Question

由于某种原因，我程序中所有以map为参数的函数均无法正常工作。此函数是调用所有函数的函数（pageAndTimestamp是一个结构btw）：

void fifo(int framesize, int numref, int* pagestream)
{
  double hit = 0, size = numref;

  map<int, pageAndTimestamp> frames = frameMaker(framesize);

  for (int time = 0; time < numref; time++)
  {
    if (pageLoaded(pagestream[time], frames))
    {
      hit++;
      output(time, pagestream[time], size, hit, frames);
    }
    else
    {
      int loc = findPageToReplace(frames);
      replacePage(loc, pagestream[time], time, frames);
      output(time, pagestream[time], size, hit, frames);
    }
  }
}

这些功能无法正常工作：

bool pageLoaded(int page, map<int, pageAndTimestamp> m)
{
  for (const auto& it : m)
  {
    if (it.second.a[0] == page)
    return true;
  }
  return false;
} 

int findPageToReplace(map<int, pageAndTimestamp> m)
{
  int timestamp = INT_MAX;
  int replaceLoc = 0;
  for (const auto& it : m)
  {
    if (it.second.a[1] == -1)
    return it.first;
    else
    {
      if (it.second.a[1] < timestamp)
      {
        timestamp = it.second.a[1];
        replaceLoc = it.first;
      }
    }
  }
  return replaceLoc;
}

void replacePage(int loc, int page, int time, map<int, pageAndTimestamp> m)
{
  m.at(loc).a[0] = page;
  m.at(loc).a[1] = time;
} 

void output(int t, int p, double s, double h, map<int, pageAndTimestamp> m)
{
  cout << "Time: " << t << endl << "Page: " << p << endl;
  for(const auto& it : m)
    cout << "Frame" << it.first << ": " << it.second.a[0] << endl;
  cout << "Hit ratio: " << h << " / " << s << " (" << h / s << ")" << endl 
 << endl << endl;
}

当我在Visual Studio 2017调试器中运行程序时，当我进入上述任何功能时，调试器会将我带到地图标准标头中的该函数标头：

map(const map& _Right)
    : _Mybase(_Right, _Alnode_traits::select_on_container_copy_construction(_Right._Getal()))
    {   // construct map by copying _Right
    }

我不知道问题出在哪里，或者为什么调试器将我带到此函数头。我该如何解决？

Answer 1

例如，函数import UIKit import FirebaseDatabase class ViewController: UIViewController, UIPickerViewDelegate, UIPickerViewDataSource { @IBOutlet weak var countryPicker: UIPickerView! //let country = ["USA", "Canada"] I am no longer using this hardcoded array var countrySelected = String() var action = 0 override func viewDidLoad() { super.viewDidLoad() //this is where I reach out to the database let database = Database.database().reference() database.child("countries").observeSingleEvent(of: .value) { (Snapshot) in print(Snapshot) } //add code here that takes the snapshot and appends the the array below var country = string() countrySelected = country[0] countryPicker.selectRow(action, inComponent: 0, animated: false) //print(countrySelected) } func numberOfComponents(in pickerView: UIPickerView) -> Int { return 1 } func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int { return country.count } func pickerView(_ pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String? { return country[row] } func pickerView(_ pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) { countrySelected = country[row] action = country.firstIndex(of: countrySelected)! //print(countrySelected) } }定义为：

replacePage

此函数将地图作为值，而不是引用或指针。因此，当您按如下方式调用它时：

void replacePage(int loc, int page, int time, map<int, pageAndTimestamp> m)

然后将映射replacePage(loc, pagestream[time], time, frames);复制到函数中的变量frames中。例如，这就是调试器将您带到复制构造函数来映射的原因。

进一步，它表示m代码

replacePage

正在更改m.at(loc).a[0] = page; m.at(loc).a[1] = time;本身而不是frames的副本。

您可能想要带有以下形式的签名的函数：

frames

其中大多数功能都采用常量引用，而bool pageLoaded(int page, const map<int, pageAndTimestamp>& m) int findPageToReplace(const map<int, pageAndTimestamp>& m) void replacePage(int loc, int page, int time, map<int, pageAndTimestamp>& m) void output(int t, int p, double s, double h, const map<int, pageAndTimestamp>& m)需要使用（非常量）引用。

以地图为参数的功能不起作用

1 个答案: