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.
@ -328,6 +398,30 @@ 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 +613,29 @@ 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 +662,29 @@ 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 +711,32 @@ 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::starts_with_bom
Available in version 2.3 and later. Replaces deprecated `is_bom()` function.
Checks whether an octet sequence starts with a UTF-8 byte order mark (BOM)
@ -678,15 +943,24 @@ 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:
@ -1029,7 +1303,9 @@ 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.
@ -1062,26 +1338,6 @@ 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.
Nemanja Trifunovic
5 years ago