort is available. In the meantime, developers use 3rd party libraries like ICU, OS specific capabilities, or simply roll out their own solutions.

In order to easily handle UTF-8 encoded Unicode strings, I have come up with a set of template functions. For anybody used to work with STL algorithms, they should be easy and natural to use. The code is freely available for any purpose - check out the license at the beginning of the utf8.h file. If you run into bugs or performance issues, please let me know and I'll do my best to address them.

The purpose of this article is not to offer an introduction to Unicode in general, and UTF-8 in particular. If you are not familiar with Unicode, be sure to check out Unicode Home Page or some other source of information for Unicode. Also, it is not my aim to advocate the use of UTF-8 encoded strings in C++ programs; if you want to handle UTF-8 encoded strings from C++, I am sure you have good reasons for it.

Examples of use

To illustrate the use of Boost UTF8 library, we shall open a file containing UTF-8 encoded text, check whether it starts with a byte order mark, read each line into a std::string, check it for validity, convert the text to UTF-16, and back to UTF-8: 

#include <fstream>
#include <iostream>
#include <string>
#include <vector>

#include <boost/utf8.hpp>

using namespace std;
using namespace boost;

int main()
{
    if (argc != 2) {
        cout << "\nUsage: docsample filename\n";
        return 0;
    }
    const char* test_file_path = argv[1];
    // Open the test file (must be UTF-8 encoded)
    ifstream fs8(test_file_path);
    if (!fs8.is_open()) {
    cout << "Could not open " << test_file_path << endl;
    return 0;
    }
    // Read the first line of the file
    unsigned line_count = 1;
    string line;
    if (!getline(fs8, line)) 
        return 0;
    // Look for utf-8 byte-order mark at the beginning
    if (line.size() > 2) {
        if (utf8::is_bom(line.c_str()))
            cout << "There is a byte order mark at the beginning of the file\n";
    }
    // Play with all the lines in the file
    do {
       // check for invalid utf-8 (for a simple yes/no check, there is also utf8::is_valid function)
        string::iterator end_it = utf8::find_invalid(line.begin(), line.end());
        if (end_it != line.end()) {
            cout << "Invalid UTF-8 encoding detected at line " << line_count << "\n";
            cout << "This part is fine: " << string(line.begin(), end_it) << "\n";
        }
        // Get the line length (at least for the valid part)
        int length = utf8::distance(line.begin(), end_it);
        cout << "Length of line " << line_count << " is " << length <<  "\n";
        // Convert it to utf-16
        vector<unsigned short> utf16line;
        utf8::utf8to16(line.begin(), end_it, back_inserter(utf16line));
        // And back to utf-8
        string utf8line; 
        utf8::utf16to8(utf16line.begin(), utf16line.end(), back_inserter(utf8line));
        // Confirm that the conversion went OK:
        if (utf8line != string(line.begin(), end_it))
            cout << "Error in UTF-16 conversion at line: " << line_count << "\n";        
        getline(fs8, line);
        line_count++;
    } while (!fs8.eof());
    return 0;
}

In the previous code sample, we have seen the use of the following functions from utf8 namespace: first we used is_bom function to detect UTF-8 byte order mark at the beginning of the file; then for each line we performed a detection of invalid UTF-8 sequences with find_invalid; the number of characters (more precisely - the number of Unicode code points) in each line was determined with a use of utf8::distance; finally, we have converted each line to UTF-16 encoding with utf8to16 and back to UTF-8 with utf16to8.

Reference

Functions From utf8 Namespace

utf8::append

Encodes a 32 bit code point as a UTF-8 sequence of octets and appends the sequence to a UTF-8 string. 

template <typename octet_iterator>
octet_iterator append(uint32_t cp, octet_iterator result);

cp: A 32 bit integer representing a code point to append to the sequence.
result: An output iterator to the place in the sequence where to append the code point.
Return value: An iterator pointing to the place after the newly appended sequence.

