1# UTF8-CPP: UTF-8 with C++ in a Portable Way 2 3 4## Introduction 5 6Many 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. C++11 provides some support for Unicode on core language and library level: u8, u, and U character and string literals, char16_t and char32_t character types, u16string and u32string library classes, and codecvt support for conversions between Unicode encoding forms. In the meantime, developers use third party libraries like ICU, OS specific capabilities, or simply roll out their own solutions. 7 8In order to easily handle UTF-8 encoded Unicode strings, I came up with a small, C++98 compatible 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. The library has been used a lot in the past ten years both in commercial and open-source projects and is considered feature-complete now. If you run into bugs or performance issues, please let me know and I'll do my best to address them. 9 10The 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](http://www.unicode.org/) 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. 11 12## Examples of use 13 14### Introductionary Sample 15 16To 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: 17 18```cpp 19#include <fstream> 20#include <iostream> 21#include <string> 22#include <vector> 23#include "utf8.h" 24using namespace std; 25int main(int argc, char** argv) 26{ 27 if (argc != 2) { 28 cout << "\nUsage: docsample filename\n"; 29 return 0; 30 } 31 32 const char* test_file_path = argv[1]; 33 // Open the test file (contains UTF-8 encoded text) 34 ifstream fs8(test_file_path); 35 if (!fs8.is_open()) { 36 cout << "Could not open " << test_file_path << endl; 37 return 0; 38 } 39 40 unsigned line_count = 1; 41 string line; 42 // Play with all the lines in the file 43 while (getline(fs8, line)) { 44 // check for invalid utf-8 (for a simple yes/no check, there is also utf8::is_valid function) 45 string::iterator end_it = utf8::find_invalid(line.begin(), line.end()); 46 if (end_it != line.end()) { 47 cout << "Invalid UTF-8 encoding detected at line " << line_count << "\n"; 48 cout << "This part is fine: " << string(line.begin(), end_it) << "\n"; 49 } 50 51 // Get the line length (at least for the valid part) 52 int length = utf8::distance(line.begin(), end_it); 53 cout << "Length of line " << line_count << " is " << length << "\n"; 54 55 // Convert it to utf-16 56 vector<unsigned short> utf16line; 57 utf8::utf8to16(line.begin(), end_it, back_inserter(utf16line)); 58 59 // And back to utf-8 60 string utf8line; 61 utf8::utf16to8(utf16line.begin(), utf16line.end(), back_inserter(utf8line)); 62 63 // Confirm that the conversion went OK: 64 if (utf8line != string(line.begin(), end_it)) 65 cout << "Error in UTF-16 conversion at line: " << line_count << "\n"; 66 67 line_count++; 68 } 69 return 0; 70} 71``` 72 73In 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`. 74 75### Checking if a file contains valid UTF-8 text 76 77Here is a function that checks whether the content of a file is valid UTF-8 encoded text without reading the content into the memory: 78 79```cpp 80bool valid_utf8_file(const char* file_name) 81{ 82 ifstream ifs(file_name); 83 if (!ifs) 84 return false; // even better, throw here 85 86 istreambuf_iterator<char> it(ifs.rdbuf()); 87 istreambuf_iterator<char> eos; 88 89 return utf8::is_valid(it, eos); 90} 91``` 92 93Because 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. 94 95Note 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: 96 97```cpp 98 utf8::utf8to16(it, eos, back_inserter(u16string)); 99``` 100 101### Ensure that a string contains valid UTF-8 text 102 103If 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: 104 105```cpp 106void fix_utf8_string(std::string& str) 107{ 108 std::string temp; 109 utf8::replace_invalid(str.begin(), str.end(), back_inserter(temp)); 110 str = temp; 111} 112``` 113 114The 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. 115 116## Reference 117 118### Functions From utf8 Namespace 119 120#### utf8::append 121 122Available in version 1.0 and later. 123 124Encodes a 32 bit code point as a UTF-8 sequence of octets and appends the sequence to a UTF-8 string. 125 126```cpp 127template <typename octet_iterator> 128octet_iterator append(uint32_t cp, octet_iterator result); 129``` 130 131`octet_iterator`: an output iterator. 132`cp`: a 32 bit integer representing a code point to append to the sequence. 133`result`: an output iterator to the place in the sequence where to append the code point. 134Return value: an iterator pointing to the place after the newly appended sequence. 135 136Example of use: 137 138```cpp 139unsigned char u[5] = {0,0,0,0,0}; 140unsigned char* end = append(0x0448, u); 141assert (u[0] == 0xd1 && u[1] == 0x88 && u[2] == 0 && u[3] == 0 && u[4] == 0); 142``` 143 144Note 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. 