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. 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.
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.
In 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](./LICENSE). 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.
@ -28,50 +28,74 @@ int main(int argc, char** argv)
cout << "\nUsage: docsample filename\n";
return 0;
}
const char* test_file_path = argv[1];
// Open the test file (contains UTF-8 encoded text)
// Open the test file (must be UTF-8 encoded)
ifstream fs8(test_file_path);
if (!fs8.is_open()) {
cout << "Could not open " <<test_file_path<<endl;
return 0;
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)
// check for invalid utf-8 (for a simple yes/no check, there is also utf8::is_valid function)
#if __cplusplus >= 201103L // C++ 11 or later
auto end_it = utf8::find_invalid(line.begin(), line.end());
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`.
Note a different pattern of usage for old compilers. For instance, this is how we convert
a UTF-8 encoded string to a UTF-16 encoded one with a pre - C++11 compiler:
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.
## Points of interest
#### Design goals and decisions
The library was designed to be:
1. Generic: for better or worse, there are many C++ string classes out there, and the library should work with as many of them as possible.
2. Portable: the library should be portable both accross different platforms and compilers. The only non-portable code is a small section that declares unsigned integers of different sizes: three typedefs. They can be changed by the users of the library if they don't match their platform. The default setting should work for Windows (both 32 and 64 bit), and most 32 bit and 64 bit Unix derivatives. Support for post C++03 language features is included for modern compilers at API level only, so the library should work even with pretty old compilers.
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](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.
2. C++11 language and library features. Still far from complete, and not easy to use.
3. [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.
4. 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.
## Reference
### Functions From utf8 Namespace
#### utf8::append
Available in version 3.0 and later. Requires a C++ 11 compliant compiler.
Encodes a 32 bit code point as a UTF-8 sequence of octets and appends the sequence to a UTF-8 string.
```cpp
void append(char32_t cp, std::string& s);
```
`cp`: a code point to append to the string.
`s`: a utf-8 encoded string to append the code point to.
In case of an invalid code point, a `utf8::invalid_code_point` exception is thrown.
#### utf8::append
Available in version 1.0 and later.
@ -238,39 +308,6 @@ In case `start` is reached before a UTF-8 lead octet is hit, or if an invalid UT
In case `start` equals `it`, a `not_enough_room` 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 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.
`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:
```cpp
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_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
`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.
`n`: number of code points `it` should be advanced. A negative value means decrement.
`end`: limit of the UTF-8 sequence to be processed. If `n` is positive and`it` gets equal to `end` during the extraction of a code point, an `utf8::not_enough_room` exception is thrown. If`n` is negative and `it` reaches `end` while `it` points t a trail byte of a UTF-8 sequence, a `utf8::invalid_code_point` exception is thrown.
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
@ -328,6 +365,30 @@ This function is used to find the length (in code points) of a UTF-8 encoded str
In case of an invalid UTF-8 seqence, a `utf8::invalid_utf8` exception is thrown. If `last` does not point to the past-of-end of a UTF-8 seqence, a `utf8::not_enough_room` exception is thrown.
#### utf8::utf16to8
Available in version 3.0 and later. Requires a C++ 11 compliant compiler.
In case of invalid UTF-32 string, a `utf8::invalid_code_point` exception is thrown.
#### utf8::utf8to32
Available in version 3.0 and later. Requires a C++ 11 compliant compiler.
Converts a UTF-8 encoded string to UTF-32.
```cpp
std::u32string utf8to32(const std::string& s);
```
`s`: a UTF-8 encoded string.
Return value: a UTF-32 encoded string.
Example of use:
```cpp
const char* twochars = "\xe6\x97\xa5\xd1\x88";
u32string utf32result = utf8to32(twochars);
assert (utf32result.size() == 2);
```
In case of an invalid UTF-8 seqence, a `utf8::invalid_utf8` exception is thrown.
#### utf8::utf8to32
Available in version 1.0 and later.
@ -447,6 +580,29 @@ In case of an invalid UTF-8 seqence, a `utf8::invalid_utf8` exception is thrown.
#### utf8::find_invalid
Available in version 3.0 and later. Requires a C++ 11 compliant compiler.
Detects an invalid sequence within a UTF-8 string.
```cpp
std::size_t find_invalid(const std::string& s);
```
`s`: a UTF-8 encoded string.
Return value: the index of the first invalid octet in the UTF-8 string. In case none were found, equals `std::string::npos`.
Example of use:
```cpp
string utf_invalid = "\xe6\x97\xa5\xd1\x88\xfa";
auto invalid = find_invalid(utf_invalid);
assert (invalid == 5);
```
This function is typically used to make sure a UTF-8 string is valid before processing it with other functions. It is especially important to call it if before doing any of the _unchecked_ operations on it.
#### utf8::find_invalid
Available in version 1.0 and later.
Detects an invalid sequence within a UTF-8 string.
@ -473,6 +629,29 @@ This function is typically used to make sure a UTF-8 string is valid before proc
#### utf8::is_valid
Available in version 3.0 and later. Requires a C++ 11 compliant compiler.
Checks whether a string object contains valid UTF-8 encoded text.
```cpp
bool is_valid(const std::string& s);
```
`s`: a UTF-8 encoded string.
Return value: `true` if the string contains valid UTF-8 encoded text; `false` if not.
Example of use:
```cpp
char utf_invalid[] = "\xe6\x97\xa5\xd1\x88\xfa";
bool bvalid = is_valid(utf_invalid);
assert (bvalid == false);
```
You may want to use `is_valid` to make sure that a string contains valid UTF-8 text without the need to know where it fails if it is not valid.
#### utf8::is_valid
Available in version 1.0 and later.
Checks whether a sequence of octets is a valid UTF-8 string.
@ -499,6 +678,32 @@ assert (bvalid == false);
#### utf8::replace_invalid
Available in version 3.0 and later. Requires a C++ 11 compliant compiler.
Replaces all invalid UTF-8 sequences within a string with a replacement marker.
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.
#### utf8::is_bom
Available in version 1.0 and later. Deprecated in version 2.3\. `starts_with_bom()` should be used instead.
#### utf8::starts_with_bom
Checks whether a sequence of three octets is a UTF-8 byte order mark (BOM)
Available in version 2.3 and later.
Checks whether an octet sequence starts with a UTF-8 byte order mark (BOM)
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.
### Types From utf8 Namespace
#### utf8::exception
@ -676,15 +881,24 @@ class iterator;
##### Member functions
`iterator();` the deafult constructor; the underlying octet_iterator is constructed with its default constructor.
`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 underlaying iterators are equal.
`bool operator != (const iterator& rhs) const;` returns `true` if the two underlaying 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:
@ -822,34 +1036,6 @@ 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 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.
```cpp
template <typenameoctet_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:
```cpp
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.
`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.
`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`: number of code points `it` should be advanced. A negative value means decrement.
Example of use:
@ -873,8 +1059,6 @@ 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
@ -1027,7 +1211,9 @@ 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);` a constructor that initializes the underlying octet_iterator with `octet_it`
`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 underlaying iterators are equal.
@ -1060,26 +1246,6 @@ 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 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.
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](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.
2. C++11 language and library features. Still far from complete, and not easy to use.
3. [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.
4. 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.
nemtrif
5 years ago