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

5 files changed, 39 insertions, 4 deletions

diff --git a/src/support/filesystem_context.h b/src/support/filesystem_context.h
index f3875a3..c00fe29 100644
--- a/src/support/filesystem_context.h
+++ b/src/support/filesystem_context.h
@@ -4,7 +4,7 @@
 #include <xalanc/XalanDOM/XalanDOMString.hpp>
 #include <xalanc/XPath/Function.hpp>
 
-#include "boost/filesystem.hpp"
+#include <boost/filesystem.hpp>
 
 #include <string>
 #include <functional>
diff --git a/src/support/include_entity_resolver.cc b/src/support/include_entity_resolver.cc
index deac1d5..197b217 100644
--- a/src/support/include_entity_resolver.cc
+++ b/src/support/include_entity_resolver.cc
@@ -2,7 +2,7 @@
 
 #include <xercesc/framework/LocalFileInputSource.hpp>
 
-#include "boost/filesystem.hpp"
+#include <boost/filesystem.hpp>
 
 #include "support/xalan_string.h"
 #include "support/xerces_string_guard.h"
diff --git a/src/support/include_entity_resolver.h b/src/support/include_entity_resolver.h
index ea5fb7a..e7c5542 100644
--- a/src/support/include_entity_resolver.h
+++ b/src/support/include_entity_resolver.h
@@ -4,7 +4,7 @@
 #include <xercesc/sax/EntityResolver.hpp>
 #include <xercesc/sax/InputSource.hpp>
 
-#include "boost/optional.hpp"
+#include <boost/optional.hpp>
 
 #include <string>
 #include <vector>
diff --git a/src/support/type/filter.h b/src/support/type/filter.h
new file mode 100644
index 0000000..4a0e19f
--- /dev/null
+++ b/src/support/type/filter.h
@@ -0,0 +1,24 @@
+#ifndef INPUTXSLT_SRC_SUPPORT_TYPE_FILTER_H_
+#define INPUTXSLT_SRC_SUPPORT_TYPE_FILTER_H_
+
+#include <tuple>
+#include <type_traits>
+
+namespace InputXSLT {
+
+template <
+	typename BaseReference,
+	typename Head,
+	typename... Tail
+>
+struct filter_derived {
+	typedef typename std::conditional<
+		std::is_base_of<BaseReference, Head>::value,
+		std::tuple<Head, Tail...>,
+		std::tuple<Tail...>
+	>::type type;
+};
+
+}
+
+#endif  // INPUTXSLT_SRC_SUPPORT_TYPE_FILTER_H_
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(