145 146In case of an invalid code point, a `utf8::invalid_code_point` exception is thrown. 147 148#### utf8::next 149 150Available in version 1.0 and later. 151 152Given the iterator to the beginning of the UTF-8 sequence, it returns the code point and moves the iterator to the next position. 153 154```cpp 155template <typename octet_iterator> 156uint32_t next(octet_iterator& it, octet_iterator end); 157``` 158 159`octet_iterator`: an input iterator. 160`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. 161`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. 162Return value: the 32 bit representation of the processed UTF-8 code point. 163 164Example of use: 165 166```cpp 167char* twochars = "\xe6\x97\xa5\xd1\x88"; 168char* w = twochars; 169int cp = next(w, twochars + 6); 170assert (cp == 0x65e5); 171assert (w == twochars + 3); 172``` 173 174This function is typically used to iterate through a UTF-8 encoded string. 175 176In case of an invalid UTF-8 seqence, a `utf8::invalid_utf8` exception is thrown. 177 178#### utf8::peek_next 179 180Available in version 2.1 and later. 181 182Given 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. 183 184```cpp 185template <typename octet_iterator> 186uint32_t peek_next(octet_iterator it, octet_iterator end); 187``` 188 189 190`octet_iterator`: an input iterator. 191`it`: an iterator pointing to the beginning of an UTF-8 encoded code point. 192`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. 193Return value: the 32 bit representation of the processed UTF-8 code point. 194 195Example of use: 196 197```cpp 198char* twochars = "\xe6\x97\xa5\xd1\x88"; 199char* w = twochars; 200int cp = peek_next(w, twochars + 6); 201assert (cp == 0x65e5); 202assert (w == twochars); 203``` 204 205In case of an invalid UTF-8 seqence, a `utf8::invalid_utf8` exception is thrown. 206 207#### utf8::prior 208 209Available in version 1.02 and later. 210 211Given 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. 212 213```cpp 214template <typename octet_iterator> 215uint32_t prior(octet_iterator& it, octet_iterator start); 216``` 217 218`octet_iterator`: a bidirectional iterator. 219`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. 220`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. 221 Return value: the 32 bit representation of the previous code point. 222 223Example of use: 224 225```cpp 226char* twochars = "\xe6\x97\xa5\xd1\x88"; 227unsigned char* w = twochars + 3; 228int cp = prior (w, twochars); 229assert (cp == 0x65e5); 230assert (w == twochars); 231``` 232 233This 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. Note that in that case `utf8::prior` may not detect an invalid UTF-8 sequence in some scenarios: for instance if there are superfluous trail octets, it will just skip them. 234 235`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. 236 237In case `start` 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. 238 239In case `start` equals `it`, a `not_enough_room` exception is thrown. 240 241#### utf8::previous 242 243Deprecated in version 1.02 and later. 244 245Given 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. 246 247```cpp 248template <typename octet_iterator> 249uint32_t previous(octet_iterator& it, octet_iterator pass_start); 250``` 251 252`octet_iterator`: a random access iterator. 253`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. 254`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. 255Return value: the 32 bit representation of the previous code point. 256 257Example of use: 258 259```cpp 260char* twochars = "\xe6\x97\xa5\xd1\x88"; 261unsigned char* w = twochars + 3; 262int cp = previous (w, twochars - 1); 263assert (cp == 0x65e5); 264assert (w == twochars); 265``` 266 267 268`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. 269 270`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. 271 272In case `pass_start` 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 273 274#### utf8::advance 275Available in version 1.0 and later. 276 277Advances an iterator by the specified number of code points within an UTF-8 sequence. 278 279```cpp 280template <typename octet_iterator, typename distance_type> 281void advance (octet_iterator& it, distance_type n, octet_iterator end); 282``` 283 284`octet_iterator`: an input iterator. 285`distance_type`: an integral type convertible to `octet_iterator`'s difference type. 286`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. 287`n`: a positive integer that shows how many code points we want to advance. 288`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. 289 290Example of use: 291 292```cpp 293char* twochars = "\xe6\x97\xa5\xd1\x88"; 294unsigned char* w = twochars; 295advance (w, 2, twochars + 6); 296assert (w == twochars + 5); 297``` 298 299This function works only "forward". In case of a negative `n`, there is no effect. 300 301In case of an invalid code point, a `utf8::invalid_code_point` exception is thrown. 302 303#### utf8::distance 304 305Available in version 1.0 and later. 306 307Given the iterators to two UTF-8 encoded code points in a seqence, returns the number of code points between them. 308 309```cpp 310template <typename octet_iterator> 311typename std::iterator_traits<octet_iterator>::difference_type distance (octet_iterator first, octet_iterator last); 312``` 313 314`octet_iterator`: an input iterator. 315`first`: an iterator to a beginning of a UTF-8 encoded code point. 316`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. 317 Return value the distance between the iterators, in code points. 318 319Example of use: 320 321```cpp 322char* twochars = "\xe6\x97\xa5\xd1\x88"; 323size_t dist = utf8::distance(twochars, twochars + 5); 324assert (dist == 2); 325``` 326 327This 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. 328 329In 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. 330 331#### utf8::utf16to8 332 333Available in version 1.0 and later. 334 335Converts a UTF-16 encoded string to UTF-8. 336 337```cpp 338template <typename u16bit_iterator, typename octet_iterator> 339octet_iterator utf16to8 (u16bit_iterator start, u16bit_iterator end, octet_iterator result); 340``` 341 342`u16bit_iterator`: an input iterator. 343`octet_iterator`: an output iterator. 344`start`: an iterator pointing to the beginning of the UTF-16 encoded string to convert. 345`end`: an iterator pointing to pass-the-end of the UTF-16 encoded string to convert. 346`result`: an output iterator to the place in the UTF-8 string where to append the result of conversion. 347Return value: An iterator pointing to the place after the appended UTF-8 string. 348 349Example of use: 350 351```cpp 352unsigned short utf16string[] = {0x41, 0x0448, 0x65e5, 0xd834, 0xdd1e}; 353vector<unsigned char> utf8result; 354utf16to8(utf16string, utf16string + 5, back_inserter(utf8result)); 355assert (utf8result.size() == 10); 356``` 357 358In case of invalid UTF-16 sequence, a `utf8::invalid_utf16` exception is thrown. 359 360#### utf8::utf8to16 361 362Available in version 1.0 and later. 363 364Converts an UTF-8 encoded string to UTF-16 365 366```cpp 367template <typename u16bit_iterator, typename octet_iterator> 368u16bit_iterator utf8to16 (octet_iterator start, octet_iterator end, u16bit_iterator result); 369``` 370 371`octet_iterator`: an input iterator. 372`u16bit_iterator`: an output iterator. 373`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. 374`result`: an output iterator to the place in the UTF-16 string where to append the result of conversion. 375Return value: An iterator pointing to the place after the appended UTF-16 string. 376 377Example of use: 378 379```cpp 380char utf8_with_surrogates[] = "\xe6\x97\xa5\xd1\x88\xf0\x9d\x84\x9e"; 381vector <unsigned short> utf16result; 382utf8to16(utf8_with_surrogates, utf8_with_surrogates + 9, back_inserter(utf16result)); 383assert (utf16result.size() == 4); 384assert (utf16result[2] == 0xd834); 385assert (utf16result[3] == 0xdd1e); 386``` 387 388In 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. 389 390#### utf8::utf32to8 391 392Available in version 1.0 and later. 393 394Converts a UTF-32 encoded string to UTF-8. 395 396```cpp 397template <typename octet_iterator, typename u32bit_iterator> 398octet_iterator utf32to8 (u32bit_iterator start, u32bit_iterator end, octet_iterator result); 399``` 400 401`octet_iterator`: an output iterator. 402`u32bit_iterator`: an input iterator. 403`start`: an iterator pointing to the beginning of the UTF-32 encoded string to convert. 404`end`: an iterator pointing to pass-the-end of the UTF-32 encoded string to convert. 405`result`: an output iterator to the place in the UTF-8 string where to append the result of conversion. 406Return value: An iterator pointing to the place after the appended UTF-8 string. 407 408Example of use: 409 410``` 411int utf32string[] = {0x448, 0x65E5, 0x10346, 0}; 412vector<unsigned char> utf8result; 413utf32to8(utf32string, utf32string + 3, back_inserter(utf8result)); 414assert (utf8result.size() == 9); 415``` 416 417In case of invalid UTF-32 string, a `utf8::invalid_code_point` exception is thrown. 418 419#### utf8::utf8to32 420 421Available in version 1.0 and later. 422 423Converts a UTF-8 encoded string to UTF-32. 424 425```cpp 426template <typename octet_iterator, typename u32bit_iterator> 427u32bit_iterator utf8to32 (octet_iterator start, octet_iterator end, u32bit_iterator result); 428``` 429 430`octet_iterator`: an input iterator. 