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Initial import of the project tree git-svn-id: http://svn.code.sf.net/p/utfcpp/code@1 a809a056-fc17-0410-9590-b4f493f8b08e
2006-07-07 11:48:37 +00:00
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Updated the html page to link to the Sourceforge project page git-svn-id: http://svn.code.sf.net/p/utfcpp/code@6 a809a056-fc17-0410-9590-b4f493f8b08e
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<p><a href="https://sourceforge.net/projects/utfcpp">The Sourceforge project page</a></p>
Initial import of the project tree git-svn-id: http://svn.code.sf.net/p/utfcpp/code@1 a809a056-fc17-0410-9590-b4f493f8b08e
2006-07-07 11:48:37 +00:00
<h2>Introduction</h2>
<p>Many C++ developers miss an easy and portable way of handling
Unicode encoded strings. C++ Standard is currently 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 3rd party
libraries like ICU, OS specific capabilities, or simply roll out
their own solutions.</p>
<p>In order to easily handle UTF-8 encoded Unicode strings, I have
come up with a set of template functions. For anybody used to work
with STL algorithms, they 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. Be aware, though, that while I
did some testing, this library has not been used in production yet.
If you run into bugs or performance issues, please let me know and
I'll do my best to address them.</p>
<p>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 <a href=
"http://www.unicode.org/">Unicode Home Page</a> 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.</p>
<h2>Examples of use</h2>
<p>To illustrate the use of this utf8 library, we shall open a file
containing a line of UTF-8 encoded text, read the line into
<code>std::string</code>, convert the text to UTF-16, and write it
to another file:</p>
<pre>
#include &lt;fstream&gt;
#include &lt;iostream&gt;
#include &lt;string&gt;
#include &lt;vector&gt;
using namespace std;
int main()
{
// Open the file with a utf-8 encoded line of text in it
ifstream fs8("utf8.txt");
if (!fs8.is_open()) {
cout &lt;&lt; "Could not open utf8.txt" &lt;&lt; endl;
return 0;
}
// is there a utf8 marker? if yes, skip it.
fs8.seekg(0, ios::end);
ifstream::pos_type file_length = fs8.tellg();
fs8.seekg(0, ios::beg);
if (file_length &gt; 3) {
char bom[3];
fs8.read(bom, 3);
if (!utf8::is_bom(bom))
fs8.seekg(0, ios::beg);
}
// Read the line from the file
string text8;
getline(fs8, text8);
// Make sure it is valid utf-8
if (!utf8::is_valid(text8.begin(), text8.end())) {
cout &lt;&lt; "Invalid utf-8 text";
return 0;
}
// Convert the text to utf-16
vector&lt;unsigned short&gt; text16;
text16.push_back(0xfeff); // bom
utf8::utf8to16(text8.begin(), text8.end(), back_inserter(text16));
// Create the file for writing the utf-16 string
ofstream fs16("utf16.txt", ios_base::out | ios_base::binary);
if (!fs16.is_open()) {
cout &lt;&lt; "Could not open utf16.txt" &lt;&lt; endl;
return 0;
}
// Write the utf16 text to the file
fs16.write(reinterpret_cast&lt;const char*&gt;(&amp;text16[0]), text16.size() * sizeof (unsigned short));
}
</pre>
<p>In the previous code sample, we have seen the use of 3 functions
from <code>utf8</code> namespace: first we used <code>is_bom</code>
function to detect UTF-8 byte order mark at the beginning of the
file, then <code>is_valid</code> to make sure that the text we
loaded is valid UTF-8, and finally <code>utf8to16</code> to convert
the text to UTF-16 encoding. Note that the use of
<code>is_valid</code> was optional in this case;
<code>utf8to16</code> throws an exception in case of invalid UTF-8
text.</p>
<h2>Reference</h2>
<h3>Functions From utf8 Namespace</h3>
<h4>utf8::append</h4>
<p>Encodes a 32 bit code point as a UTF-8 sequence of octets and
appends the sequence to a UTF-8 string.</p>
<code>template &lt;typename octet_iterator&gt; octet_iterator
append(uint32_t cp, octet_iterator result);</code>
<p><code>cp</code>: A 32 bit integer representing a code point to
append to the sequence.<br>
<code>result</code>: An output iterator to the place in the
sequence where to append the code point.<br>
<u>Return value</u>: An iterator pointing to the place after the
newly appended sequence.</p>
<p>Example of use:</p>
<pre>
unsigned char u[5] = {0,0,0,0,0};