Example of use: 

unsigned char u[5] = {0,0,0,0,0};
unsigned char* end = append(0x0448, u);
assert (u[0] == 0xd1 && u[1] == 0x88 && u[2] == 0 && u[3] == 0 && u[4] == 0);

Note that append does not allocate any memory - it is the burden of the caller to make sure there is enough memory allocated for the operation. To make things more interesting, append can add anywhere between 1 and 4 octets to the sequence. In practice, you would most often want to use std::back_inserter to ensure that the necessary memory is allocated.

In case of an invalid code point, a utf8::invalid_code_point exception is thrown.

utf8::next

Given the iterator to the beginning of the UTF-8 sequence, it returns the code point and moves the iterator to the next position. 

template <typename octet_iterator> 
uint32_t next(octet_iterator& it, octet_iterator end);

it: a reference to an iterator pointing to the beginning of an UTF-8 encoded code point. After the function returns, it is incremented to point to the beginning of the next code point.
end: end of the UTF-8 sequence to be processed. If it gets equal to end during the extraction of a code point, an utf8::not_enough_room exception is thrown.
Return value: the 32 bit representation of the processed UTF-8 code point.

Example of use: 

char* twochars = "\xe6\x97\xa5\xd1\x88";
char* w = twochars;
int cp = next(w, twochars + 6);
assert (cp == 0x65e5);
assert (w == twochars + 3);

This function is typically used to iterate through a UTF-8 encoded string.

In case of an invalid UTF-8 seqence, a utf8::invalid_utf8 exception is thrown.

utf8::prior

Given a reference to an iterator pointing to an octet in a UTF-8 seqence, it decreases the iterator until it hits the beginning of the previous UTF-8 encoded code point and returns the 32 bits representation of the code point. 

template <typename octet_iterator> 
uint32_t prior(octet_iterator& it, octet_iterator start);

it: a reference pointing to an octet within a UTF-8 encoded string. After the function returns, it is decremented to point to the beginning of the previous code point.
start: an iterator to the beginning of the sequence where the search for the beginning of a code point is performed. It is a safety measure to prevent passing the beginning of the string in the search for a UTF-8 lead octet.
Return value: the 32 bit representation of the previous code point.

Example of use: 

char* twochars = "\xe6\x97\xa5\xd1\x88";
unsigned char* w = twochars + 3;
int cp = prior (w, twochars);
assert (cp == 0x65e5);
assert (w == twochars);

This function has two purposes: one is two iterate backwards through a UTF-8 encoded string. Note that it is usually a better idea to iterate forward instead, since utf8::next is faster. The second purpose is to find a beginning of a UTF-8 sequence if we have a random position within a string.

it will typically point to the beginning of a code point, and start will point to the beginning of the string to ensure we don't go backwards too far. it is decreased until it points to a lead UTF-8 octet, and then the UTF-8 sequence beginning with that octet is decoded to a 32 bit representation and returned.

In case pass_end is reached before a UTF-8 lead octet is hit, or if an invalid UTF-8 sequence is started by the lead octet, an invalid_utf8 exception is thrown.

utf8::advance

Advances an iterator by the specified number of code points within an UTF-8 sequence. 

template <typename octet_iterator, typename distance_type> 
void advance (octet_iterator& it, distance_type n, octet_iterator end);

it: a reference to an iterator pointing to the beginning of an UTF-8 encoded code point. After the function returns, it is incremented to point to the nth following code point.
n: a positive integer that shows how many code points we want to advance.
end: end of the UTF-8 sequence to be processed. If it gets equal to end during the extraction of a code point, an utf8::not_enough_room exception is thrown.

Example of use: 

char* twochars = "\xe6\x97\xa5\xd1\x88";
unsigned char* w = twochars;
advance (w, 2, twochars + 6);
assert (w == twochars + 5);

This function works only "forward". In case of a negative n, there is no effect.

In case of an invalid code point, a utf8::invalid_code_point exception is thrown.

utf8::distance

Given the iterators to two UTF-8 encoded code points in a seqence, returns the number of code points between them. 

template <typename octet_iterator> 
typename std::iterator_traits<octet_iterator>::difference_type distance (octet_iterator first, octet_iterator last);

first: an iterator to a beginning of a UTF-8 encoded code point.
last: an iterator to a "post-end" of the last UTF-8 encoded code point in the sequence we are trying to determine the length. It can be the beginning of a new code point, or not.
Return value the distance between the iterators, in code points.

Example of use: 

char* twochars = "\xe6\x97\xa5\xd1\x88";
size_t dist = utf8::distance(twochars, twochars + 5);
assert (dist == 2);

This function is used to find the length (in code points) of a UTF-8 encoded string. The reason it is called distance, rather than, say, length is mainly because developers are used that length is an O(1) function. Computing the length of an UTF-8 string is a linear operation, and it looked better to model it after std::distance algorithm.

In case of an invalid UTF-8 seqence, a utf8::invalid_utf8 exception is thrown. If last does not point to the past-of-end of a UTF-8 seqence, a utf8::not_enough_room exception is thrown.

utf8::utf16to8

Converts a UTF-16 encoded string to UTF-8. 

template <typename u16bit_iterator, typename octet_iterator>
octet_iterator utf16to8 (u16bit_iterator start, u16bit_iterator end, octet_iterator result);

start: an iterator pointing to the beginning of the UTF-16 encoded string to convert.
end: an iterator pointing to pass-the-end of the UTF-16 encoded string to convert.
result: an output iterator to the place in the UTF-8 string where to append the result of conversion.
Return value: An iterator pointing to the place after the appended UTF-8 string.

Example of use: 

unsigned short utf16string[] = {0x41, 0x0448, 0x65e5, 0xd834, 0xdd1e};
vector<unsigned char> utf8result;
utf16to8(utf16string, utf16string + 5, back_inserter(utf8result));
assert (utf8result.size() == 10);

In case of invalid UTF-16 sequence, a utf8::invalid_utf16 exception is thrown.

utf8::utf8to16

Converts an UTF-8 encoded string to UTF-16 

template <typename u16bit_iterator, typename octet_iterator>
u16bit_iterator utf8to16 (octet_iterator start, octet_iterator end, u16bit_iterator result);

start: an iterator pointing to the beginning of the UTF-8 encoded string to convert. < br /> end: an iterator pointing to pass-the-end of the UTF-8 encoded string to convert.
result: an output iterator to the place in the UTF-16 string where to append the result of conversion.
Return value: An iterator pointing to the place after the appended UTF-16 string.

Example of use: 

char utf8_with_surrogates[] = "\xe6\x97\xa5\xd1\x88\xf0\x9d\x84\x9e";
vector <unsigned short> utf16result;
utf8to16(utf8_with_surrogates, utf8_with_surrogates + 9, back_inserter(utf16result));
assert (utf16result.size() == 4);
assert (utf16result[2] == 0xd834);
assert (utf16result[3] == 0xdd1e);

In case of an invalid UTF-8 seqence, a utf8::invalid_utf8 exception is thrown. If end does not point to the past-of-end of a UTF-8 seqence, a utf8::not_enough_room exception is thrown.

utf8::utf32to8

Converts a UTF-32 encoded string to UTF-8. 

template <typename octet_iterator, typename u32bit_iterator>
octet_iterator utf32to8 (u32bit_iterator start, u32bit_iterator end, octet_iterator result);

start: an iterator pointing to the beginning of the UTF-32 encoded string to convert.
end: an iterator pointing to pass-the-end of the UTF-32 encoded string to convert.
result: an output iterator to the place in the UTF-8 string where to append the result of conversion.
Return value: An iterator pointing to the place after the appended UTF-8 string.

Example of use: 

int utf32string[] = {0x448, 0x65E5, 0x10346, 0};
vector<unsigned char> utf8result;
utf32to8(utf32string, utf32string + 3, back_inserter(utf8result));
assert (utf8result.size() == 9);

In case of invalid UTF-32 string, a utf8::invalid_code_point exception is thrown.

utf8::utf8to32

Converts a UTF-8 encoded string to UTF-32. 

template <typename octet_iterator, typename u32bit_iterator>
u32bit_iterator utf8to32 (octet_iterator start, octet_iterator end, u32bit_iterator result);

start: an iterator pointing to the beginning of the UTF-8 encoded string to convert.
end: an iterator pointing to pass-the-end of the UTF-8 encoded string to convert.
result: an output iterator to the place in the UTF-32 string where to append the result of conversion.
Return value: An iterator pointing to the place after the appended UTF-32 string.

