Add to_chars for writing uuid strings without allocating

doc/uuid.html

@@ -62,6 +62,7 @@
         <ul>
             <li><a href="#Synopsis_io">Synopsis</a></li>
             <li><a href="#Stream_operators">Stream Operators</a></li>
+            <li><a href="#to_chars">To Chars</a></li>
             <li><a href="#to_string">To String</a></li>
         </ul>
         <li><a href="#boost/uuid/uuid_serialize.hpp">boost/uuid/uuid_serialize.hpp</a></li>
@@ -722,6 +723,11 @@ template &lt;typename ch, typename char_traits&gt;
 template &lt;typename ch, typename char_traits&gt;
     std::basic_istream&lt;ch, char_traits&gt;&amp; operator&gt;&gt;(std::basic_istream&lt;ch, char_traits&gt; &amp;is, uuid &amp;u);
 
+template&lt;class OutputIterator&gt;
+    OutputIterator to_chars(uuid const&amp; u, OutputIterator out);
+
+bool to_chars(uuid const&amp; u, char* first, char* last);
+
 std::string to_string(uuid const&amp; u);
 std::wstring to_wstring(uuid const&amp; u);
 
@@ -757,6 +763,21 @@ boost::uuids::uuid u2 = boost::lexical_cast&lt;boost::uuids::uuid&gt;(s);
 assert(u1 == u2);
 </pre>
 
+<h4><a name="to_chars">To Chars</a></h4>
+<p>
+The function <tt>to_chars</tt> is a fast non-allocating (and non-throwing if the
+iterator does not throw) function to write a <b>uuid</b> to a string buffer.
+It writes exactly 36 characters to the iterator on success (not null-terminated).
+<pre>
+boost::uuids::uuid u; // initialize uuid
+
+char buf[36];
+boost::uuids::to_chars(u, buf);
+// OR
+bool ret = boost::uuids::to_chars(u, std::begin(buf), std::end(buf));
+assert(ret);
+</pre>
+
 <h4><a name="to_string">To String</a></h4>
 <p>
 The functions <tt>to_string</tt> and <tt>to_wstring</tt> are provided as a
@@ -845,8 +866,8 @@ mailing list provided useful comments and greatly helped to shape the library.
 <p>Revised November 8, 2017</p>
 
 <hr>
-<p>© Copyright Andy Tompkins, 2006</p>
-<p>© Copyright 2017 James E. King III</p>
+<p>© Copyright Andy Tompkins, 2006</p>
+<p>© Copyright 2017 James E. King III</p>
 <p> Distributed under the Boost Software
 License, Version 1.0. (See accompanying file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or copy at <a href="https://www.boost.org/LICENSE_1_0.txt">
 www.boost.org/LICENSE_1_0.txt</a>)</p>

include/boost/uuid/uuid_io.hpp

@@ -146,23 +146,38 @@ inline wchar_t to_wchar(size_t i) {
 
 } // namespace detail
 
-inline std::string to_string(uuid const& u)
+template<class OutputIterator>
+OutputIterator to_chars(uuid const& u, OutputIterator out)
 {
-    std::string result;
-    result.reserve(36);
-
     std::size_t i=0;
     for (uuid::const_iterator it_data = u.begin(); it_data!=u.end(); ++it_data, ++i) {
         const size_t hi = ((*it_data) >> 4) & 0x0F;
-        result += detail::to_char(hi);
+        *out++ = detail::to_char(hi);
 
         const size_t lo = (*it_data) & 0x0F;
-        result += detail::to_char(lo);
+        *out++ = detail::to_char(lo);
 
         if (i == 3 || i == 5 || i == 7 || i == 9) {
-            result += '-';
+            *out++ = '-';
         }
     }
+    return out;
+}
+
+inline bool to_chars(uuid const& u, char* first, char* last)
+{
+    if (last - first < 36) {
+        return false;
+    }
+    to_chars(u, first);
+    return true;
+}
+
+inline std::string to_string(uuid const& u)
+{
+    std::string result(36, char());
+    // string::data() returns const char* before C++17
+    to_chars(u, &result[0]);
     return result;
 }
 

test/test_io.cpp

@@ -147,6 +147,39 @@ int main(int, char*[])
 
 #endif
 
+    {
+        char buf[36] = {};
+        BOOST_TEST(to_chars(u3, &buf[0], &buf[36]));
+        BOOST_TEST(std::string(buf, 36) == std::string("12345678-90ab-cdef-1234-567890abcdef"));
+    }
+    {
+        char buf[37] = {}; // will be null-terminated
+        BOOST_TEST(to_chars(u3, &buf[0], &buf[37]));
+        BOOST_TEST(std::string(buf) == std::string("12345678-90ab-cdef-1234-567890abcdef"));
+    }
+    {
+        char buf[35] = {};
+        BOOST_TEST(!to_chars(u3, &buf[0], &buf[35]));
+    }
+    {
+        char buf[36] = {};
+        char* ret = to_chars(u3, &buf[0]);
+        BOOST_TEST(ret == &buf[36]);
+        BOOST_TEST(std::string(buf, 36) == std::string("12345678-90ab-cdef-1234-567890abcdef"));
+    }
+    {
+        char buf[36] = {};
+        char* ret = to_chars(u3, buf); // decay
+        BOOST_TEST(ret == &buf[36]);
+        BOOST_TEST(std::string(buf, 36) == std::string("12345678-90ab-cdef-1234-567890abcdef"));
+    }
+    {
+        char buf[37] = {}; // will be null-terminated
+        char* ret = to_chars(u3, buf);
+        BOOST_TEST(ret == &buf[36]);
+        BOOST_TEST(std::string(buf) == std::string("12345678-90ab-cdef-1234-567890abcdef"));
+    }
+
     BOOST_TEST(to_string(u1) == std::string("00000000-0000-0000-0000-000000000000"));
     BOOST_TEST(to_string(u3) == std::string("12345678-90ab-cdef-1234-567890abcdef"));