unsigned char* end = append(0x0448, u);
assert (u[0] == 0xd1 &amp;&amp; u[1] == 0x88 &amp;&amp; u[2] == 0 &amp;&amp; u[3] == 0 &amp;&amp; u[4] == 0);
</pre>
<p>Note that <code>append</code> 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,
<code>append</code> can add anywhere between 1 and 4 octets to the
sequence. In practice, you would most often want to use
<code>std::back_inserter</code> to ensure that the necessary memory
is allocated.</p>
<p>In case of an invalid code point, a
<code>utf8::invalid_code_point</code> exception is thrown.</p>
<h4>utf8::next</h4>
<p>Given the iterator to the beginning of the UTF-8 sequence, it
returns the code point and moves the iterator to the next
position.</p>
<code>template &lt;typename octet_iterator&gt; uint32_t
next(octet_iterator&amp; it, octet_iterator end);</code>
<p><code>it</code>: 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.<br>
<code>end</code>: end of the UTF-8 sequence to be processed. If
<code>it</code> gets equal to <code>end</code> during the
extraction of a code point, an <code>utf8::not_enough_room</code>
exception is thrown.<br>
<u>Return value</u>: the 32 bit representation of the processed
UTF-8 code point.</p>
<p>Example of use:</p>
<pre>
unsigned char twochars[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0x0};
unsigned char* w = twochars;
int cp = next(w, twochars + 6);
assert (cp == 0x65e5);
assert (w == twochars + 3);
</pre>
<p>This function is typically used to iterate through a UTF-8
encoded string.</p>
<p>In case of an invalid UTF-8 seqence, a
<code>utf8::invalid_utf8</code> exception is thrown.</p>
<h4>utf8::previous</h4>
<p>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.</p>
<code>template &lt;typename octet_iterator&gt; uint32_t
previous(octet_iterator&amp; it, octet_iterator pass_start);</code>
<p><code>it</code>: 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.<br>
<code>pass_start</code>: 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.<br>
<u>Return value</u>: the 32 bit representation of the previous code
point.</p>
<p>Example of use:</p>
<pre>
unsigned char twochars[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0x0};
unsigned char* w = twochars + 3;
int cp = previous (w, twochars - 1);
assert (cp == 0x65e5);
assert (w == twochars);
</pre>
<p>The primary purpose of this function is to iterate backwards
through a UTF-8 encoded string. Therefore, <code>it</code> will
typically point to the beginning of a code point, and
<code>pass_start</code> will point to the octet just before the
beginning of the string to ensure we don't go backwards too far.
<code>it</code> 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.</p>
<p>In case <code>pass_end</code> is reached before a UTF-8 lead
octet is hit, or if an invalid UTF-8 sequence is started by the
lead octet, an <code>invalid_utf8</code> exception is thrown</p>
<h4>utf8::advance</h4>
<p>Advances an iterator by the specified number of code points
within an UTF-8 sequence.</p>
<code>template &lt;typename octet_iterator, typename
distance_type&gt; void advance (octet_iterator&amp; it,
distance_type n, octet_iterator end);</code>
<p><code>it</code>: 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.<br>
<code>n</code>: a positive integer that shows how many code points
we want to advance.<br>
<code>end</code>: end of the UTF-8 sequence to be processed. If
<code>it</code> gets equal to <code>end</code> during the
extraction of a code point, an <code>utf8::not_enough_room</code>
exception is thrown.<br></p>
<p>Example of use:</p>
<pre>
unsigned char twochars[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0x0};
unsigned char* w = twochars;
advance (w, 2, twochars + 6);
assert (w == twochars + 5);
</pre>
<p>This function works only "forward". In case of a negative
<code>n</code>, there is no effect.</p>
<p>In case of an invalid code point, a
<code>utf8::invalid_code_point</code> exception is thrown.</p>
<h4>utf8::distance</h4>
<p>Given the iterators to two UTF-8 encoded code points in a
seqence, returns the number of code points between them.</p>
<code>template &lt;typename octet_iterator&gt; typename
std::iterator_traits&lt;octet_iterator&gt;::difference_type
distance (octet_iterator first, octet_iterator last);</code>
<p><code>first</code>: an iterator to a beginning of a UTF-8
encoded code point.<br>
<code>last</code>: 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.<br>
<u>Return value</u> the distance between the iterators, in code
points.</p>
<p>Example of use:</p>
<pre>
unsigned char twochars[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0x0};
size_t dist = utf8::distance(twochars, twochars + 5);
assert (dist == 2);
</pre>
<p>This function is used to find the length (in code points) of a
UTF-8 encoded string. The reason it is called <em>distance</em>,
rather than, say, <em>length</em> is mainly because developers are
used that <em>length</em> 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 <code>std::distance</code> algorithm.</p>
<p>In case of an invalid UTF-8 seqence, a
<code>utf8::invalid_utf8</code> exception is thrown. If
<code>last</code> does not point to the past-of-end of a UTF-8
seqence, a <code>utf8::not_enough_room</code> exception is
thrown.</p>
<h4>utf8::utf16to8</h4>
<p>Converts a UTF-16 encoded string to UTF-8.</p>
<code>template &lt;typename u16bit_iterator, typename
octet_iterator&gt; void utf16to8 (u16bit_iterator start,
u16bit_iterator end, octet_iterator result);</code>
<p><code>start</code>: an iterator pointing to the beginning of the
UTF-16 encoded string to convert.<br>
<code>end</code>: an iterator pointing to pass-the-end of the
UTF-16 encoded string to convert.<br>
<code>result</code>: an output iterator to the place in the UTF-8
string where to append the result of conversion.</p>
<p>Example of use:</p>
<pre>
unsigned short utf16string[] = {0x41, 0x0448, 0x65e5, 0xd834, 0xdd1e};
vector&lt;unsigned char&gt; utf8result;
utf16to8(utf16string, utf16string + 5, back_inserter(utf8result));
assert (utf8result.size() == 10);
</pre>
<p>In case of invalid UTF-16 sequence, a
<code>utf8::invalid_utf16</code> exception is thrown.</p>
<h4>utf8::utf8to16</h4>
<p>Converts an UTF-8 encoded string to UTF-16</p>
<code>template &lt;typename u16bit_iterator, typename
octet_iterator&gt; void utf8to16 (octet_iterator start,
octet_iterator end, u16bit_iterator result);</code>
<p><code>start</code>: an iterator pointing to the beginning of the
UTF-8 encoded string to convert. &lt; br /&gt; <code>end</code>: an
iterator pointing to pass-the-end of the UTF-8 encoded string to
convert.<br>
<code>result</code>: an output iterator to the place in the UTF-16
string where to append the result of conversion.</p>
<p>Example of use:</p>
<pre>
unsigned char utf8_with_surrogates[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88,
0xf0, 0x9d, 0x84, 0x9e};
vector &lt;unsigned short&gt; utf16result;
utf8to16(utf8_with_surrogates, utf8_with_surrogates + 9, back_inserter(utf16result));
assert (utf16result.size() == 4);
assert (utf16result[2] == 0xd834);
assert (utf16result[3] == 0xdd1e);
</pre>
<p>In case of an invalid UTF-8 seqence, a
<code>utf8::invalid_utf8</code> exception is thrown. If
<code>last</code> does not point to the past-of-end of a UTF-8
seqence, a <code>utf8::not_enough_room</code> exception is
thrown.</p>
<h4>utf8::utf32to8</h4>
<p>Converts a UTF-32 encoded string to UTF-8.</p>
<code>template &lt;typename octet_iterator, typename
u32bit_iterator&gt; void utf32to8 (u32bit_iterator start,
u32bit_iterator end, octet_iterator result);</code>
<p><code>start</code>: an iterator pointing to the beginning of the
UTF-32 encoded string to convert.<br>
<code>end</code>: an iterator pointing to pass-the-end of the
UTF-32 encoded string to convert.<br>
<code>result</code>: an output iterator to the place in the UTF-8
string where to append the result of conversion.</p>
<p>Example of use:</p>
<pre>
int utf32string[] = {0x448, 0x65E5, 0x10346, 0};
vector&lt;unsigned char&gt; utf8result;
utf32to8(utf32string, utf32string + 3, back_inserter(utf8result));
assert (utf8result.size() == 9);
</pre>
<p>In case of invalid UTF-32 string, a
<code>utf8::invalid_code_point</code> exception is thrown.</p>
<h4>utf8::utf8to32</h4>
<p>Converts a UTF-8 encoded string to UTF-32.</p>
<code>template &lt;typename octet_iterator, typename
u32bit_iterator&gt; void utf8to32 (octet_iterator start,
octet_iterator end, u32bit_iterator result);</code>
<p><code>start</code>: an iterator pointing to the beginning of the
UTF-8 encoded string to convert.<br>
<code>end</code>: an iterator pointing to pass-the-end of the UTF-8
encoded string to convert.<br>
<code>result</code>: an output iterator to the place in the UTF-32
string where to append the result of conversion.</p>
<p>Example of use:</p>
<pre>
unsigned char twochars[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0x0};
vector&lt;int&gt; utf32result;
utf8to32(twochars, twochars + 5, back_inserter(utf32result));
assert (utf32result.size() == 2);
</pre>
<p>In case of an invalid UTF-8 seqence, a
<code>utf8::invalid_utf8</code> exception is thrown. If
<code>last</code> does not point to the past-of-end of a UTF-8
seqence, a <code>utf8::not_enough_room</code> exception is
thrown.</p>
<h4>utf8::find_invalid</h4>
<p>Detects an invalid sequence within a UTF-8 string.</p>
<code>template &lt;typename octet_iterator&gt; octet_iterator
find_invalid(octet_iterator start, octet_iterator end);</code>
<p><code>start</code>: an iterator pointing to the beginning of the
UTF-8 string to test for validity.<br>
<code>end</code>: an iterator pointing to pass-the-end of the UTF-8
string to test for validity.<br>
<u>Return value</u>: an iterator pointing to the first invalid
octet in the UTF-8 string. In case none were found, equals
<code>end</code>.</p>
<p>Example of use:</p>
<pre>
unsigned char utf_invalid[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0xfa};
unsigned char* invalid = find_invalid(utf_invalid, utf_invalid + 6);
assert (invalid == utf_invalid + 5);
</pre>
<p>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 <em>unchecked</em>
operations on it.</p>
<h4>utf8::is_valid</h4>
<p>Checks whether a sequence of octets is a valid UTF-8 string.</p>
<code>template &lt;typename octet_iterator&gt; bool
is_valid(octet_iterator start, octet_iterator end);</code>
<p><code>start</code>: an iterator pointing to the beginning of the
UTF-8 string to test for validity.<br>
<code>end</code>: an iterator pointing to pass-the-end of the UTF-8
string to test for validity.<br>
<u>Return value</u>: <code>true</code> if the sequence is a valid
UTF-8 string; <code>false</code> if not.</p>
Example of use:
<pre>
unsigned char utf_invalid[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0xfa};
bool bvalid = is_valid(utf_invalid, utf_invalid + 6);
assert (bvalid == false);
</pre>
<p><code>is_valid</code> is a shorthand for
<code>find_invalid(start, end) == end;</code>. 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.</p>
<h4>utf8::is_bom</h4>
<p>Checks whether a sequence of three octets is a UTF-8 byte order
mark (BOM)</p>
<code>template &lt;typename octet_iterator&gt; bool is_bom
(octet_iterator it);</code>
<p><code>it</code> Beginning of the 3-octet sequence to check<br>
<u>Return value</u>: <code>true</code> if the sequence is UTF-8
byte order mark; <code>false</code> if not.</p>
<p>Example of use:</p>
<pre>
unsigned char byte_order_mark[] = {0xef, 0xbb, 0xbf};
bool bbom = is_bom(byte_order_mark);
assert (bbom == true);
</pre>
<p>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.</p>
<h3>Functions From utf8::unchecked Namespace</h3>
<h4>utf8::unchecked::append</h4>
<p>Encodes a 32 bit code point as a UTF-8 sequence of octets and
appends the sequence to a UTF-8 string.</p>
<code>template &lt;typename octet_iterator&gt; octet_iterator
append(uint32_t cp, octet_iterator result);</code>
<p><code>cp</code>: A 32 bit integer representing a code point to
append to the sequence.<br>
<code>result</code>: An output iterator to the place in the
sequence where to append the code point.<br>
<u>Return value</u>: An iterator pointing to the place after the
newly appended sequence.</p>
<p>Example of use:</p>
<pre>
unsigned char u[5] = {0,0,0,0,0};
unsigned char* end = unchecked::append(0x0448, u);
assert (u[0] == 0xd1 &amp;&amp; u[1] == 0x88 &amp;&amp; u[2] == 0 &amp;&amp; u[3] == 0 &amp;&amp; u[4] == 0);
</pre>
<p>This is a quicker but less safe version of
<code>utf8::append</code>. It does not check for validity of the
supplied code point, and may produce an invalid UTF-8 sequence.</p>
<h4>utf8::unchecked::next</h4>
<p>Given the iterator to the beginning of a UTF-8 sequence, it
returns the code point and moves the iterator to the next
position.</p>
<code>template &lt;typename octet_iterator&gt; uint32_t
next(octet_iterator&amp; it);</code>
<p><code>it</code>: 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.<br>
<u>Return value</u>: the 32 bit representation of the processed
UTF-8 code point.</p>
<p>Example of use:</p>
<pre>
unsigned char twochars[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0x0};
unsigned char* w = twochars;
int cp = unchecked::next(w);
assert (cp == 0x65e5);
assert (w == twochars + 3);
</pre>
<p>This is a quicker but less safe version of
<code>utf8::next</code>. It does not check for validity of the
supplied UTF-8 sequence.</p>
<h4>utf8::unchecked::previous</h4>
<p>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.</p>
<code>template &lt;typename octet_iterator&gt; uint32_t
previous(octet_iterator&amp; it);</code>
<p><code>it</code>: 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.<br>
<u>Return value</u>: the 32 bit representation of the previous code
point.</p>
<p>Example of use:</p>
<pre>
unsigned char twochars[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0x0};
unsigned char* w = twochars + 3;
int cp = unchecked::previous (w);
assert (cp == 0x65e5);
assert (w == twochars);
</pre>
<p>This is a quicker but less safe version of
<code>utf8::previous</code>. It does not check for validity of the
supplied UTF-8 sequence and offers no boundary checking.</p>
<h4>utf8::unchecked::advance</h4>
<p>Advances an iterator by the specified number of code points
within an UTF-8 sequence.</p>
<code>template &lt;typename octet_iterator, typename
distance_type&gt; void advance (octet_iterator&amp; it,
distance_type n);</code>
<p><code>it</code>: 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.<br>
<code>n</code>: a positive integer that shows how many code points
we want to advance.<br></p>
<p>Example of use:</p>
<pre>
unsigned char twochars[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0x0};
unsigned char* w = twochars;
unchecked::advance (w, 2);
assert (w == twochars + 5);
</pre>
<p>This function works only "forward". In case of a negative
<code>n</code>, there is no effect.</p>
<p>This is a quicker but less safe version of
<code>utf8::advance</code>. It does not check for validity of the
supplied UTF-8 sequence and offers no boundary checking.</p>
<h4>utf8::unchecked::distance</h4>
<p>Given the iterators to two UTF-8 encoded code points in a
seqence, returns the number of code points between them.</p>
<code>template &lt;typename octet_iterator&gt; typename
std::iterator_traits&lt;octet_iterator&gt;::difference_type
distance (octet_iterator first, octet_iterator last);</code>
<p><code>first</code>: an iterator to a beginning of a UTF-8
encoded code point.<br>
<code>last</code>: 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.<br>
<u>Return value</u> the distance between the iterators, in code
points.</p>
<p>Example of use:</p>
<pre>
unsigned char twochars[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0x0};
size_t dist = utf8::unchecked::distance(twochars, twochars + 5);
assert (dist == 2);
</pre>
<p>This is a quicker but less safe version of
<code>utf8::distance</code>. It does not check for validity of the
supplied UTF-8 sequence.</p>
<h4>utf8::unchecked::utf16to8</h4>
<p>Converts a UTF-16 encoded string to UTF-8.</p>
<code>template &lt;typename u16bit_iterator, typename
octet_iterator&gt; void utf16to8 (u16bit_iterator start,
u16bit_iterator end, octet_iterator result);</code>
<p><code>start</code>: an iterator pointing to the beginning of the
UTF-16 encoded string to convert.<br>
<code>end</code>: an iterator pointing to pass-the-end of the
UTF-16 encoded string to convert.<br>
<code>result</code>: an output iterator to the place in the UTF-8
string where to append the result of conversion.</p>
<p>Example of use:</p>
<pre>
unsigned short utf16string[] = {0x41, 0x0448, 0x65e5, 0xd834, 0xdd1e};
vector&lt;unsigned char&gt; utf8result;
unchecked::utf16to8(utf16string, utf16string + 5, back_inserter(utf8result));
assert (utf8result.size() == 10);
</pre>
<p>This is a quicker but less safe version of
<code>utf8::utf16to8</code>. It does not check for validity of the
supplied UTF-16 sequence.</p>
<h4>utf8::unchecked::utf8to16</h4>
<p>Converts an UTF-8 encoded string to UTF-16</p>
<code>template &lt;typename u16bit_iterator, typename
octet_iterator&gt; void utf8to16 (octet_iterator start,
octet_iterator end, u16bit_iterator result);</code>
<p><code>start</code>: an iterator pointing to the beginning of the
UTF-8 encoded string to convert. &lt; br /&gt; <code>end</code>: an
iterator pointing to pass-the-end of the UTF-8 encoded string to
convert.<br>
<code>result</code>: an output iterator to the place in the UTF-16
string where to append the result of conversion.</p>
<p>Example of use:</p>
<pre>
unsigned char utf8_with_surrogates[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88,
0xf0, 0x9d, 0x84, 0x9e};
vector &lt;unsigned short&gt; 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);
</pre>
<p>This is a quicker but less safe version of
<code>utf8::utf8to16</code>. It does not check for validity of the
supplied UTF-8 sequence.</p>
<h4>utf8::unchecked::utf32to8</h4>
<p>Converts a UTF-32 encoded string to UTF-8.</p>
<code>template &lt;typename octet_iterator, typename
u32bit_iterator&gt; void utf32to8 (u32bit_iterator start,
u32bit_iterator end, octet_iterator result);</code>
<p><code>start</code>: an iterator pointing to the beginning of the
UTF-32 encoded string to convert.<br>
<code>end</code>: an iterator pointing to pass-the-end of the
UTF-32 encoded string to convert.<br>
<code>result</code>: an output iterator to the place in the UTF-8
string where to append the result of conversion.</p>
<p>Example of use:</p>
<pre>
int utf32string[] = {0x448, 0x65E5, 0x10346, 0};
vector&lt;unsigned char&gt; utf8result;
utf32to8(utf32string, utf32string + 3, back_inserter(utf8result));
assert (utf8result.size() == 9);
</pre>
<p>This is a quicker but less safe version of
<code>utf8::utf32to8</code>. It does not check for validity of the
supplied UTF-32 sequence.</p>
<h4>utf8::unchecked::utf8to32</h4>
<p>Converts a UTF-8 encoded string to UTF-32.</p>
<code>template &lt;typename octet_iterator, typename
u32bit_iterator&gt; void utf8to32 (octet_iterator start,
octet_iterator end, u32bit_iterator result);</code>
<p><code>start</code>: an iterator pointing to the beginning of the
UTF-8 encoded string to convert.<br>
<code>end</code>: an iterator pointing to pass-the-end of the UTF-8
encoded string to convert.<br>
<code>result</code>: an output iterator to the place in the UTF-32
string where to append the result of conversion.</p>
<p>Example of use:</p>
<pre>
unsigned char twochars[] = {0xE6, 0x97, 0xA5, 0xd1, 0x88, 0x0};
vector&lt;int&gt; utf32result;
unchecked::utf8to32(twochars, twochars + 5, back_inserter(utf32result));
assert (utf32result.size() == 2);
</pre>
<p>This is a quicker but less safe version of
<code>utf8::utf8to32</code>. It does not check for validity of the
supplied UTF-8 sequence.</p>
<h2>Points of interest</h2>
<h4>Design goals and decisions</h4>
<p>The library was designed to be:</p>
<ol>
<li>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.</li>
<li>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.</li>
<li>Lightweight: follow the "pay only for what you use"
guidline.</li>
<li>Unintrusive: avoid forcing any particular design or even
programming style on the user. This is a library, not a
framework.</li>
</ol>
<h4>Alternatives</h4>
<p>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:</p>
<ol>
<li><a href="http://icu.sourceforge.net/">ICU Library</a>. 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.</li>
<li><a href=
"http://www.gtkmm.org/gtkmm2/docs/tutorial/html/ch03s04.html">Glib::ustring</a>.
A class specifically made to work with UTF-8 strings, and also feel
like <code>std::string</code>. 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.</li>
<li>Platform dependent solutions: Windows and POSIX have functions
to convert strings from one encoding to another. That is only a
subset of what my library offers, but if that is all you need it
may be good enough, especially given the fact that these functions
are mature and tested in production.</li>
</ol>
<h2>Conclusion</h2>
<p>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.</p>
<h2>References</h2>
<ol>
<li><a href="http://www.unicode.org/">The Unicode
Consortium</a>.</li>
<li><a href="http://icu.sourceforge.net/">ICU Library</a>.</li>
<li><a href="http://en.wikipedia.org/wiki/UTF-8">UTF-8 at
Wikipedia</a></li>
</ol>
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