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UTF-8 With C++ in a Portable Way

, 8 Jul 2009
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A simple, portable, and lightweight generic library for handling UTF-8 encoded strings.

Table of Contents

Introduction

Many C++ developers miss an easy and portable way of handling Unicode encoded strings. The original C++ Standard (known as C++98 or C++03) is 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 third 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 came up with a small generic library. For anybody used to working with STL algorithms and iterators, it 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 the 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

Introductory sample

To illustrate the use of the library, let's start with a small but complete program that opens a file containing UTF-8 encoded text, reads it line by line, checks each line for invalid UTF-8 byte sequences, and converts it to UTF-16 encoding and back to UTF-8:

#include <fstream>
#include <iostream>
#include <string>
#include <vector>
#include "utf8.h"
using namespace std;
int main(int argc, char** argv)
{
    if (argc != 2) {
        cout << "\nUsage: docsample filename\n";
        return 0;
    }

    const char* test_file_path = argv[1];
    // Open the test file (contains UTF-8 encoded text)
    ifstream fs8(test_file_path);
    if (!fs8.is_open()) {
    cout << "Could not open " << test_file_path << endl;
    return 0;
    }

    unsigned line_count = 1;
    string line;
    // Play with all the lines in the file
    while (getline(fs8, line)) {
        // 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";        

        line_count++;
    }
    return 0;
}

In the previous code sample, 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, including the end of line and even BOM if there is one) in each line was determined with the use of utf8::distance; finally, we have converted each line to UTF-16 encoding with utf8to16 and back to UTF-8 with utf16to8.

Checking if a file contains valid UTF-8 text

Here is a function that checks whether the content of a file is valid UTF-8 encoded text without reading the content into the memory:

bool valid_utf8_file(iconst char* file_name)
{
    ifstream ifs(file_name);
    if (!ifs)
        return false; // even better, throw here

    istreambuf_iterator<char> it(ifs.rdbuf());
    istreambuf_iterator<char> eos;

    return utf8::is_valid(it, eos);
}

Because the function utf8::is_valid() works with input iterators, we were able to pass an istreambuf_iterator to it and read the contents of the file directly without loading it to the memory first.

Note that other functions that take input iterator arguments can be used in a similar way. For instance, to read the contents of a UTF-8 encoded text file and convert the text to UTF-16, just do something like:

utf8::utf8to16(it, eos, back_inserter(u16string));

Ensure that a string contains valid UTF-8 text

If we have some text that "probably" contains UTF-8 encoded text and we want to replace any invalid UTF-8 sequence with a replacement character, something like the following function may be used:

void fix_utf8_string(std::string& str)
{
    std::string temp;
    utf8::replace_invalid(str.begin(), str.end(), back_inserter(temp));
    str = temp;
}

The function will replace any invalid UTF-8 sequence with a Unicode replacement character. There is an overloaded function that enables the caller to supply their own replacement character.

Reference

Functions from the utf8 namespace

utf8::append

Available in version 1.0 and later.

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

Available in version 1.0 and later.

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, a 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 sequence, a utf8::invalid_utf8 exception is thrown.

utf8::peek_next

Available in version 2.1 and later.

Given the iterator to the beginning of the UTF-8 sequence, it returns the code point for the following sequence without changing the value of the iterator.

template <typename octet_iterator> 
uint32_t peek_next(octet_iterator it, octet_iterator end);
  • it: an iterator pointing to the beginning of a UTF-8 encoded 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, a 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 = peek_next(w, twochars + 6);
assert (cp == 0x65e5);
assert (w == twochars);

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

utf8::prior

Available in version 1.02 and later.

Given a reference to an iterator pointing to an octet in a UTF-8 sequence, it decreases the iterator until it hits the beginning of the previous UTF-8 encoded code point and returns the 32 bit 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 to 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 the 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::previous

Deprecated in version 1.02 and later.

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

template <typename octet_iterator> 
uint32_t previous(octet_iterator& it, octet_iterator pass_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.
  • pass_start: an iterator to the point in the sequence where the search for the beginning of a code point is aborted if no result was reached. 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 = previous (w, twochars - 1);
assert (cp == 0x65e5);
assert (w == twochars);

utf8::previous is deprecated, and utf8::prior should be used instead, although existing code can continue using this function. The problem is the parameter pass_start that points to the position just before the beginning of the sequence. Standard containers don't have the concept of "pass start" and the function can not be used with their iterators.

it will typically point to the beginning of a code point, and pass_start will point to the octet just before 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

Available in version 1.0 and later.

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 a 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, a 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

Available in version 1.0 and later.

Given the iterators to two UTF-8 encoded code points in a sequence, 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 the 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 to length as an O(1) function. Computing the length of a UTF-8 string is a linear operation, and it looked better to model it after the std::distance algorithm.

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

utf8::utf16to8

Available in version 1.0 and later.

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 past-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 an invalid UTF-16 sequence, a utf8::invalid_utf16 exception is thrown.

utf8::utf8to16

Available in version 1.0 and later.

Converts a 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.
  • end: an iterator pointing to past-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 sequence, a utf8::invalid_utf8 exception is thrown. If end does not point to the past-of-end of a UTF-8 sequence, a utf8::not_enough_room exception is thrown.

utf8::utf32to8

Available in version 1.0 and later.

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 past-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 an invalid UTF-32 string, a utf8::invalid_code_point exception is thrown.

utf8::utf8to32

Available in version 1.0 and later.

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 past-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 sequence, a utf8::invalid_utf8 exception is thrown. If end does not point to the past-of-end of a UTF-8 sequence, a utf8::not_enough_room exception is thrown.

utf8::find_invalid

Available in version 1.0 and later.

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 past-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 before doing any of the unchecked operations on it.

utf8::is_valid

Available in version 1.0 and later.

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 past-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 sequence is a valid UTF-8 string without the need to know where it fails if it is not valid.

utf8::replace_invalid

Available in version 2.0 and later.

Replaces all invalid UTF-8 sequences within a string with a replacement marker.

template <typename octet_iterator, typename output_iterator>
output_iterator replace_invalid(octet_iterator start, octet_iterator end, 
                output_iterator out, uint32_t replacement);
template <typename octet_iterator, typename output_iterator>
output_iterator replace_invalid(octet_iterator start, 
                octet_iterator end, output_iterator out);
  • start: an iterator pointing to the beginning of the UTF-8 string to look for invalid UTF-8 sequences.
  • end: an iterator pointing to past-the-end of the UTF-8 string to look for invalid UTF-8 sequences.
  • out: an output iterator to the range where the result of replacement is stored.
  • replacement: a Unicode code point for the replacement marker. The version without this parameter assumes the value 0xfffd.
  • Return value: an iterator pointing to the place after the UTF-8 string with replaced invalid sequences.

Example of use:

char invalid_sequence[] = "a\x80\xe0\xa0\xc0\xaf\xed\xa0\x80z";
vector<char> replace_invalid_result;
replace_invalid (invalid_sequence, invalid_sequence + 
        sizeof(invalid_sequence), back_inserter(replace_invalid_result), '?');
bvalid = is_valid(replace_invalid_result.begin(), 
                  replace_invalid_result.end());
assert (bvalid);
char* fixed_invalid_sequence = "a????z";
assert (std::equal(replace_invalid_result.begin(), 
        replace_invalid_result.end(), fixed_invalid_sequence));

replace_invalid does not perform in-place replacement of invalid sequences. Rather, it produces a copy of the original string with the invalid sequences replaced with a replacement marker. Therefore, out must not be in the [start, end] range.

If end does not point to the past-of-end of a UTF-8 sequence, a utf8::not_enough_room exception is thrown.

utf8::is_bom

Available in version 1.0 and later.

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.

Types from the utf8 namespace

utf8::iterator

Available in version 2.0 and later.