Example of use: 

char* twochars = "\xe6\x97\xa5\xd1\x88";
vector<int> utf32result;
utf8to32(twochars, twochars + 5, back_inserter(utf32result));
assert (utf32result.size() == 2);

In case of an invalid UTF-8 seqence, a utf8::invalid_utf8 exception is thrown. If end does not point to the past-of-end of a UTF-8 seqence, a utf8::not_enough_room exception is thrown.

utf8::find_invalid

Detects an invalid sequence within a UTF-8 string. 

template <typename octet_iterator> 
octet_iterator find_invalid(octet_iterator start, octet_iterator end);

start: an iterator pointing to the beginning of the UTF-8 string to test for validity.
end: an iterator pointing to pass-the-end of the UTF-8 string to test for validity.
Return value: an iterator pointing to the first invalid octet in the UTF-8 string. In case none were found, equals end.

Example of use: 

char utf_invalid[] = "\xe6\x97\xa5\xd1\x88\xfa";
char* invalid = find_invalid(utf_invalid, utf_invalid + 6);
assert (invalid == utf_invalid + 5);

This function is typically used to make sure a UTF-8 string is valid before processing it with other functions. It is especially important to call it if before doing any of the unchecked operations on it.

utf8::is_valid

Checks whether a sequence of octets is a valid UTF-8 string. 

template <typename octet_iterator> 
bool is_valid(octet_iterator start, octet_iterator end);

start: an iterator pointing to the beginning of the UTF-8 string to test for validity.
end: an iterator pointing to pass-the-end of the UTF-8 string to test for validity.
Return value: true if the sequence is a valid UTF-8 string; false if not.

Example of use: 
char utf_invalid[] = "\xe6\x97\xa5\xd1\x88\xfa";
bool bvalid = is_valid(utf_invalid, utf_invalid + 6);
assert (bvalid == false);

is_valid is a shorthand for find_invalid(start, end) == end;. You may want to use it to make sure that a byte seqence is a valid UTF-8 string without the need to know where it fails if it is not valid.

utf8::is_bom

Checks whether a sequence of three octets is a UTF-8 byte order mark (BOM) 

template <typename octet_iterator> 
bool is_bom (octet_iterator it);

it: beginning of the 3-octet sequence to check
Return value: true if the sequence is UTF-8 byte order mark; false if not.

Example of use: 

unsigned char byte_order_mark[] = {0xef, 0xbb, 0xbf};
bool bbom = is_bom(byte_order_mark);
assert (bbom == true);

The typical use of this function is to check the first three bytes of a file. If they form the UTF-8 BOM, we want to skip them before processing the actual UTF-8 encoded text.

Functions From utf8::unchecked Namespace

utf8::unchecked::append

Encodes a 32 bit code point as a UTF-8 sequence of octets and appends the sequence to a UTF-8 string. 

template <typename octet_iterator>
octet_iterator append(uint32_t cp, octet_iterator result);

cp: A 32 bit integer representing a code point to append to the sequence.
result: An output iterator to the place in the sequence where to append the code point.
Return value: An iterator pointing to the place after the newly appended sequence.

Example of use: 

unsigned char u[5] = {0,0,0,0,0};
unsigned char* end = unchecked::append(0x0448, u);
assert (u[0] == 0xd1 && u[1] == 0x88 && u[2] == 0 && u[3] == 0 && u[4] == 0);

This is a faster but less safe version of utf8::append. It does not check for validity of the supplied code point, and may produce an invalid UTF-8 sequence.

utf8::unchecked::next

Given the iterator to the beginning of a UTF-8 sequence, it returns the code point and moves the iterator to the next position. 

template <typename octet_iterator>
uint32_t next(octet_iterator& it);

it: a reference to an iterator pointing to the beginning of an UTF-8 encoded code point. After the function returns, it is incremented to point to the beginning of the next code point.
Return value: the 32 bit representation of the processed UTF-8 code point.

