Use explicit streaming ops instead of Boost.LexicalCast

doc/example/patterns_quick_constraint.cpp

@@ -35,13 +35,15 @@ namespace
 //]
 
 //[ quick_constraint_solution
-#include <boost/lexical_cast.hpp>
+#include <sstream>
 
 namespace // in the same namespace as 'my_class'
 {
     bool operator==( const my_class& actual, const std::string& expected ) // the first part of the trick is to compare to a string
     {
-        return boost::lexical_cast< std::string >( actual ) == expected;
+        std::ostringstream s;
+        s << actual;
+        return s.str() == expected;
     }
 } // mock