431`u32bit_iterator`: an output iterator. 432`start`: an iterator pointing to the beginning of the UTF-8 encoded string to convert. 433`end`: an iterator pointing to pass-the-end of the UTF-8 encoded string to convert. 434`result`: an output iterator to the place in the UTF-32 string where to append the result of conversion. 435Return value: An iterator pointing to the place after the appended UTF-32 string. 436 437Example of use: 438 439```cpp 440char* twochars = "\xe6\x97\xa5\xd1\x88"; 441vector<int> utf32result; 442utf8to32(twochars, twochars + 5, back_inserter(utf32result)); 443assert (utf32result.size() == 2); 444``` 445 446In 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. 447 448#### utf8::find_invalid 449 450Available in version 1.0 and later. 451 452Detects an invalid sequence within a UTF-8 string. 453 454```cpp 455template <typename octet_iterator> 456octet_iterator find_invalid(octet_iterator start, octet_iterator end); 457``` 458 459`octet_iterator`: an input iterator. 460`start`: an iterator pointing to the beginning of the UTF-8 string to test for validity. 461`end`: an iterator pointing to pass-the-end of the UTF-8 string to test for validity. 462Return value: an iterator pointing to the first invalid octet in the UTF-8 string. In case none were found, equals `end`. 463 464Example of use: 465 466```cpp 467char utf_invalid[] = "\xe6\x97\xa5\xd1\x88\xfa"; 468char* invalid = find_invalid(utf_invalid, utf_invalid + 6); 469assert (invalid == utf_invalid + 5); 470``` 471 472This 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. 473 474#### utf8::is_valid 475 476Available in version 1.0 and later. 477 478Checks whether a sequence of octets is a valid UTF-8 string. 479 480```cpp 481template <typename octet_iterator> 482bool is_valid(octet_iterator start, octet_iterator end); 483``` 484 485`octet_iterator`: an input iterator. 486`start`: an iterator pointing to the beginning of the UTF-8 string to test for validity. 487`end`: an iterator pointing to pass-the-end of the UTF-8 string to test for validity. 488Return value: `true` if the sequence is a valid UTF-8 string; `false` if not. 489 490Example of use: 491 492```cpp 493char utf_invalid[] = "\xe6\x97\xa5\xd1\x88\xfa"; 494bool bvalid = is_valid(utf_invalid, utf_invalid + 6); 495assert (bvalid == false); 496``` 497 498`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. 499 500#### utf8::replace_invalid 501 502Available in version 2.0 and later. 503 504Replaces all invalid UTF-8 sequences within a string with a replacement marker. 505 506```cpp 507template <typename octet_iterator, typename output_iterator> 508output_iterator replace_invalid(octet_iterator start, octet_iterator end, output_iterator out, uint32_t replacement); 509template <typename octet_iterator, typename output_iterator> 510output_iterator replace_invalid(octet_iterator start, octet_iterator end, output_iterator out); 511``` 512 513`octet_iterator`: an input iterator. 514`output_iterator`: an output iterator. 515`start`: an iterator pointing to the beginning of the UTF-8 string to look for invalid UTF-8 sequences. 516`end`: an iterator pointing to pass-the-end of the UTF-8 string to look for invalid UTF-8 sequences. 517`out`: An output iterator to the range where the result of replacement is stored. 518`replacement`: A Unicode code point for the replacement marker. The version without this parameter assumes the value `0xfffd` 519Return value: An iterator pointing to the place after the UTF-8 string with replaced invalid sequences. 520 521Example of use: 522 523```cpp 524char invalid_sequence[] = "a\x80\xe0\xa0\xc0\xaf\xed\xa0\x80z"; 525vector<char> replace_invalid_result; 526replace_invalid (invalid_sequence, invalid_sequence + sizeof(invalid_sequence), back_inserter(replace_invalid_result), '?'); 527bvalid = is_valid(replace_invalid_result.begin(), replace_invalid_result.end()); 528assert (bvalid); 529char* fixed_invalid_sequence = "a????z"; 530assert (std::equal(replace_invalid_result.begin(), replace_invalid_result.end(), fixed_invalid_sequence)); 531``` 532 533`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. 534 535If `end` does not point to the past-of-end of a UTF-8 sequence, a `utf8::not_enough_room` exception is thrown. 536 537#### utf8::starts_with_bom 538 539Available in version 2.3 and later. Relaces deprecated `is_bom()` function. 540 541Checks whether an octet sequence starts with a UTF-8 byte order mark (BOM) 542 543```cpp 544template <typename octet_iterator> 545bool starts_with_bom (octet_iterator it, octet_iterator end); 546``` 547 548`octet_iterator`: an input iterator. 549`it`: beginning of the octet sequence to check 550`end`: pass-end of the sequence to check 551Return value: `true` if the sequence starts with a UTF-8 byte order mark; `false` if not. 552 553Example of use: 554 555```cpp 556unsigned char byte_order_mark[] = {0xef, 0xbb, 0xbf}; 557bool bbom = starts_with_bom(byte_order_mark, byte_order_mark + sizeof(byte_order_mark)); 558assert (bbom == true); 559``` 560 561The 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. 562 563#### utf8::is_bom 564 565Available in version 1.0 and later. Deprecated in version 2.3\. `starts_with_bom()` should be used instead. 566 567Checks whether a sequence of three octets is a UTF-8 byte order mark (BOM) 568 569```cpp 570template <typename octet_iterator> 571bool is_bom (octet_iterator it); // Deprecated 572``` 573 574`octet_iterator`: an input iterator. 575`it`: beginning of the 3-octet sequence to check 576Return value: `true` if the sequence is UTF-8 byte order mark; `false` if not. 577 578Example of use: 579 580```cpp 581unsigned char byte_order_mark[] = {0xef, 0xbb, 0xbf}; 582bool bbom = is_bom(byte_order_mark); 583assert (bbom == true); 584``` 585 586The 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. 587 588If 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. 589 590### Types From utf8 Namespace 591 592#### utf8::exception 593 594Available in version 2.3 and later. 595 596Base class for the exceptions thrown by UTF CPP library functions. 597 598```cpp 599class exception : public std::exception {}; 600``` 601 602Example of use: 603 604```cpp 605try { 606 code_that_uses_utf_cpp_library(); 607} 608catch(const utf8::exception& utfcpp_ex) { 609 cerr << utfcpp_ex.what(); 610} 611``` 612 613#### utf8::invalid_code_point 614 615Available in version 1.0 and later. 616 617Thrown by UTF8 CPP functions such as `advance` and `next` if an UTF-8 sequence represents and invalid code point. 618 619```cpp 620class invalid_code_point : public exception { 621public: 622 uint32_t code_point() const; 623}; 624``` 625 626Member function `code_point()` can be used to determine the invalid code point that caused the exception to be thrown. 627 628#### utf8::invalid_utf8 629 630Available in version 1.0 and later. 631 632Thrown by UTF8 CPP functions such as `next` and `prior` if an invalid UTF-8 sequence is detected during decoding. 633 634```cpp 635class invalid_utf8 : public exception { 636public: 637 uint8_t utf8_octet() const; 638}; 639``` 640 641Member function `utf8_octet()` can be used to determine the beginning of the byte sequence that caused the exception to be thrown. 642 643#### utf8::invalid_utf16 644 645Available in version 1.0 and later. 646 647Thrown by UTF8 CPP function `utf16to8` if an invalid UTF-16 sequence is detected during decoding. 648 649```cpp 650class invalid_utf16 : public exception { 651public: 652 uint16_t utf16_word() const; 653}; 654``` 655 656Member function `utf16_word()` can be used to determine the UTF-16 code unit that caused the exception to be thrown. 657 658#### utf8::not_enough_room 659 660Available in version 1.0 and later. 661 662Thrown by UTF8 CPP functions such as `next` if the end of the decoded UTF-8 sequence was reached before the code point was decoded. 663 664```cpp 665class not_enough_room : public exception {}; 666``` 667 668#### utf8::iterator 669 670Available in version 2.0 and later. 671 672Adapts the underlying octet iterator to iterate over the sequence of code points, rather than raw octets. 673 674```cpp 675template <typename octet_iterator> 676class iterator; 677``` 678 679##### Member functions 680 681`iterator();` the deafult constructor; the underlying octet_iterator is constructed with its default constructor. 682`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. 683`octet_iterator base () const;` returns the underlying octet_iterator. 684`uint32_t operator * () const;` decodes the utf-8 sequence the underlying octet_iterator is pointing to and returns the code point. 685`bool operator == (const iterator& rhs) const;` returns `true` if the two underlaying iterators are equal. 686`bool operator != (const iterator& rhs) const;` returns `true` if the two underlaying iterators are not equal. 687`iterator& operator ++ ();` the prefix increment - moves the iterator to the next UTF-8 encoded code point. 688`iterator operator ++ (int);` the postfix increment - moves the iterator to the next UTF-8 encoded code point and returns the current one. 689`iterator& operator -- ();` the prefix decrement - moves the iterator to the previous UTF-8 encoded code point. 690`iterator operator -- (int);` the postfix decrement - moves the iterator to the previous UTF-8 encoded code point and returns the current one. 691 692Example of use: 693 694```cpp 695char* threechars = "\xf0\x90\x8d\x86\xe6\x97\xa5\xd1\x88"; 696utf8::iterator<char*> it(threechars, threechars, threechars + 9); 697utf8::iterator<char*> it2 = it; 698assert (it2 == it); 699assert (*it == 0x10346); 700assert (*(++it) == 0x65e5); 701assert ((*it++) == 0x65e5); 702assert (*it == 0x0448); 703assert (it != it2); 704utf8::iterator<char*> endit (threechars + 9, threechars, threechars + 9); 705assert (++it == endit); 706assert (*(--it) == 0x0448); 707assert ((*it--) == 0x0448); 708assert (*it == 0x65e5); 709assert (--it == utf8::iterator<char*>(threechars, threechars, threechars + 9)); 710assert (*it == 0x10346); 711``` 712 713The 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. 714 715Note 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.: 716 717```cpp 718std::string s = "example"; 719utf8::iterator i (s.begin(), s.begin(), s.end()); 720``` 721 722### Functions From utf8::unchecked Namespace 723 724#### utf8::unchecked::append 725 726Available in version 1.0 and later. 727 728Encodes a 32 bit code point as a UTF-8 sequence of octets and appends the sequence to a UTF-8 string. 729 730```cpp 731template <typename octet_iterator> 732octet_iterator append(uint32_t cp, octet_iterator result); 733``` 734 735`cp`: A 32 bit integer representing a code point to append to the sequence. 736`result`: An output iterator to the place in the sequence where to append the code point. 737Return value: An iterator pointing to the place after the newly appended sequence. 738 739Example of use: 740 741```cpp 742unsigned char u[5] = {0,0,0,0,0}; 743unsigned char* end = unchecked::append(0x0448, u); 744assert (u[0] == 0xd1 && u[1] == 0x88 && u[2] == 0 && u[3] == 0 && u[4] == 0); 745``` 746 747This 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. 748 749#### utf8::unchecked::next 750 751Available in version 1.0 and later. 752 753Given the iterator to the beginning of a UTF-8 sequence, it returns the code point and moves the iterator to the next position. 754 755```cpp 756template <typename octet_iterator> 757uint32_t next(octet_iterator& it); 758``` 759 760`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. 761 Return value: the 32 bit representation of the processed UTF-8 code point. 762 763Example of use: 764 765```cpp 766char* twochars = "\xe6\x97\xa5\xd1\x88"; 767char* w = twochars; 768int cp = unchecked::next(w); 769assert (cp == 0x65e5); 770assert (w == twochars + 3); 771``` 772 773This is a faster but less safe version of `utf8::next`. It does not check for validity of the supplied UTF-8 sequence. 774 775#### utf8::unchecked::peek_next 776 777Available in version 2.1 and later. 778 779Given the iterator to the beginning of a UTF-8 sequence, it returns the code point. 780 781```cpp 782template <typename octet_iterator> 783uint32_t peek_next(octet_iterator it); 784``` 785 786`it`: an iterator pointing to the beginning of an UTF-8 encoded code point. 787Return value: the 32 bit representation of the processed UTF-8 code point. 788 789Example of use: 790 791```cpp 792char* twochars = "\xe6\x97\xa5\xd1\x88"; 793char* w = twochars; 794int cp = unchecked::peek_next(w); 795assert (cp == 0x65e5); 796assert (w == twochars); 797``` 798 799This is a faster but less safe version of `utf8::peek_next`. It does not check for validity of the supplied UTF-8 sequence. 800 801#### utf8::unchecked::prior 802 803Available in version 1.02 and later. 804 805Given 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. 806 807```cpp 808template <typename octet_iterator> 809uint32_t prior(octet_iterator& it); 810``` 811 812`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. 813 Return value: the 32 bit representation of the previous code point. 814 815Example of use: 816 817```cpp 818char* twochars = "\xe6\x97\xa5\xd1\x88"; 819char* w = twochars + 3; 820int cp = unchecked::prior (w); 821assert (cp == 0x65e5); 822assert (w == twochars); 823``` 824 825This 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. 826 827#### utf8::unchecked::previous (deprecated, see utf8::unchecked::prior) 828 829Deprecated in version 1.02 and later. 830 831Given 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. 832 833```cpp 834template <typename octet_iterator> 835uint32_t previous(octet_iterator& it); 836``` 837 838`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. 839Return value: the 32 bit representation of the previous code point. 840 841Example of use: 842 843```cpp 844char* twochars = "\xe6\x97\xa5\xd1\x88"; 845char* w = twochars + 3; 846int cp = unchecked::previous (w); 847assert (cp == 0x65e5); 848assert (w == twochars); 849``` 850 851The 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` 852 853This 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. 854 855#### utf8::unchecked::advance 856 857Available in version 1.0 and later. 858 859Advances an iterator by the specified number of code points within an UTF-8 sequence. 