Example of use: 

char* twochars = "\xe6\x97\xa5\xd1\x88";
char* w = twochars;
int cp = unchecked::next(w);
assert (cp == 0x65e5);
assert (w == twochars + 3);

This is a faster but less safe version of utf8::next. It does not check for validity of the supplied UTF-8 sequence.

utf8::unchecked::prior

Given a reference to an iterator pointing to an octet in a UTF-8 seqence, it decreases the iterator until it hits the beginning of the previous UTF-8 encoded code point and returns the 32 bits representation of the code point. 

template <typename octet_iterator>
uint32_t prior(octet_iterator& it);

it: a reference pointing to an octet within a UTF-8 encoded string. After the function returns, it is decremented to point to the beginning of the previous code point.
Return value: the 32 bit representation of the previous code point.

Example of use: 

char* twochars = "\xe6\x97\xa5\xd1\x88";
char* w = twochars + 3;
int cp = unchecked::prior (w);
assert (cp == 0x65e5);
assert (w == twochars);

This is a faster but less safe version of utf8::prior. It does not check for validity of the supplied UTF-8 sequence and offers no boundary checking.

utf8::unchecked::advance

Advances an iterator by the specified number of code points within an UTF-8 sequence. 

template <typename octet_iterator, typename distance_type>
void advance (octet_iterator& it, distance_type n);

it: a reference to an iterator pointing to the beginning of an UTF-8 encoded code point. After the function returns, it is incremented to point to the nth following code point.
n: a positive integer that shows how many code points we want to advance.

Example of use: 

char* twochars = "\xe6\x97\xa5\xd1\x88";
char* w = twochars;
unchecked::advance (w, 2);
assert (w == twochars + 5);

This function works only "forward". In case of a negative n, there is no effect.

This is a faster but less safe version of utf8::advance. It does not check for validity of the supplied UTF-8 sequence and offers no boundary checking.

utf8::unchecked::distance

Given the iterators to two UTF-8 encoded code points in a seqence, returns the number of code points between them. 

template <typename octet_iterator>
typename std::iterator_traits<octet_iterator>::difference_type distance (octet_iterator first, octet_iterator last);

first: an iterator to a beginning of a UTF-8 encoded code point.
last: an iterator to a "post-end" of the last UTF-8 encoded code point in the sequence we are trying to determine the length. It can be the beginning of a new code point, or not.
Return value the distance between the iterators, in code points.

Example of use: 

char* twochars = "\xe6\x97\xa5\xd1\x88";
size_t dist = utf8::unchecked::distance(twochars, twochars + 5);
assert (dist == 2);

This is a faster but less safe version of utf8::distance. It does not check for validity of the supplied UTF-8 sequence.

utf8::unchecked::utf16to8

Converts a UTF-16 encoded string to UTF-8. 

template <typename u16bit_iterator, typename octet_iterator>
octet_iterator utf16to8 (u16bit_iterator start, u16bit_iterator end, octet_iterator result);

start: an iterator pointing to the beginning of the UTF-16 encoded string to convert.
end: an iterator pointing to pass-the-end of the UTF-16 encoded string to convert.
result: an output iterator to the place in the UTF-8 string where to append the result of conversion.
Return value: An iterator pointing to the place after the appended UTF-8 string.

Example of use: 

unsigned short utf16string[] = {0x41, 0x0448, 0x65e5, 0xd834, 0xdd1e};
vector<unsigned char> utf8result;
unchecked::utf16to8(utf16string, utf16string + 5, back_inserter(utf8result));
assert (utf8result.size() == 10);

This is a faster but less safe version of utf8::utf16to8. It does not check for validity of the supplied UTF-16 sequence.

utf8::unchecked::utf8to16

Converts an UTF-8 encoded string to UTF-16 

template <typename u16bit_iterator, typename octet_iterator>
u16bit_iterator utf8to16 (octet_iterator start, octet_iterator end, u16bit_iterator result);

start: an iterator pointing to the beginning of the UTF-8 encoded string to convert. < br /> end: an iterator pointing to pass-the-end of the UTF-8 encoded string to convert.
result: an output iterator to the place in the UTF-16 string where to append the result of conversion.
Return value: An iterator pointing to the place after the appended UTF-16 string.

Example of use: 

char utf8_with_surrogates[] = "\xe6\x97\xa5\xd1\x88\xf0\x9d\x84\x9e";
vector <unsigned short> utf16result;
unchecked::utf8to16(utf8_with_surrogates, utf8_with_surrogates + 9, back_inserter(utf16result));
assert (utf16result.size() == 4);
assert (utf16result[2] == 0xd834);
assert (utf16result[3] == 0xdd1e);

This is a faster but less safe version of utf8::utf8to16. It does not check for validity of the supplied UTF-8 sequence.

utf8::unchecked::utf32to8

Converts a UTF-32 encoded string to UTF-8. 

template <typename octet_iterator, typename u32bit_iterator>
octet_iterator utf32to8 (u32bit_iterator start, u32bit_iterator end, octet_iterator result);

start: an iterator pointing to the beginning of the UTF-32 encoded string to convert.
end: an iterator pointing to pass-the-end of the UTF-32 encoded string to convert.
result: an output iterator to the place in the UTF-8 string where to append the result of conversion.
Return value: An iterator pointing to the place after the appended UTF-8 string.

Example of use: 

int utf32string[] = {0x448, 0x65e5, 0x10346, 0};
vector<unsigned char> utf8result;
utf32to8(utf32string, utf32string + 3, back_inserter(utf8result));
assert (utf8result.size() == 9);

This is a faster but less safe version of utf8::utf32to8. It does not check for validity of the supplied UTF-32 sequence.

utf8::unchecked::utf8to32

Converts a UTF-8 encoded string to UTF-32. 

template <typename octet_iterator, typename u32bit_iterator>
u32bit_iterator utf8to32 (octet_iterator start, octet_iterator end, u32bit_iterator result);

start: an iterator pointing to the beginning of the UTF-8 encoded string to convert.
end: an iterator pointing to pass-the-end of the UTF-8 encoded string to convert.
result: an output iterator to the place in the UTF-32 string where to append the result of conversion.
Return value: An iterator pointing to the place after the appended UTF-32 string.

Example of use: 

char* twochars = "\xe6\x97\xa5\xd1\x88";
vector<int> utf32result;
unchecked::utf8to32(twochars, twochars + 5, back_inserter(utf32result));
assert (utf32result.size() == 2);

This is a faster but less safe version of utf8::utf8to32. It does not check for validity of the supplied UTF-8 sequence.

Points of interest

Design goals and decisions

The library was designed to be:

  1. Generic: for better or worse, there are many C++ string classes out there, and the library should work with as many of them as possible.
  2. Portable: the library should be portable both accross different platforms and compilers. The only non-portable code is a small section that declares unsigned integers of different sizes: three typedefs. They can be changed by the users of the library if they don't match their platform. The default setting should work for Windows (both 32 and 64 bit), and most 32 bit and 64 bit Unix derivatives.
  3. Lightweight: follow the "pay only for what you use" guidline.
  4. Unintrusive: avoid forcing any particular design or even programming style on the user. This is a library, not a framework.

Alternatives

In case you want to look into other means of working with UTF-8 strings from C++, here is the list of solutions I am aware of:

  1. ICU Library. It is very powerful, complete, feature-rich, mature, and widely used. Also big, intrusive, non-generic, and doesn't play well with the Standard Library. I definitelly recommend looking at ICU even if you don't plan to use it.
  2. Glib::ustring. A class specifically made to work with UTF-8 strings, and also feel like std::string. If you prefer to have yet another string class in your code, it may be worth a look. Be aware of the licensing issues, though.
  3. Platform dependent solutions: Windows and POSIX have functions to convert strings from one encoding to another. That is only a subset of what my library offers, but if that is all you need it may be good enough, especially given the fact that these functions are mature and tested in production.

Conclusion

Until Unicode becomes officially recognized by the C++ Standard Library, we need to use other means to work with UTF-8 strings. Template functions I describe in this article may be a good step in this direction.

References

  1. The Unicode Consortium.
  2. ICU Library.
  3. UTF-8 at Wikipedia
  4. UTF-8 and Unicode FAQ for Unix/Linux