Hello Hartmut,<div><br></div><div>as discussed @irc channel I am posting the example of my problem (I use boost 1.47):</div><div><br></div><div><br></div><div><div><div>#include &lt;boost/config/warning_disable.hpp&gt;</div>

<div>#include &lt;boost/spirit/include/karma.hpp&gt;</div><div>#include &lt;boost/spirit/include/karma_char_class.hpp&gt;</div><div><br></div><div>#include &lt;boost/fusion/include/std_pair.hpp&gt;</div><div><br></div><div>

#include &lt;iterator&gt;</div><div>#include &lt;cassert&gt;</div><div>#include &lt;utility&gt;</div><div>#include &lt;string&gt;</div><div><br></div><div>namespace karma = boost::spirit::karma;</div><div><br></div><div>
typedef std::pair&lt;unsigned char, unsigned char&gt; utf16_t;</div>
<div><br></div><div>template&lt;class OutIter&gt;</div><div>struct json_char_encoder2</div><div>  : boost::spirit::karma::grammar&lt;OutIter, std::string()&gt;</div><div>{</div><div>  json_char_encoder2()</div><div>    : json_char_encoder2::base_type(encoded_)</div>

<div>  {</div><div>    json_sym_.add</div><div>      (&#39;&quot;&#39;, &quot;\\\&quot;&quot;)</div><div>      (&#39;/&#39;, &quot;\\/&quot;)</div><div>      (&#39;\\&#39;, &quot;\\\\&quot;)</div><div>      (&#39;\b&#39;, &quot;\\b&quot;)</div>

<div>      (&#39;\f&#39;, &quot;\\f&quot;)</div><div>      (&#39;\n&#39;, &quot;\\n&quot;)</div><div>      (&#39;\r&#39;, &quot;\\r&quot;)</div><div>      (&#39;\t&#39;, &quot;\\t&quot;)</div><div>    ;</div><div><br></div>

<div>    using boost::spirit::iso8859_1::print;</div><div><br></div><div>    encoded_=*(json_sym_ | print | uchar_);</div><div>    uchar_ = karma::lit(&quot;\\u&quot;) &lt;&lt; karma::hex &lt;&lt; karma::hex;  //also tried to use unicode_gen_;</div>

<div>  }</div><div><br></div><div>  karma::rule&lt;OutIter, std::string()&gt; encoded_;</div><div>  karma::symbols&lt;char, char const*&gt; json_sym_;</div><div>  karma::rule&lt;OutIter, utf16_t()&gt; uchar_;</div><div>  karma::uint_generator&lt;unsigned short, 16&gt; unicode_gen_;</div>

<div>};</div><div><br></div><div>int main()</div><div>{</div><div>  assert(2==sizeof(short));</div><div><br></div><div>  typedef std::back_insert_iterator&lt;std::string&gt; iter_t;</div><div>  json_char_encoder2&lt;iter_t&gt; encoder2;</div>

<div><br></div><div>  std::string generated;</div><div>  iter_t sink(generated);</div><div><br></div><div>  std::string sinput=&quot;abc&quot;;</div><div>  sinput.push_back(char(0xAA));</div><div>  sinput.push_back(char(0xBB));</div>

<div>  bool result = karma::generate(sink, encoder2, sinput);</div><div><br></div><div>  return 0;</div><div>}</div></div></div><div><br></div><div><br></div><div>The problem here is that I would like the generator in case of uchar_ to retrieve 2 subsequent string bytes and write them out as \uXXXX as specified at JSON.org. What I get is:</div>

<div>a. IF: replacing utf16_t with unsigned short =&gt; and removing 1 of the karma::hex terminals =&gt; 2 calls where each byte is converted to unsigned short</div><div>b. IF: having the source as it is =&gt; fails the generation, where the generated string is empty.</div>

<div><br></div><div>One strange behavior as well: result is true for case b. Why?</div><div><br></div><div><br></div><div>Regards,</div><div>Ovanes</div><div><br></div><div><br></div>