我有这个问题，因为我正在阅读import android.content.Context;
import android.database.Cursor;
import android.graphics.Point;
import android.net.Uri;
import android.os.Bundle;
import android.provider.MediaStore;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Display;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.FrameLayout;
import android.widget.GridView;
import android.widget.ImageView;
import java.util.HashSet;
import java.util.Set;
import #####.chromio.com.#####.R;
import #####.chromio.com.#####.polypicker.model.Image;
import #####.chromio.com.#####.polypicker.utils.ImageInternalFetcher;
public class EnhancedGallery extends AppCompatActivity implements
View.OnScrollChangeListener{
public ImageInternalFetcher mImageFetcher;
private ImageGalleryAdapter mGalleryAdapter;
private Set<Image> mSelectedImages;
Display display;
Toolbar toolbar;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_enhanced_gallery);
toolbar = (Toolbar) findViewById(R.id.toolbar_transparent);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
mGalleryAdapter = new ImageGalleryAdapter(this);
GridView galleryGridView = (GridView)findViewById(R.id.pp__gallery_grid);
mSelectedImages = new HashSet<Image>();
mImageFetcher = new ImageInternalFetcher(this, 500);
Cursor imageCursor = null;
try {
final String[] columns = {MediaStore.Images.Media.DATA, MediaStore.Images.ImageColumns.ORIENTATION};
final String orderBy = MediaStore.Images.Media.DATE_ADDED + " DESC";
imageCursor = getContentResolver().query(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, columns, null, null, orderBy);
while (imageCursor.moveToNext()) {
Uri uri = Uri.parse(imageCursor.getString(imageCursor.getColumnIndex(MediaStore.Images.Media.DATA)));
int orientation = imageCursor.getInt(imageCursor.getColumnIndex(MediaStore.Images.ImageColumns.ORIENTATION));
mGalleryAdapter.add(new Image(uri, orientation));
}
} catch (Exception e) {
e.printStackTrace();
} finally {
if(imageCursor != null && !imageCursor.isClosed()) {
imageCursor.close();
}
}
galleryGridView.setAdapter(mGalleryAdapter);
galleryGridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
Image image = mGalleryAdapter.getItem(i);
if (!containsImage(image)) {
//addImage(image);
} else {
//removeImage(image);
}
// refresh the view to
// mGalleryAdapter.getView(i, view, adapterView);
mGalleryAdapter.notifyDataSetChanged();
}
});
}
@Override
public void onScrollChange(View view, int i, int i1, int i2, int i3) {
toolbar.setAlpha(getAlphaforActionBar(view.getScrollY()));
}
private int getAlphaforActionBar(int scrollY) {
int minDist = 0,maxDist = 650;
if(scrollY>maxDist){
return 255;
}
else if(scrollY<minDist){
return 0;
}
else {
int alpha = 0;
alpha = (int) ((255.0/maxDist)*scrollY);
return alpha;
}
}
class ViewHolder {
ImageView mThumbnail;
// This is like storing too much data in memory.
// find a better way to handle this
Image mImage;
}
public class ImageGalleryAdapter extends ArrayAdapter<Image> {
public ImageGalleryAdapter(Context context) {
super(context, 0);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder;
if (convertView == null) {
convertView = LayoutInflater.from(getContext()).inflate(R.layout.enhanced_gallery_adapter, null);
holder = new ViewHolder();
holder.mThumbnail = (ImageView) convertView.findViewById(R.id.pp__thumbnail_image);
convertView.setTag(holder);
} else {
holder = (ViewHolder) convertView.getTag();
}
Image image = getItem(position);
boolean isSelected = containsImage(image);
convertView.setForeground(isSelected ? getResources().getDrawable(R.drawable.gallery_photo_selected) : null);
if (holder.mImage == null || !holder.mImage.equals(image)) {
mImageFetcher.loadImage(image.mUri, holder.mThumbnail);
holder.mImage = image;
}
return convertView;
}
}
public boolean containsImage(Image image) {
return mSelectedImages.contains(image);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case android.R.id.home:
finish();
return true;
case R.id.action_settings:
return true;
}
return super.onOptionsItemSelected(item);
}
}
的部分。它说auto_ptr无法保存在向量中，因为它具有破坏性的复制和赋值。我能以某种方式理解它，我能想到的一个例子就像auto_ptr会让我感到惊讶。但是，如果我小心翼翼地使用它并且不这样做，我可以将auto_ptr ap = vec[0]存储在向量中吗？有没有更深层次的原因我无法存储？也许内部向量的实现需要这样的假设：元素需要是可复制构造的吗？
感谢。
答案 0 :(得分：0)
调整向量大小时，会将其元素复制到新的后备数组，这是使用复制构造函数的位置。如果你有一个足够新的编译器，请考虑使用std :: unique_ptr。
答案 1 :(得分：0)
因为vector必须在其存储中存储已构造的对象。因此，它必须将对象复制到自己的存储中。
也是确保scope-safety的向量，它必须复制/移动传递的对象。
请参阅push_back文档
std::vector::push_back
void push_back( const T& value ); (1)
void push_back( T&& value ); (2) (since C++11)
将给定元素值追加到容器的末尾。
1）新元素初始化为值的副本。
2）将值移入新元素。
参数
value - 要追加的元素的值
类型要求
T必须符合
CopyInsertable的要求才能使用重载T必须符合
MoveInsertable的要求才能使用重载
CopyInsertable ID定义为
指定可以在未初始化的存储中就地复制构造类型的实例。