Implemented primitive optional parameter support for external functions

* renamed FunctionExternalTextFormatter into FunctionExternalCommand
** the goal is to provide a general interface to a variety of external commands
*** e.g. not just text formatters but system utilities for file management and so on
** this requires the stdin parameter to be optional as not all external commands require stdin input
* implemented "filter_derived" helper template to determine amount of optional parameters
** optional parameters are defined as "boost::optional" specializations
*** they in turn can be detected by checking if "boost::optional_detail::optional_tag" is a base class
* "callConstructDocument" member method of "FunctionBase" performs additional bounds checking of parameter vector
* "boost::optional<std::string>" specific member overload was added to "XObjectValue" helper class
** we will have to provide full specializations for all optional types as C++ prohibits partial member function template specialization
* renamed the external function in "PlattformGuard"
* changed README.md and test cases accordingly

1 files changed, 12 insertions, 1 deletions

diff --git a/src/support/type/xobject_value.cc b/src/support/type/xobject_value.cc
index 9bb6648..e457a69 100644
--- a/src/support/type/xobject_value.cc
+++ b/src/support/type/xobject_value.cc
@@ -4,7 +4,8 @@
 #include <xalanc/XalanDOM/XalanDocumentFragment.hpp>
 
 #include <boost/algorithm/string.hpp>
-#include "boost/filesystem.hpp"
+#include <boost/filesystem.hpp>
+#include <boost/optional.hpp>
 
 #include <string>
 
@@ -27,6 +28,16 @@ std::string XObjectValue::get<std::string>(
 }
 
 template <>
+boost::optional<std::string> XObjectValue::get<boost::optional<std::string>>(
+	const xalan::XObjectPtr& ptr) const {
+	if ( ptr.null() ) {
+		return boost::optional<std::string>();
+	} else {
+		return this->get<std::string>(ptr);
+	}
+}
+
+template <>
 boost::filesystem::path XObjectValue::get<boost::filesystem::path>(
 	const xalan::XObjectPtr& ptr) const {
 	const std::string rawPath(