860 861```cpp 862template <typename octet_iterator, typename distance_type> 863void advance (octet_iterator& it, distance_type n); 864``` 865 866`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. 867`n`: a positive integer that shows how many code points we want to advance. 868 869Example of use: 870 871```cpp 872char* twochars = "\xe6\x97\xa5\xd1\x88"; 873char* w = twochars; 874unchecked::advance (w, 2); 875assert (w == twochars + 5); 876``` 877 878This function works only "forward". In case of a negative `n`, there is no effect. 879 880This 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. 881 882#### utf8::unchecked::distance 883 884Available in version 1.0 and later. 885 886Given the iterators to two UTF-8 encoded code points in a seqence, returns the number of code points between them. 887 888```cpp 889template <typename octet_iterator> 890typename std::iterator_traits<octet_iterator>::difference_type distance (octet_iterator first, octet_iterator last); 891``` 892 893`first`: an iterator to a beginning of a UTF-8 encoded code point. 894`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. 895Return value: the distance between the iterators, in code points. 896 897Example of use: 898 899```cpp 900char* twochars = "\xe6\x97\xa5\xd1\x88"; 901size_t dist = utf8::unchecked::distance(twochars, twochars + 5); 902assert (dist == 2); 903``` 904 905This is a faster but less safe version of `utf8::distance`. It does not check for validity of the supplied UTF-8 sequence. 906 907#### utf8::unchecked::utf16to8 908 909Available in version 1.0 and later. 910 911Converts a UTF-16 encoded string to UTF-8. 912 913```cpp 914template <typename u16bit_iterator, typename octet_iterator> 915octet_iterator utf16to8 (u16bit_iterator start, u16bit_iterator end, octet_iterator result); 916``` 917 918`start`: an iterator pointing to the beginning of the UTF-16 encoded string to convert. 919`end`: an iterator pointing to pass-the-end of the UTF-16 encoded string to convert. 920`result`: an output iterator to the place in the UTF-8 string where to append the result of conversion. 921Return value: An iterator pointing to the place after the appended UTF-8 string. 922 923Example of use: 924 925```cpp 926unsigned short utf16string[] = {0x41, 0x0448, 0x65e5, 0xd834, 0xdd1e}; 927vector<unsigned char> utf8result; 928unchecked::utf16to8(utf16string, utf16string + 5, back_inserter(utf8result)); 929assert (utf8result.size() == 10); 930``` 931 932This is a faster but less safe version of `utf8::utf16to8`. It does not check for validity of the supplied UTF-16 sequence. 933 934#### utf8::unchecked::utf8to16 935 936Available in version 1.0 and later. 937 938Converts an UTF-8 encoded string to UTF-16 939 940```cpp 941template <typename u16bit_iterator, typename octet_iterator> 942u16bit_iterator utf8to16 (octet_iterator start, octet_iterator end, u16bit_iterator result); 943``` 944 945`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. 946`result`: an output iterator to the place in the UTF-16 string where to append the result of conversion. 947Return value: An iterator pointing to the place after the appended UTF-16 string. 948 949Example of use: 950 951```cpp 952char utf8_with_surrogates[] = "\xe6\x97\xa5\xd1\x88\xf0\x9d\x84\x9e"; 953vector <unsigned short> utf16result; 954unchecked::utf8to16(utf8_with_surrogates, utf8_with_surrogates + 9, back_inserter(utf16result)); 955assert (utf16result.size() == 4); 956assert (utf16result[2] == 0xd834); 957assert (utf16result[3] == 0xdd1e); 958``` 959 960This is a faster but less safe version of `utf8::utf8to16`. It does not check for validity of the supplied UTF-8 sequence. 961 962#### utf8::unchecked::utf32to8 963 964Available in version 1.0 and later. 965 966Converts a UTF-32 encoded string to UTF-8. 967 968```cpp 969template <typename octet_iterator, typename u32bit_iterator> 970octet_iterator utf32to8 (u32bit_iterator start, u32bit_iterator end, octet_iterator result); 971``` 972 973`start`: an iterator pointing to the beginning of the UTF-32 encoded string to convert. 974`end`: an iterator pointing to pass-the-end of the UTF-32 encoded string to convert. 975`result`: an output iterator to the place in the UTF-8 string where to append the result of conversion. 976Return value: An iterator pointing to the place after the appended UTF-8 string. 977 978Example of use: 979 980```cpp 981int utf32string[] = {0x448, 0x65e5, 0x10346, 0}; 982vector<unsigned char> utf8result; 983utf32to8(utf32string, utf32string + 3, back_inserter(utf8result)); 984assert (utf8result.size() == 9); 985``` 986 987This is a faster but less safe version of `utf8::utf32to8`. It does not check for validity of the supplied UTF-32 sequence. 988 989#### utf8::unchecked::utf8to32 990 991Available in version 1.0 and later. 