c++ - 删除指向 C++ 列表的指针非常非常慢。为什么？

Question

我正试图快速摆脱 STL 列表。所以我声明了一个指向该列表的指针。我进行所有操作，然后删除指针以释放 RAM。但是删除指向列表的指针的过程很慢，和我做 list.clear() 时一样慢。所以它很慢。为什么会这样？如何快速删除分配的 RAM？当我处理 vector 和 deque 时，删除速度很快。下面是一个演示这一点的程序。

//============//
// STL delete //
//============//

#include <iostream>     
#include <algorithm>    
#include <vector>       
#include <list>
#include <deque>
#include <cmath>
#include <iomanip>
#include <ctime>

using std::cout;
using std::cin;
using std::endl;
using std::list;
using std::vector;
using std::deque;
using std::fixed;
using std::setprecision;
using std::showpoint;
using std::sort;

// the main program

int main() 
{
  // variables and parameters

  const long int I_MAX = static_cast<long int>(pow(10.0, 7.5));
  const long int K_MAX = static_cast<long int>(pow(10.0, 6.0));
  long int i;
  long int k;
  clock_t t1;
  clock_t t2;
  double tall;

  // set the output

  cout << fixed;
  cout << setprecision(5);
  cout << showpoint;

  // main bench loop

  for (k = 0; k < K_MAX; k++)
  {
    list<double>   * listA  = new list<double>   [1];
    vector<double> * vecA   = new vector<double> [1];
    deque<double>  * deqA   = new deque<double>  [1];

    cout << endl;
    cout << "------------------------------->>> " << k << endl;
    cout << endl;

    // build the vector

    t1 = clock();

    cout << "  1 --> build the vector ..." << endl;

    for (i = 0; i < I_MAX; i++)
    { vecA->push_back(static_cast<double>(cos(i))); }

    t2 = clock();

    tall = (t2-t1)/static_cast<double>(CLOCKS_PER_SEC);

    cout << "  2 --> done with the vector --> " << tall << endl;

    // build the list

    t1 = clock();

    cout << "  3 --> build the list ..." << endl;

    for (i = 0; i < I_MAX; i++)
    { listA->push_back(static_cast<double>(cos(i))); }

    t2 = clock();

    tall = (t2-t1)/static_cast<double>(CLOCKS_PER_SEC);

    cout << "  4 --> done with the list   --> " << tall << endl;

    // build the deque

    t1 = clock();

    cout << "  5 --> build the deque ..." << endl;

    for (i = 0; i < I_MAX; i++)
    { deqA->push_back(static_cast<double>(cos(i))); }

    t2 = clock();

    tall = (t2-t1)/static_cast<double>(CLOCKS_PER_SEC);

    cout << "  6 --> done with the deque  --> " << tall << endl;

    // sort the vector

    t1 = clock();

    cout << "  7 --> sort the vector ..." << endl;

    sort(vecA->begin(), vecA->end());

    t2 = clock();

    tall = (t2-t1)/static_cast<double>(CLOCKS_PER_SEC);

    cout << "  8 --> done with the vector --> " << tall << endl;

    // sort the list

    t1 = clock();

    cout << "  9 --> sort the list ..." << endl;

    listA->sort();

    t2 = clock();

    tall = (t2-t1)/static_cast<double>(CLOCKS_PER_SEC);

    cout << " 10 --> done with the list   --> " << tall << endl;

    // sort the deque

    t1 = clock();

    cout << " 11 --> sort the deque ..." << endl;

    sort(deqA->begin(), deqA->end());

    t2 = clock();

    tall = (t2-t1)/static_cast<double>(CLOCKS_PER_SEC);

    cout << " 12 --> done with the deque  --> " << tall << endl;

    // delete the vector

    t1 = clock();

    cout << " 13 --> delete the vector ..." << endl;

    delete [] vecA;

    t2 = clock();

    tall = (t2-t1)/static_cast<double>(CLOCKS_PER_SEC);

    cout << " 14 --> done with the vector --> " << tall << endl;

    // delete the list

    t1 = clock();

    cout << " 15 --> delete the list ..." << endl;

    delete [] listA;

    t2 = clock();

    tall = (t2-t1)/static_cast<double>(CLOCKS_PER_SEC);

    cout << " 16 --> done with the list   --> " << tall << endl;

    t1 = clock();

    // delete the deque

    cout << " 17 --> delete the deque ..." << endl;

    delete [] deqA;

    t2 = clock();

    tall = (t2-t1)/static_cast<double>(CLOCKS_PER_SEC);

    cout << " 18 --> done with the deque  --> " << tall << endl;
  }

  int sentinel;
  cin >> sentinel; 

  return 0;
}

Answer 1

列表中的每个元素都有自己的节点，这意味着必须释放额外的分配。

如果你想真正快速地摆脱它并使用具有普通析构函数的成员(不需要调用)，请为列表使用自定义分配器，它为此进行了优化。
顺便说一句:在堆上分配容器是一种悲观。

无论如何，根据您的用例，另一个容器如 std::vector 可能更有意义。