Question

在＆＃34;短＆＃34;，我priority_queue选择满足特定条件的k unordered_set<int>。我想将它们（哈希集）作为queue返回。

由于priority_queue的创建和维护期望元素交换，我使用指针而不是unordered_set<int>作为priority_queue的条目。

因此，返回类型应为queue< smart_ptr<unordered_set<int>> >。

如果我使用shared_ptr代码正常，但我希望使用unique_ptr，因为它更经济，客户承诺将其用作unique_ptr。

如何使用unique_ptr来实现以下代码？

-------------------详细说明-------------------------- -----

我有一个函数可以从文件中读取保留k行，这些行的大小最接近参考大小。比方说，如果k=2，引用大小为5，则文件包含6行大小（此行中的整数数）3,5,6,20,2,1。 k-closest行分别是大小为5和6的两行。

我使用priority_queue大小为k的自定义比较器来实现目标。我决定返回包含所选queue行的k-closest，因为客户端不想知道比较器是如何实现的（比较器是priority_queue模板的参数）。 / p>

using ptr_type = shared_ptr<unordered_set<int>>;
// ???????????????????????????????????????
// using ptr_type = unique_ptr<unordered_set<int>>; // unique_ptr does not work
// ???????????????????????????????????????
// Is it possible to transfer unique_ptr entries from a priority_queue to a queue?
using pair_comm_type = pair<int,ptr_type>;

queue<pair_comm_type> f() {
    // myFile.txt is a space separated file of integers.
    // Different lines may have different lengths (number of integers)
  string inputFile = "myFile.txt";
  const int TOP_K_LINE = 3;

    // to open a file
  ifstream fin(inputFile.c_str());
  string readBuffer;
  // The file opened

    // to define a priority_queue
    // define customized compare function, such that retained lines have size
    // closest to the reference value.
  double referenceSize = log10(10.0);
  auto comp = [&referenceSize](const pair_comm_type &LHS, const pair_comm_type &RHS)
      { return abs(log10(LHS.first)-referenceSize) 
      < abs(log10(RHS.first)-referenceSize); };
  priority_queue<pair_comm_type, vector<pair_comm_type>, decltype(comp)> myHeap(comp);
  // the priority_queue defined

  int bufIntValue = -1;
  int curMinArraySize = -1; // auxilliary variable, to reduce heap top access
    // to read the file line by line
  while (getline(fin,readBuffer)) {
      // to read int in each line to a hash set
    istringstream S(readBuffer);
    ptr_type lineBufferPtr(new unordered_set<int>);
    while (S>>bufIntValue) lineBufferPtr->insert(bufIntValue);
// one line read

      // to decide retain or not based on the length of this line
    int arraySize = lineBufferPtr->size();
    if (myHeap.size() < TOP_K_LINE) {
      // We can add new lines as long as top-k is not reached
      myHeap.emplace(arraySize,std::move(lineBufferPtr));
      curMinArraySize = myHeap.top().first;
      continue;
    }
    if (arraySize <= curMinArraySize) continue;
    myHeap.emplace(arraySize,std::move(lineBufferPtr));
    myHeap.pop();
    curMinArraySize = myHeap.top().first;
  }
  // all lines read
  fin.close();

    // to transfer values from the priority_queue to a queue
    // ???????????????????????????????????????
    // Is it possible that we can make changes here such that unique_ptr can also work??????
    // ???????????????????????????????????????
  queue<pair_comm_type> Q;
  while (!myHeap.empty()) {
    auto temp = myHeap.top();
    myHeap.pop();
    Q.emplace(temp.first,std::move(temp.second));
  }

  /*
  while (!Q.empty()) {
    printf("%d, ",Q.front().first);
    Q.pop();
  }
  printf("\n");
  */
  return Q;
}

Answer 1

STL容器旨在移动，当您这样做时，它与使用指针一样高效。事实上，他们在内部使用指针，所以你不必。

我会考虑使用这样的值：

using pair_comm_type = pair<int, unordered_set<int>>;

queue<pair_comm_type> f() {

  string inputFile = "myFile.txt";
  const int TOP_K_LINE = 3;

  ifstream fin(inputFile.c_str());
  string readBuffer;

  double referenceSize = log10(10.0);
  auto comp = [&referenceSize](const pair_comm_type &LHS, const pair_comm_type &RHS)
      { return abs(log10(LHS.first)-referenceSize)
      < abs(log10(RHS.first)-referenceSize); };
  priority_queue<pair_comm_type, vector<pair_comm_type>, decltype(comp)> myHeap(comp);

  int bufIntValue = -1;
  int curMinArraySize = -1;

  while (getline(fin,readBuffer)) {
    istringstream S(readBuffer);

    // no need to use pointers here
    unordered_set<int> lineBufferPtr;
    while (S>>bufIntValue)
        lineBufferPtr.insert(bufIntValue);

    int arraySize = lineBufferPtr.size();
    if (myHeap.size() < TOP_K_LINE) {

      myHeap.emplace(arraySize,std::move(lineBufferPtr));
      curMinArraySize = myHeap.top().first;
      continue;
    }
    if (arraySize <= curMinArraySize) continue;
    myHeap.emplace(arraySize,std::move(lineBufferPtr));
    myHeap.pop();
    curMinArraySize = myHeap.top().first;
  }

  fin.close();

  // Use std::move to transfer the top() element which will be
  // just as efficient as using pointers

  queue<pair_comm_type> Q;

  while (!myHeap.empty()) {
    auto temp = std::move(myHeap.top()); // USE MOVES
    myHeap.pop();
    Q.push(std::move(temp));
  }

  return Q;
}

Answer 2

@Galik的解决方案有效。

至于原来的问题，简单的答案是否定的。我们无法从priority_queue中传输unique_ptr。

unique_ptr的{{3}}，其参数是const引用，将被删除。 priority_queue::top()的返回类型为copy constructor。因此，我们不能使用返回值来创建新的unique_ptr对象。

如何将unique_ptr从优先级队列传输到队列

2 个答案: