[C++] Iterator Patterns 例子

Autostart registry entries: contains values used to load applications when Windows is started.

Example: TROJ_DASMIN.C 在 HKLM\Software\Microsoft\Windows\CurrentVersion\Run 寫入 MSConfigr = "%System%\JDBGMRG.EXE" 和 VirusCheckII = "%System%\AVIRCHK.EXE"，因此每次系統啟動時，TROJ_DASMIN.C 就會被執行。

怎麼找出這些威脅呢？當然就是列舉所有的 autostart registry entries。這讓我想到 iterator patterns: provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. [GoF]

有想法之後，該怎麼用 C++ 實做出來呢？首先先偷 [chrome]/trunk/src/base/win/registry.h，稍微修改一下，接著寫自己的 autostart_reg_iterator.h:

// The author disclaims copyright to this source code.

#ifndef MODULE_AUTOSTART_REG_ITERATOR_H_
#define MODULE_AUTOSTART_REG_ITERATOR_H_
#pragma once

#include <windows.h>

#include "base/basictypes.h"

#include <vector>

namespace base {
namespace win {
class RegValueIterator;
}
}

namespace module {

// Iterates the autostart entries.
class AutostartRegIterator {
 public:
  AutostartRegIterator();

  ~AutostartRegIterator();

  DWORD ValueCount() const;

  // True while the iterator is valid.
  bool Valid() const;

  // Advances to the next entry.
  void Next();

  const wchar_t* Key() const;
  const wchar_t* Name() const;
  const wchar_t* Value() const;
  DWORD ValueSize() const;
  DWORD Type() const;

 private:
  // All iterators of autostart registry iterator.
  std::vector<base::win::RegValueIterator*> iterators_;

  // Current index of iterators. 
  int index_;

  DISALLOW_COPY_AND_ASSIGN(AutostartRegIterator);
};

}  // namespace module

#endif  // MODULE_AUTOSTART_REG_ITERATOR_H_

底下是 .cc 檔:

// The author disclaims copyright to this source code.

#include "module/autostart_reg_iterator.h"

#include "module/reg_value_iterator_factory.h"
#include "base/win/reg.h"

namespace module {

using base::win::RegValueIterator;

const wchar_t* kAutostartRegKey[] = { ... };

AutostartRegIterator::AutostartRegIterator() {
  RegValueIteratorFactory factory;
  for (int i = 0; i != arraysize(kAutostartRegKey); ++i) {
    iterators_.push_back(factory.Create(kAutostartRegKey[i]));
  }
  index_ = iterators_.size() - 1;
}

AutostartRegIterator::~AutostartRegIterator() {
  for (int i = 0; i != arraysize(kAutostartRegKey); ++i) {
    delete iterators_.at(i);
  }
  iterators_.clear();
}

DWORD AutostartRegIterator::ValueCount() const {
  DWORD count = 0;
  for (int i = 0; i != arraysize(kAutostartRegKey); ++i) {
    count += iterators_.at(i)->ValueCount();
  }
  return count;
}

bool AutostartRegIterator::Valid() const {
  return index_ > 0 || iterators_.at(0)->Valid();
}

void AutostartRegIterator::Next() {
  if (index_ > 0 && !iterators_.at(index_)->Valid()) {
    --index_;
  } else {
    iterators_.at(index_)->Next();
  }
}

const wchar_t* AutostartRegIterator::Key() const {
  return kAutostartRegKey[index_];
}

const wchar_t* AutostartRegIterator::Name() const { 
  return iterators_.at(index_)->Name();
}

const wchar_t* AutostartRegIterator::Value() const { 
  return iterators_.at(index_)->Value();
}

DWORD AutostartRegIterator::ValueSize() const { 
  return iterators_.at(index_)->ValueSize();
}

DWORD AutostartRegIterator::Type() const { 
  return iterators_.at(index_)->Type(); 
}

}  // namespace module

有些細節就略過了，重點就是如何用大的 iterator 包住小的 base::win::RegValueIterator iterators，如何實做 multiple traversals of aggregate objects。