992 993Converts a UTF-8 encoded string to UTF-32. 994 995```cpp 996template <typename octet_iterator, typename u32bit_iterator> 997u32bit_iterator utf8to32 (octet_iterator start, octet_iterator end, u32bit_iterator result); 998``` 999 1000`start`: an iterator pointing to the beginning of the UTF-8 encoded string to convert. 1001`end`: an iterator pointing to pass-the-end of the UTF-8 encoded string to convert. 1002`result`: an output iterator to the place in the UTF-32 string where to append the result of conversion. 1003Return value: An iterator pointing to the place after the appended UTF-32 string. 1004 1005Example of use: 1006 1007```cpp 1008char* twochars = "\xe6\x97\xa5\xd1\x88"; 1009vector<int> utf32result; 1010unchecked::utf8to32(twochars, twochars + 5, back_inserter(utf32result)); 1011assert (utf32result.size() == 2); 1012``` 1013 1014This is a faster but less safe version of `utf8::utf8to32`. It does not check for validity of the supplied UTF-8 sequence. 1015 1016### Types From utf8::unchecked Namespace 1017 1018#### utf8::iterator 1019 1020Available in version 2.0 and later. 1021 1022Adapts the underlying octet iterator to iterate over the sequence of code points, rather than raw octets. 1023 1024```cpp 1025template <typename octet_iterator> 1026class iterator; 1027``` 1028 1029##### Member functions 1030 1031`iterator();` the deafult constructor; the underlying octet_iterator is constructed with its default constructor. 1032`explicit iterator (const octet_iterator& octet_it);` a constructor that initializes the underlying octet_iterator with `octet_it` 1033`octet_iterator base () const;` returns the underlying octet_iterator. 1034`uint32_t operator * () const;` decodes the utf-8 sequence the underlying octet_iterator is pointing to and returns the code point. 1035`bool operator == (const iterator& rhs) const;` returns `true` if the two underlaying iterators are equal. 1036`bool operator != (const iterator& rhs) const;` returns `true` if the two underlaying iterators are not equal. 1037`iterator& operator ++ ();` the prefix increment - moves the iterator to the next UTF-8 encoded code point. 1038`iterator operator ++ (int);` the postfix increment - moves the iterator to the next UTF-8 encoded code point and returns the current one. 1039`iterator& operator -- ();` the prefix decrement - moves the iterator to the previous UTF-8 encoded code point. 1040`iterator operator -- (int);` the postfix decrement - moves the iterator to the previous UTF-8 encoded code point and returns the current one. 1041 1042Example of use: 1043 1044```cpp 1045char* threechars = "\xf0\x90\x8d\x86\xe6\x97\xa5\xd1\x88"; 1046utf8::unchecked::iterator<char*> un_it(threechars); 1047utf8::unchecked::iterator<char*> un_it2 = un_it; 1048assert (un_it2 == un_it); 1049assert (*un_it == 0x10346); 1050assert (*(++un_it) == 0x65e5); 1051assert ((*un_it++) == 0x65e5); 1052assert (*un_it == 0x0448); 1053assert (un_it != un_it2); 1054utf8::::unchecked::iterator<char*> un_endit (threechars + 9); 1055assert (++un_it == un_endit); 1056assert (*(--un_it) == 0x0448); 1057assert ((*un_it--) == 0x0448); 1058assert (*un_it == 0x65e5); 1059assert (--un_it == utf8::unchecked::iterator<char*>(threechars)); 1060assert (*un_it == 0x10346); 1061``` 1062 1063This is an unchecked version of `utf8::iterator`. It is faster in many cases, but offers no validity or range checks. 1064 1065## Points of interest 1066 1067#### Design goals and decisions 1068 1069The library was designed to be: 1070 10711. 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. 10722. 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. At this point I don't plan to use any post C++03 features, so the library should work even with pretty old compilers. 10733. Lightweight: follow the "pay only for what you use" guideline. 10744. Unintrusive: avoid forcing any particular design or even programming style on the user. This is a library, not a framework. 1075 1076#### Alternatives 1077 1078In 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: 1079 10801. [ICU Library](http://icu.sourceforge.net/). 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. 10812. C++11 language and library features. Still far from complete, and not easy to use. 10823. [Glib::ustring](http://www.gtkmm.org/gtkmm2/docs/tutorial/html/ch03s04.html). 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. 10834. 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. 1084 1085## Links 1086 10871. [The Unicode Consortium](http://www.unicode.org/). 10882. [ICU Library](http://icu.sourceforge.net/). 10893. [UTF-8 at Wikipedia](http://en.wikipedia.org/wiki/UTF-8) 10904. [UTF-8 and Unicode FAQ for Unix/Linux](http://www.cl.cam.ac.uk/~mgk25/unicode.html) 1091