我的项目有问题。我相信来自新c ++ stabdards的共享/弱指针可以解决它，但我不清楚如何。在我读到的关于标准库的书中，没有评论我的想法如何使用这些指针。
在我的应用程序中，我有Library课程。此类具有内部vector个Book个对象。这些对象通常在Library类中创建，但外部代码使用函数从中查询Books对象，并在以后使用它们。每个Book实例都有一个指向“host”Library实例的公共指针。
有时外部代码会删除Library并创建新代码，但它仍然可以使用旧库中以前查询过的Book个对象。我的问题是Book类应该删除不再有效的指针，因此book不会包含指向已删除库实例的无效指针。
我不清楚如何实现书类的弱指针来实现这种行为。
答案 0 :(得分：1)
有可能。
由于Library需要知道如何添加图书，因此需要知道如何为自己获取weak_ptr。幸运的是，有一个名为enable_shared_from_this的辅助基类允许你这样做。
#include <memory>
#include <iostream>
#include <vector>
class Library;
struct Book {
std::weak_ptr<Library> host_library;
};
class Library : public std::enable_shared_from_this<Library> {
std::vector<Book> books;
public:
const Book& getBook(size_t index) const { return books.at(index); }
void addBook() {
books.emplace_back();
books.back().host_library = shared_from_this();
}
};
int main() {
auto library = std::make_shared<Library>();
library->addBook();
auto book_copy = library->getBook(0);
library.reset(); // Destroy the library
auto old_library = book_copy.host_library.lock(); // Try to get a shared_ptr
if (!old_library) // to the library from the book.
std::cout << "Library gone!\n";
}
要使其正常工作，Library始终需要创建为shared_ptr。您可以通过使构造函数为私有并具有返回shared_ptr的静态创建函数来强制执行该操作。
答案 1 :(得分：0)
我认为弱点不会帮助你。
而是更好地实现Library类。当然图书馆应该知道哪些书属于它。如果删除库，它应该清理其内存并更新属于它的所有对象。
删除库后，本书会发生什么？
而是使用public class Categorii_LIst_Item_Clicked extends ActionBarActivity {
static Context context;
static Bundle extras;
SectionsPagerAdapter mSectionsPagerAdapter;
static ImageLoader imageLoader;
static DisplayImageOptions options;
ViewPager mViewPager;
@Override
protected void onCreate (Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_categorii__list__item__clicked);
context = this;
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
extras = getIntent().getExtras();
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mSectionsPagerAdapter);
//Setup the ImageLoader, we'll use this to display our images
ImageLoaderConfiguration config = new ImageLoaderConfiguration.Builder(this).build();
imageLoader = ImageLoader.getInstance();
imageLoader.init(config);
//Setup options for ImageLoader so it will handle caching for us.
options = new DisplayImageOptions.Builder()
.cacheInMemory()
.cacheOnDisc()
.build();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_categorii__list__item__clicked, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
public class SectionsPagerAdapter extends FragmentPagerAdapter {
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
if (position == 0) {
return PlaceholderFragment.newInstance(position);
} else if (position == 1) {
return VideoFragment.newInstance1(position);
}
return PlaceholderFragment.newInstance(position);
}
@Override
public int getCount() {
return 2;
}
@Override
public CharSequence getPageTitle(int position) {
Locale l = Locale.getDefault();
switch (position) {
case 0:
return getString(R.string.title_section4).toUpperCase(l);
case 1:
return getString(R.string.title_section5).toUpperCase(l);
}
return null;
}
}
public static class PlaceholderFragment extends Fragment {
private static final String ARG_SECTION_NUMBER = "section_number";
public static PlaceholderFragment newInstance(int sectionNumber) {
PlaceholderFragment fragment = new PlaceholderFragment();
Bundle args = new Bundle();
args.putInt(ARG_SECTION_NUMBER, sectionNumber);
fragment.setArguments(args);
return fragment;
}
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_list_item_clicked, container, false);
TextView pDate = (TextView) rootView.findViewById(R.id.textView);
pDate.setText(extras.getString("pdate"));
TextView ptitle = (TextView) rootView.findViewById(R.id.section_label);
ptitle.setText(extras.getString("pname"));
TextView pnText = (TextView) rootView.findViewById(R.id.textView2);
pnText.setText(extras.getString("pText"));
//Setup a listener we can use to swtich from the loading indicator to the Image once it's ready
ImageLoadingListener listener = new ImageLoadingListener() {
@Override
public void onLoadingStarted(String arg0, View arg1) {
// TODO Auto-generated method stub
}
@Override
public void onLoadingCancelled(String arg0, View arg1) {
// TODO Auto-generated method stub
}
@Override
public void onLoadingComplete(String arg0, View arg1, Bitmap arg2) {
// i/ndicator.setVisibility(View.INVISIBLE);
// iconImg.setVisibility(View.VISIBLE);
}
@Override
public void onLoadingFailed(String arg0, View arg1, FailReason arg2) {
// TODO Auto-generated method stub
}
};
//Load the image and use our options so caching is handled.
final ImageView iconImg = (ImageView) rootView.findViewById(R.id.imageView);
imageLoader.displayImage(extras.getString("pImage"), iconImg, options, listener);
return rootView;
}
}
public static class VideoFragment extends Fragment {
private static final String ARG_SECTION_NUMBER = "section_number";
public static VideoFragment newInstance1(int sectionNumber) {
VideoFragment fragment = new VideoFragment();
Bundle args = new Bundle();
args.putInt(ARG_SECTION_NUMBER, sectionNumber);
fragment.setArguments(args);
return fragment;
}
public VideoFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.video_fragment, container, false);
VideoView myVideoView = (VideoView) rootView.findViewById(R.id.videoView);
Uri video = Uri.parse(extras.getString("pVideo"));
myVideoView.setMediaController(new MediaController(context));
myVideoView.setVideoURI(video);
myVideoView.requestFocus();
//myVideoView.start();
return rootView;
}
}
}