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 work 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 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.
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 a use of utf8::distance; finally, we have converted
each line to UTF-16 encoding with utf8to16 and back to UTF-8 with
utf16to8.
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 content 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 content 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));
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.
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);
octet_iterator: an output iterator.
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.
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);
octet_iterator: an input iterator.
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.
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);
octet_iterator: an input iterator.
it: an iterator pointing to the beginning of an 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, 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 = peek_next(w, twochars + 6); assert (cp == 0x65e5); assert (w == twochars);
In case of an invalid UTF-8 seqence, a utf8::invalid_utf8 exception is
thrown.
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, octet_iterator start);
octet_iterator: a bidirectional iterator.
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.
Deprecated in version 1.02 and later.
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 previous(octet_iterator& it, octet_iterator pass_start);
octet_iterator: a random access iterator.
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 the 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
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);
octet_iterator: an input iterator.
distance_type: an integral type convertible to octet_iterator's difference type.
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.
Available in version 1.0 and later.
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);
octet_iterator: an input iterator.
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.
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);
u16bit_iterator: an input iterator.
octet_iterator: an output iterator.
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.
Available in version 1.0 and later.
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);
octet_iterator: an input iterator.
u16bit_iterator: an output iterator.
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.
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);
octet_iterator: an output iterator.
u32bit_iterator: an input iterator.
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.
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);
octet_iterator: an input iterator.
u32bit_iterator: an output iterator.
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.
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);
octet_iterator: an input iterator.
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.
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);
octet_iterator: an input iterator.
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.
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.
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);
octet_iterator: an input iterator.
output_iterator: an output iterator.
start: an iterator pointing to the beginning of the UTF-8 string to
look for invalid UTF-8 sequences.
end: an iterator pointing to pass-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.
Available in version 2.3 and later. Relaces deprecated is_bom() function.
Checks whether an octet sequence starts with a UTF-8 byte order mark (BOM)
template <typename octet_iterator> bool starts_with_bom (octet_iterator it, octet_iterator end);
octet_iterator: an input iterator.
it: beginning of the octet sequence to check
end: pass-end of the sequence to check
Return value: true if the sequence
starts with a UTF-8 byte order mark; false if not.
Example of use:
unsigned char byte_order_mark[] = {0xef, 0xbb, 0xbf}; bool bbom = starts_with_bom(byte_order_mark, byte_order_mark + sizeof(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.
Available in version 1.0 and later. Deprecated in version 2.3. starts_with_bom() should be used
instead.
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); // Deprecated
octet_iterator: an input iterator.
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.
If a sequence is
shorter than three bytes, an invalid iterator will be dereferenced. Therefore, this function is deprecated
in favor of starts_with_bom()that takes the end of sequence as an argument.
Available in version 2.3 and later.
Base class for the exceptions thrown by UTF CPP library functions.
class exception : public std::exception {};
Example of use:
try { code_that_uses_utf_cpp_library(); } catch(const utf8::exception& utfcpp_ex) { cerr << utfcpp_ex.what(); }
Available in version 1.0 and later.
Thrown by UTF8 CPP functions such as advance and next if an UTF-8 sequence represents and invalid code point.
class invalid_code_point : public exception { public: uint32_t code_point() const; };
Member function code_point() can be used to determine the invalid code point that
caused the exception to be thrown.
Available in version 1.0 and later.
Thrown by UTF8 CPP functions such as next and prior if an invalid UTF-8 sequence
is detected during decoding.
class invalid_utf8 : public exception { public: uint8_t utf8_octet() const; };
Member function utf8_octet() can be used to determine the beginning of the byte
sequence that caused the exception to be thrown.
Available in version 1.0 and later.
Thrown by UTF8 CPP function utf16to8 if an invalid UTF-16 sequence
is detected during decoding.
class invalid_utf16 : public exception { public: uint16_t utf16_word() const; };
Member function utf16_word() can be used to determine the UTF-16 code unit
that caused the exception to be thrown.
Available in version 1.0 and later.
Thrown by UTF8 CPP functions such as next if the end of the decoded UTF-8 sequence
was reached before the code point was decoded.
class not_enough_room : public exception {};
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;
iterator(); 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); octet_iterator with octet_it
and sets the range in which the iterator is considered valid.
octet_iterator base () const; octet_iterator.
uint32_t operator * () const; octet_iterator is pointing to and returns the code point.
bool operator == (const iterator& rhs)
const; bool operator != (const iterator& rhs)
const; iterator& operator ++ (); iterator operator ++ (int); iterator& operator -- (); iterator operator -- (int); 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 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
utf8::next() and utf8::prior() functions.
Note that 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 object to be constructed against the same range - otherwise an exception is thrown. Typically,
the range will be determined by sequence container functions begin and end, i.e.:
std::string s = "example";
utf8::iterator i (s.begin(), s.begin(), s.end());
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.
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 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.
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 an 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.
Available in version 1.02 and later.
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.
Deprecated in version 1.02 and later.
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 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.
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 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.
Available in version 1.0 and later.
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.
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 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.
Available in version 1.0 and later.
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.
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 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.
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 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.
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;
iterator(); octet_iterator is
constructed with its default constructor.
explicit iterator (const octet_iterator& octet_it);
octet_iterator with octet_it
octet_iterator base () const; octet_iterator.
uint32_t operator * () const; octet_iterator is pointing to and returns the code point.
bool operator == (const iterator& rhs)
const; bool operator != (const iterator& rhs)
const; iterator& operator ++ (); iterator operator ++ (int); iterator& operator -- (); iterator operator -- (int); 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.
The library was designed to be:
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:
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.
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.