Issue #276 - introduce classes BSTR_ParamArgument and BSTR_ParamResult

src/plugin/COM_Value.cpp

Index: src/plugin/COM_Value.cpp
===================================================================
new file mode 100644
--- /dev/null
+++ b/src/plugin/COM_Value.cpp
@@ -0,0 +1,70 @@
+/**
+ * \file COM_Value.cpp Support for the values used in COM and Automation.
+ */
+#include "COM_Value.h"
+#include <stdexcept>
+
+using namespace AdblockPlus::COM;
+
+/*
+ * MSDN SysFreeString function http://msdn.microsoft.com/en-us/library/windows/desktop/ms221481%28v=vs.85%29.aspx
+ */
+BSTR_Param::~BSTR_Param()
+{
+  // There's no need to check for null; SysFreeString already handles that case.
+  SysFreeString(bstr);
+}
+
+namespace {
+  /*
+   * MSDN SysAllocStringLen function http://msdn.microsoft.com/en-us/library/windows/desktop/ms221639%28v=vs.85%29.aspx
+   */
+  BSTR construct_BSTR_ParamArgument(const std::wstring& s)
+  {
+    auto length = s.length();
+    if (length == 0)
+    {
+      /*
+       * BSTR makes no distinction between null pointers and non-null pointers to the empty string.
+       * In order to detect allocation failure reliably, we simply don't allocate empty strings.
+       * Compare with the default constructor.
+       */
+      return nullptr;
+    }
+    // Assert length >= 1
+    BSTR b = SysAllocStringLen(s.c_str(), length);
+    /*
+     * Since the string is non-empty,
+     *   if the return value of SysAllocStringLen is null it means that it had an allocation failure.
+     */
+    if (!b)
+    {
+      throw std::bad_alloc();
+    }
+    return b;
+  }
+}
+
+BSTR_ParamArgument::BSTR_ParamArgument(const std::wstring& s)
+  : BSTR_Param(construct_BSTR_ParamArgument(s))
+{}
+
+BSTR* BSTR_ParamResult::operator&()
+{
+  // Assign the empty string, if not already empty.
+  if (bstr)
+  {
+    SysFreeString(bstr);
+    bstr = nullptr;
+  }
+  return &bstr;
+}
+
+BSTR_ParamResult::operator std::wstring() const
+{
+  if (!bstr)
+  {
+    return std::wstring();
+  }
+  return std::wstring(bstr, SysStringLen(bstr));
+}