You should use karma::_val
. For example you can write some binders (simple example)
<< lit("<id>")
<< karma::string[_1 = phoenix::bind(&Item::getId, karma::_val)]
<< lit("</id>") // Item ID
<< lit("<values>")
<< (int_ % ';')[_1 = phoenix::bind(&Item::getValues, karma::_val)]
<< lit("</values>") // List of Item values