Issue #41 - Bring method of determining IE version up to date

src/shared/Registry.cpp

+#include "Registry.h"
+#include <memory>
+
+using namespace AdblockPlus;
+
+RegistryKey::RegistryKey(HKEY parent, const std::wstring& key_name)
+{
+  if (key_name.empty())
+  {
+    throw std::runtime_error("key_name may not be empty");
+  }
+  HRESULT hr = RegOpenKeyExW(parent, key_name.c_str(), 0, KEY_QUERY_VALUE, &key);
+  if (hr != ERROR_SUCCESS || !key)
+  {
+    throw std::runtime_error("Failure in RegOpenKeyExW");
+  }
+}
+
+RegistryKey::~RegistryKey()
+{
+  RegCloseKey(key);
+}
+
+std::wstring RegistryKey::value_wstring(const std::wstring& name) const
+{
+  /*
+   * Step one is to determine the presence of the value, along with its type and byte size.
+   */
+  DWORD type;
+  DWORD size = 0;
+  HRESULT hr = ::RegQueryValueExW(static_cast<HKEY>(key), name.c_str(), 0, &type, 0, &size);
+  if (hr != ERROR_SUCCESS)
+  {
+    throw std::runtime_error("Failure in RegQueryValueEx to query name");
+  }
+  if (type != REG_SZ)
+  {
+    throw std::runtime_error("Value is not string type");
+  }
+
+  /*
+   * Step two is to allocate a buffer for the string and query for its value.
+   * Note that 'size' is the size in bytes, which we need for the system call,
+   *   but that 'psize' is the size in words, which we need to manipulate the wchar_t array 'p'.
+   */
+  size_t psize = (size + 1)/2;
+  // Round the byte size up to the nearest multiple of two.
+  size = 2 * psize;
+  // We need to allocate a temporary buffer to receive the value, because there's no interface to write directly into the buffer of std::basic_string.
+  std::unique_ptr<wchar_t[]> p(new wchar_t[psize]);
+  hr = RegQueryValueExW(key, name.c_str(), 0, 0, reinterpret_cast<BYTE*>(p.get()), &size);
+  if (hr != ERROR_SUCCESS)
+  {
+    throw std::runtime_error("Failure in RegQueryValueExW to retrieve value");
+  }
+
+  /*
+   * Step three is to construct a return string.
+   *
+   * There's the possibility of an extra terminating null character in the return value of the query.
+   * If so, we have to decrement the length of the return value to eliminate it.
+   * If it persists, it will interfere with later string operations such as concatenation.
+   */
+  if (p[psize - 1] == L'\0')
+  {
+    --psize;
+  }
+  return std::wstring(p.get(), psize);
+}
+