Adapts the underlying octet iterator to iterate over the sequence of code points, rather than raw octets.

template <typename octet_iterator>
class iterator;
Member functions
  • iterator();

    The deafult constructor; the underlying octet_iterator is constructed with its default constructor.

  • explicit iterator (const octet_iterator& octet_it, const octet_iterator& range_start, const octet_iterator& range_end);

    A constructor that initializes the underlying octet_iterator with octet_it and sets the range in which the iterator is considered valid.

  • octet_iterator base () const;

    Returns the underlying octet_iterator.

  • uint32_t operator * () const;

    Decodes the UTF-8 sequence the underlying octet_iterator is pointing to and returns the code point.

  • bool operator == (const iterator& rhs) const;

    Returns true if the two underlying iterators are equal.

  • bool operator != (const iterator& rhs) const;

    Returns true if the two underlying iterators are not equal.

  • iterator& operator ++ ();

    The prefix increment - moves the iterator to the next UTF-8 encoded code point.

  • iterator operator ++ (int);

    The postfix increment - moves the iterator to the next UTF-8 encoded code point and returns the current one.

  • iterator& operator -- ();

    The prefix decrement - moves the iterator to the previous UTF-8 encoded code point.

  • iterator operator -- (int);

    The postfix decrement - moves the iterator to the previous UTF-8 encoded code point and returns the current one.

Example of use:

char* threechars = "\xf0\x90\x8d\x86\xe6\x97\xa5\xd1\x88";
utf8::iterator<char*> it(threechars, threechars, threechars + 9);
utf8::iterator<char*> it2 = it;
assert (it2 == it);
assert (*it == 0x10346);
assert (*(++it) == 0x65e5);
assert ((*it++) == 0x65e5);
assert (*it == 0x0448);
assert (it != it2);
utf8::iterator<char*> endit (threechars + 9, 
                threechars, threechars + 9);
assert (++it == endit);
assert (*(--it) == 0x0448);
assert ((*it--) == 0x0448);
assert (*it == 0x65e5);
assert (--it == utf8::iterator<char*>(threechars, 
                    threechars, threechars + 9));
assert (*it == 0x10346);

The purpose of the utf8::iterator adapter is to enable easy iteration as well as the use of STL algorithms with UTF-8 encoded strings. Increment and decrement operators are implemented in terms of the utf8::next() and utf8::prior() functions.

Note that the utf8::iterator adapter is a checked iterator. It operates on the range specified in the constructor; any attempt to go out of that range will result in an exception. Even the comparison operators require both iterator objects to be constructed against the same range; otherwise, an exception is thrown. Typically, the range will be determined by the sequence container functions begin and end, i.e.:

std::string s = "example";
utf8::iterator i (s.begin(), s.begin(), s.end());

Functions from the utf8::unchecked namespace

utf8::unchecked::append

Available in version 1.0 and later.

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

Available in version 1.0 and later.

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 a 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::peek_next

Available in version 2.1 and later.

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

template <typename octet_iterator>
uint32_t peek_next(octet_iterator it);
  • it: an iterator pointing to the beginning of a UTF-8 encoded 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::peek_next(w);
assert (cp == 0x65e5);
assert (w == twochars);

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

utf8::unchecked::prior

Available in version 1.02 and later.

Given a reference to an iterator pointing to an octet in a UTF-8 sequence, 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::previous (deprecated, see utf8::unchecked::prior)

Deprecated in version 1.02 and later.

Given a reference to an iterator pointing to an octet in a UTF-8 sequence, 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 previous(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::previous (w);
assert (cp == 0x65e5);
assert (w == twochars);

The reason this function is deprecated is just the consistency with the "checked" versions, where prior should be used instead of previous. In fact, unchecked::previous behaves exactly the same as unchecked::prior.

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

utf8::unchecked::advance

Available in version 1.0 and later.

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 a 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

Available in version 1.0 and later.

Given the iterators to two UTF-8 encoded code points in a sequence, 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 the beginning of a UTF-8 encoded code point.
  • last: an iterator to the "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

Available in version 1.0 and later.

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 past-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

Available in version 1.0 and later.

Converts a 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.
  • end: an iterator pointing to past-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

Available in version 1.0 and later.

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 past-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

Available in version 1.0 and later.

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 past-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.

Types from the utf8::unchecked namespace

utf8::iterator

Available in version 2.0 and later.

Adapts the underlying octet iterator to iterate over the sequence of code points, rather than raw octets.

template <typename octet_iterator>
class iterator;
Member functions
  • iterator();

    The default constructor; the underlying octet_iterator is constructed with its default constructor.

  • explicit iterator (const octet_iterator& octet_it);

    A constructor that initializes the underlying octet_iterator with octet_it.

  • octet_iterator base () const;

    Returns the underlying octet_iterator.

  • uint32_t operator * () const;

    Decodes the UTF-8 sequence the underlying octet_iterator is pointing to, and returns the code point.

  • bool operator == (const iterator& rhs) const;

    Returns true if the two underlying iterators are equal.

  • bool operator != (const iterator& rhs) const;

    Returns true if the two underlying iterators are not equal.

  • iterator& operator ++ ();

    The prefix increment - moves the iterator to the next UTF-8 encoded code point.

  • iterator operator ++ (int);

    The postfix increment - moves the iterator to the next UTF-8 encoded code point and returns the current one.

  • iterator& operator -- ();

    The prefix decrement - moves the iterator to the previous UTF-8 encoded code point.

  • iterator operator -- (int);

    The postfix decrement - moves the iterator to the previous UTF-8 encoded code point and returns the current one.

Example of use:

char* threechars = "\xf0\x90\x8d\x86\xe6\x97\xa5\xd1\x88";
utf8::unchecked::iterator<char*> un_it(threechars);
utf8::unchecked::iterator<char*> un_it2 = un_it;
assert (un_it2 == un_it);
assert (*un_it == 0x10346);
assert (*(++un_it) == 0x65e5);
assert ((*un_it++) == 0x65e5);
assert (*un_it == 0x0448);
assert (un_it != un_it2);
utf8::::unchecked::iterator<char*> un_endit (threechars + 9);  
assert (++un_it == un_endit);
assert (*(--un_it) == 0x0448);
assert ((*un_it--) == 0x0448);
assert (*un_it == 0x65e5);
assert (--un_it == utf8::unchecked::iterator<char*>(threechars));
assert (*un_it == 0x10346);

This is an unchecked version of utf8::iterator. It is faster in many cases, but offers no validity or range checks.

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 across 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" guideline.
  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 definitely 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 feels 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.

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

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

Nemanja Trifunovic
Software Developer (Senior) SAP
United States United States
Born in Kragujevac, Serbia. Now lives in Boston area with his wife and daughters.
 
Wrote his first program at the age of 13 on a Sinclair Spectrum, became a professional software developer after he graduated.
 
Very passionate about programming and software development in general.

Comments and Discussions

 
Generalconvert_from_utf32 PinmvpN a v a n e e t h27-Oct-09 1:12 
GeneralRe: convert_from_utf32 PinmemberNemanja Trifunovic27-Oct-09 3:43 
GeneralRe: convert_from_utf32 PinmvpN a v a n e e t h27-Oct-09 7:59 
Questiononly convert between UTFs? Pinmemberedger13-Jul-09 20:51 
AnswerRe: only convert between UTFs? Pinmembermokanel13-Jul-09 22:32 
AnswerRe: only convert between UTFs? PinmemberNemanja Trifunovic14-Jul-09 0:55 
GeneralVersion 2.2 released. PinmemberNemanja Trifunovic7-Jul-09 3:59 
GeneralRe: Version 2.2 released. PinmvpN a v a n e e t h8-Jul-09 17:33 
GeneralRe: Version 2.2 released. PinmemberNemanja Trifunovic9-Jul-09 6:15 
QuestionWarning in Visual C++ Pinmemberkoloko4-May-09 6:44 
AnswerRe: Warning in Visual C++ PinmemberNemanja Trifunovic4-May-09 7:15 
GeneralRe: Warning in Visual C++ Pinmemberkoloko4-May-09 17:13 
Generalutf16to8 and next() seem not to play well together (VS2005) PinmemberBill Davy5-Jan-09 22:17 
GeneralRe: utf16to8 and next() seem not to play well together (VS2005) PinmemberNemanja Trifunovic6-Jan-09 3:55 
QuestionCan I use this with VC6? PinmemberNeville Franks4-Mar-08 19:23 
GeneralRe: Can I use this with VC6? PinmemberNemanja Trifunovic5-Mar-08 3:22 
GeneralRe: Can I use this with VC6? PinmemberNeville Franks5-Mar-08 12:42 
GeneralComparing characters/strings... Pinmembermilaks18-Jan-07 16:24 
AnswerRe: Comparing characters/strings... PinmemberNemanja Trifunovic19-Jan-07 2:04 
GeneralRe: Comparing characters/strings... Pinmembermilaks19-Jan-07 4:51 
GeneralRe: Comparing characters/strings... PinmemberNemanja Trifunovic19-Jan-07 10:05 
GeneralRe: Comparing characters/strings... Pinmembermilaks19-Jan-07 12:08 
GeneralRe: Comparing characters/strings... PinmemberNemanja Trifunovic19-Jan-07 13:03 
GeneralRe: Comparing characters/strings... Pinmembermilaks19-Mar-07 14:35 
GeneralRe: Comparing characters/strings... PinmemberNemanja Trifunovic20-Mar-07 2:24 
GeneralRe: Comparing characters/strings... Pinmembermilaks20-Mar-07 6:13 
GeneralRe: Comparing characters/strings... PinmemberNemanja Trifunovic20-Mar-07 6:54 
GeneralRe: Comparing characters/strings... Pinmembermilaks20-Mar-07 18:18 
GeneralFYI PinmemberSceptic Mole14-Oct-06 8:26 
GeneralRe: FYI PinmemberNemanja Trifunovic14-Oct-06 12:35 
NewsVersion 1.0 Final released PinmemberNemanja Trifunovic9-Sep-06 11:53 
GeneralRe: Version 1.0 Final released Pinmembereoo23-Oct-06 21:26 
GeneralRe: Version 1.0 Final released PinmemberNemanja Trifunovic24-Oct-06 2:08 
GeneralVersion 1.01 released PinmemberNemanja Trifunovic24-Oct-06 7:46 
GeneralRe: Version 1.01 released Pinmembereoo24-Oct-06 19:33 
QuestionWhat's new ... PinmemberAnonymuos9-Sep-06 0:16 
AnswerRe: What's new ... PinmemberNemanja Trifunovic9-Sep-06 2:34 
QuestionBOM Pinmemberpink_fish1-Aug-06 8:17 
AnswerRe: BOM [modified] PinmemberNemanja Trifunovic1-Aug-06 8:29 
GeneralRe: BOM Pinmemberpink_fish1-Aug-06 13:13 
NewsSourceforge project PinmemberNemanja Trifunovic16-Jul-06 4:34 
NewsRe: Sourceforge project [modified] PinmemberNemanja Trifunovic17-Jul-06 2:38 
NewsBeta 2 is out PinmemberNemanja Trifunovic29-Jul-06 4:02 
GeneralAnother approach [modified] Pinmemberj337y d0nut5-Jul-06 8:39 
GeneralRe: Another approach PinmemberNemanja Trifunovic5-Jul-06 9:28 
GeneralRe: Another approach PinmemberNemanja Trifunovic5-Jul-06 9:58 
QuestionGood idea PinmemberRoland Pibinger2-Jul-06 0:40 
AnswerRe: Good idea PinmemberNemanja Trifunovic2-Jul-06 1:41 
GeneralRe: Good idea PinmemberRoland Pibinger2-Jul-06 6:37 
GeneralRe: Good idea [modified] PinmemberNemanja Trifunovic2-Jul-06 9:11 

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