考虑这个例子（https://ideone.com/RpFRTZ）
这将有效地使用不相关类型Foo::comp (const Foo& a)的参数调用Bar。
这不仅会编译，如果我注释掉std::cout << "a = " << a.s << std::endl;它也会以某种方式工作并打印Result: 0
如果我确实打印出了值，而不是段错误，这是公平的......但为什么它首先编译？
#include <functional>
#include <string>
#include <iostream>
struct Foo
{
bool comp(const Foo& a)
{
std::cout << "a = " << a.s << std::endl;
return a.s == s;
}
std::string s;
};
struct Bar
{
int a;
};
template <class F, class T>
void execute (F f, T a)
{
std::cout << "Result: " << f (a) << std::endl;
}
int main()
{
Foo* f1 = new Foo;
f1->s = "Hello";
Foo f2;
f2.s = "Bla";
Bar b;
b.a = 100;
execute (std::bind2nd (std::mem_fun(&Foo::comp), b), f1);
return 0;
}
答案 0 :(得分：1)
答案在于std :: bind2nd：
的实现package com.example.mi.mikpiadmin;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.util.ArrayList;
import java.util.HashMap;
public class Store_KPI extends AppCompatActivity implements ListView.OnItemClickListener {
private PreferenceHelper preferenceHelper;
ListView listView1;
public static final String URL_GET_ISSUE = "http://dcoder.in/mi/two.php";
public static final String TAG_JSON_ARRAY="result";
public static final String TAG_ID = "id";
public static final String TAG_PARAMETER = "parameter";
public static final String TAG_ACHIEVEMENT = "achievement";
public static final String TAG_TARGET_WEEKLY = "target_weekly";
public static final String TAG_TARGET_MONTHLY = "target_monthly";
public static final String TAG_PER_ACHIEVEMENT = "per_achievement";
private String JSON_ISSUE_STRING;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_store_kpi);
preferenceHelper = new PreferenceHelper(this);
listView1=(ListView)findViewById(R.id.list_view_store_kpi) ;
listView1.setOnItemClickListener(this);
getJSON();
}
private void showEmployee(){
JSONObject jsonObject = null;
ArrayList<HashMap<String,String>> list = new ArrayList<HashMap<String, String>>();
try {
jsonObject = new JSONObject(JSON_ISSUE_STRING);
JSONArray result = jsonObject.getJSONArray(TAG_JSON_ARRAY);
for(int i = 0; i<result.length(); i++){
JSONObject jo = result.getJSONObject(i);
String parameter = jo.getString(TAG_PARAMETER);
String achievement = jo.getString(TAG_ACHIEVEMENT);
String target = jo.getString(TAG_TARGET_WEEKLY);
String mtarget = jo.getString(TAG_TARGET_MONTHLY);
String per_achievement = jo.getString(TAG_PER_ACHIEVEMENT);
HashMap<String,String> employees = new HashMap<>();
employees.put(TAG_PARAMETER,parameter);
employees.put(TAG_ACHIEVEMENT,achievement);
employees.put(TAG_TARGET_WEEKLY,target);
employees.put(TAG_TARGET_MONTHLY,mtarget);
employees.put(TAG_PER_ACHIEVEMENT,per_achievement);
list.add(employees);
}
} catch (JSONException e) {
e.printStackTrace();
}
ListAdapter adapter = new SimpleAdapter(
Store_KPI.this, list, R.layout.list_item,
new String[]{TAG_PARAMETER,TAG_ACHIEVEMENT,TAG_TARGET_WEEKLY,TAG_TARGET_MONTHLY,TAG_PER_ACHIEVEMENT},
new int[]{R.id.id, R.id.name, R.id.feedback,R.id.parmeter,R.id.target});
listView1.setAdapter(adapter);
((SimpleAdapter) listView1.getAdapter()).notifyDataSetChanged();
}
private void getJSON(){
class GetJSON extends AsyncTask<Void,Void,String> {
private ProgressDialog loading;
@Override
protected void onPreExecute() {
super.onPreExecute();
loading = ProgressDialog.show(Store_KPI.this,"Fetching Data","Wait...",false,false);
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
loading.dismiss();
JSON_ISSUE_STRING = s;
showEmployee();
}
@Override
protected String doInBackground(Void... params) {
RequestHandler rh = new RequestHandler();
String s = rh.sendGetRequest(URL_GET_ISSUE+"?store="+preferenceHelper.getStore());
Log.d("123",s);
return s;
}
}
GetJSON gj = new GetJSON();
gj.execute();
}
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Intent intent = new Intent(this, Store_KPI.class);
HashMap<String,String> map =(HashMap)parent.getItemAtPosition(position);
//String empId = map.get(Config.TAG_ID).toString();
//intent.putExtra(Config.EMP_ID,empId);
startActivity(intent);
}
}
Logcat
09-11 12:41:57.852 30353-30353/? D/AndroidRuntime: Calling main entry com.android.commands.pm.Pm
09-11 12:41:57.860 30353-30363/? W/MessageQueue: Handler (android.os.Handler) {bda2ab} sending message to a Handler on a dead thread
java.lang.IllegalStateException: Handler (android.os.Handler) {bda2ab} sending message to a Handler on a dead thread
at android.os.MessageQueue.enqueueMessage(MessageQueue.java:543)
at android.os.Handler.enqueueMessage(Handler.java:643)
at android.os.Handler.sendMessageAtTime(Handler.java:612)
at android.os.Handler.sendMessageDelayed(Handler.java:582)
at android.os.Handler.post(Handler.java:338)
at android.os.ResultReceiver$MyResultReceiver.send(ResultReceiver.java:57)
at com.android.internal.os.IResultReceiver$Stub.onTransact(IResultReceiver.java:58)
at android.os.Binder.execTransact(Binder.java:565)
09-11 12:41:57.861 30353-30353/? I/art: System.exit called, status: 0
09-11 12:41:57.861 30353-30353/? I/AndroidRuntime: VM exiting with result code 0.
09-11 12:43:26.885 1579-1586/? I/art: Background sticky concurrent mark sweep GC freed 25454(2MB) AllocSpace objects, 2(136KB) LOS objects, 16% free, 14MB/17MB, paused 2.151ms total 142.176ms
09-11 12:46:48.997 1355-1573/? D/MDnsDS: MDnsSdListener::Monitor poll timed out
09-11 12:46:48.997 1355-1573/? D/MDnsDS: Going to poll with pollCount 1
--------- beginning of system
09-11 12:46:56.827 1579-1628/? D/ConnectivityService: NetworkAgentInfo [MOBILE (LTE) - 100] validation failed
09-11 12:50:54.788 1579-1586/? I/art: Background sticky concurrent mark sweep GC freed 30451(3MB) AllocSpace objects, 4(224KB) LOS objects, 19% free, 13MB/17MB, paused 1.593ms total 146.983ms
09-11 12:58:17.038 1579-1628/? D/ConnectivityService: NetworkAgentInfo [MOBILE (LTE) - 100] validation failed
09-11 13:00:02.319 1579-1624/? D/WifiNative-HAL: Failing getSupportedFeatureset because HAL isn't started
09-11 13:00:02.320 1579-1595/? E/BluetoothAdapter: Bluetooth binder is null
09-11 13:00:02.320 1579-1595/? E/BatteryStatsService: no controller energy info supplied
09-11 13:00:02.322 1579-1595/? E/KernelCpuSpeedReader: Failed to read cpu-freq: /sys/devices/system/cpu/cpu0/cpufreq/stats/time_in_state (No such file or directory)
09-11 13:00:02.322 1579-1595/? E/KernelUidCpuTimeReader: Failed to read uid_cputime: /proc/uid_cputime/show_uid_stat (No such file or directory)
09-11 13:00:02.323 1579-1595/? E/BatteryStatsService: modem info is invalid: ModemActivityInfo{ mTimestamp=0 mSleepTimeMs=0 mIdleTimeMs=0 mTxTimeMs[]=[0, 0, 0, 0, 0] mRxTimeMs=0 mEnergyUsed=0}
09-11 13:00:12.608 1579-1579/? I/EntropyMixer: Writing entropy...
09-11 13:00:51.190 23018-23055/? D/EGL_emulation: eglMakeCurrent: 0xaab05720: ver 2 0 (tinfo 0xaab03740)
09-11 13:00:51.399 1579-1613/? I/ActivityManager: START u0 {act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10200000 cmp=com.android.launcher3/.Launcher (has extras)} from uid 1000 on display 0
09-11 13:00:51.400 1346-1369/? W/audio_hw_generic: Not supplying enough data to HAL, expected position 2278264 , only wrote 2278080
09-11 13:00:51.424 1304-1338/? D/gralloc_ranchu: gralloc_alloc: Creating ashmem region of size 2650112
[ 09-11 13:00:51.432 1579: 1613 D/ ]
HostConnection::get() New Host Connection established 0x9a6e7d40, tid 1613
[ 09-11 13:00:51.433 1579: 1613 W/ ]
Unrecognized GLES max version string in extensions: ANDROID_EMU_CHECKSUM_HELPER_v1 ANDROID_EMU_native_sync_v2 ANDROID_EMU_dma_v1
09-11 13:00:51.467 27945-27974/? D/EGL_emulation: eglMakeCurrent: 0xa8270320: ver 2 0 (tinfo 0xa820a7e0)
09-11 13:00:51.592 1985-2257/? D/EGL_emulation: eglMakeCurrent: 0xaab05480: ver 2 0 (tinfo 0xaab03660)
09-11 13:00:52.091 1985-2257/? W/OpenGLRenderer: Incorrectly called buildLayer on View: ShortcutAndWidgetContainer, destroying layer...
09-11 13:00:54.627 1346-1370/? W/audio_hw_generic: Not supplying enough data to HAL, expected position 2585797 , only wrote 2432880
09-11 13:01:00.022 23018-23055/? D/EGL_emulation: eglMakeCurrent: 0xaab05720: ver 2 0 (tinfo 0xaab03740)
09-11 13:01:00.049 1579-1592/? I/ProcessStatsService: Prepared write state in 7ms
09-11 13:01:00.073 1579-1591/? I/ProcessStatsService: Pruning old procstats: /data/system/procstats/state-2017-09-07-13-05-30.bin
你可以看到有一种不安全的C风格演员＆＃34; _Arg2_type（__ x）＆＃34;到了正确的类型，所以你的例子就像编写它一样编译：
template<typename _Operation, typename _Tp>
inline binder2nd<_Operation>
bind2nd(const _Operation& __fn, const _Tp& __x)
{
typedef typename _Operation::second_argument_type _Arg2_type;
return binder2nd<_Operation>(__fn, _Arg2_type(__x));
}
，遗憾的是有效的C ++代码。