Question

这可能是一个重复的问题，但我找不到它，可能是由于可能缺乏术语。

问题在于：我想将一个lambda函数赋给稍后会调用的变量。

class MyClass
{
  public:
    MyClass( QWdiget* widget );

    void doSomething();

  private:
    QWidget* my_widget;
    std::function<QString()> my_text;
}

MyClass::MyClass( QWdiget* widget )
    : my_text( [=](){return QString();} )
{
  if ( qobject_cast<QLabel*>( widget ) )
  {
    my_text = [=](){return my_widget ? qobject_cast<QLabel*>( my_widget )->text() : Qstring(); };
  }
  else if ( qobject_cast<QGroupBox*>( widget ) )
  {
    my_text = [=](){return my_widget ? qobject_cast<QGroupBox*>( my_widget )->title() : QString(); };
  }
  else
  {
    return;
  }
  my_widget = widget;
}

MyClass::DoSomething()
{
  QString str = my_text();
  ....whatever
}

我遇到了崩溃但无法找到。

修改

正确实施默认值可部分解决问题。现在，qobject_cast崩溃。

Answer 1

添加如下默认实现：

public class overview extends TabActivity {

private MyApplication app;
private JSONArray v;
private TabWidget m_tabs;

@Override
public void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_overview);
    app = ((MyApplication) getApplicationContext());
    app.ActivityMode = true;
    NotificationManager nMgr = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    nMgr.cancel(gvendorservice.msgID);
    int pending=0;
    int delivered=0;

    TabHost tabHost = getTabHost();
    // Tab for ShowOrders
    TabSpec ShowOrders = tabHost.newTabSpec("Normal Order");
    // setting Title and Icon for the Tab
    ShowOrders.setIndicator("Normal Order");
    Intent ShowOrdersIntent = new Intent(this, Orders.class);
    ShowOrders.setContent(ShowOrdersIntent);

    // Tab for Showchatlist
    TabSpec Showchatlist = tabHost.newTabSpec("Chat");
    Intent ShowchatlistIntent = new Intent(this, activity_chatlist.class);
    Showchatlist.setIndicator("CHATS\n\t"+app.getNewMsgCount().length());
    Showchatlist.setContent(ShowchatlistIntent);

    // Tab for AcceptOrder
    TabSpec AcceptOrder = tabHost.newTabSpec("Order");
    Intent AcceptOrderIntent = new Intent(this, activity_acceptorder.class);
    AcceptOrder.setIndicator("OrderS\n\t"+app.getOpenOrders().length());
    AcceptOrder.setContent(AcceptOrderIntent);

    //Tab for MyChatOrder
    TabSpec MyChatOrder = tabHost.newTabSpec("Pending");

    Intent MyChatOrdertIntent = new Intent(this, activity_mychatorders.class);
    MyChatOrdertIntent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
    JSONArray orderlistArray = app.getMyChatOrders();
    try {
        TextView tcp = (TextView) findViewById(R.id.txtChatOrdersPending);
        for (int i = 0; i < orderlistArray.length(); i++) {
            JSONObject cat = orderlistArray.getJSONObject(i);
            if (cat.getInt("delivered") == 0) {
                pending++;
            } else {
                delivered++;
            }
        }

        tcp.setText(String.valueOf(pending));
    }
    catch(JSONException e)
    {

    }

    MyChatOrder.setIndicator("Pending\n\t"+String.valueOf(pending));
    MyChatOrder.setContent(MyChatOrdertIntent);

    // Adding all TabSpec to TabHost
    tabHost.addTab(ShowOrders); // Adding normal order tab
    tabHost.addTab(Showchatlist); // Adding chat tab
    tabHost.addTab(AcceptOrder); // Adding orders tab
    tabHost.addTab(MyChatOrder);//Adding pending tab
    for (int i = 0; i < tabHost.getTabWidget().getTabCount(); i++) {
        TabWidget tw = (TabWidget) tabHost.findViewById(android.R.id.tabs);
        View tabView = tw.getChildTabViewAt(i);
        TextView tv = (TextView) tabView.findViewById(android.R.id.title);
        tv.setTextSize(12);
        tv.setGravity(Gravity.CENTER_VERTICAL);
        tv.setAllCaps(true);
    }
}

@Override
protected void onResume() {
    super.onResume();
    app.ActivityMode = true;
    NotificationManager nMgr = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    nMgr.cancel(gvendorservice.msgID);
    int pending=0;
    int delivered=0;

    JSONArray orderlistArray = app.getMyChatOrders();
    try {

        TextView tcp = (TextView) findViewById(R.id.txtChatOrdersPending);
        for (int i = 0; i < orderlistArray.length(); i++) {
            JSONObject cat = orderlistArray.getJSONObject(i);
            if (cat.getInt("delivered") == 0) {
                pending++;
            } else {
                delivered++;
            }
        }

        tcp.setText(String.valueOf(pending));
    }
    catch(JSONException e)
    {
    }
}

@Override
protected void onDestroy() {
    super.onDestroy();
    app.ActivityMode = false;
}

@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_overview, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    Intent intent;
    //noinspection SimplifiableIfStatement
    if (id == R.id.action_logout) {
        app = ((MyApplication) getApplicationContext());
        app.logOut();
        intent = new Intent(overview.this, LoginActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_NEW_TASK);
        overview.this.startActivity(intent);
        overview.this.finish();
        return true;
    }

    return super.onOptionsItemSelected(item);
}

将lambda赋给变量并稍后调用它

1 个答案: