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

src/shared/Registry.cpp

+#include "Registry.h"
+#include <memory>
+
+using namespace AdblockPlus;
+
+static HKEY predefined_hkeys[5] =
+{
+  HKEY_CLASSES_ROOT,
+  HKEY_CURRENT_CONFIG,
+  HKEY_CURRENT_USER,
+  HKEY_LOCAL_MACHINE,
+  HKEY_USERS
+};
+
+Registry_Key::Registry_Key(Predefined root, std::wstring key_name)
+  : is_predefined(key_name.empty())
+{
+  auto predefined_key(predefined_hkeys[(unsigned int) root ]);
+  if (is_predefined)
+  {
+    key = predefined_key;
+    return;
+  }
+  HRESULT hr = RegOpenKeyExW(predefined_key, key_name.c_str(), 0, KEY_QUERY_VALUE, &key);
+  if (hr != ERROR_SUCCESS || !key)
+  {
+    throw std::runtime_error("Failure in RegOpenKeyExW");
+  }
+}
+
+Registry_Key::Registry_Key(Predefined root)
+  : key(predefined_hkeys[(unsigned int) root ]), is_predefined(true)
+{
+}
+
+Registry_Key::~Registry_Key()
+{
+  if (!is_predefined)
+  {
+    RegCloseKey(key);
+  }
+}
+
+std::wstring Registry_Key::value_wstring(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'.
+   */
+  // Round the byte size up to the nearest multiple of two.
+  size = (size + 1) & ~ DWORD(1);
+  size_t psize = size >> 1;
+  // 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);
+}
+