c++ - 带有 boost directory_iterator 的 Max_element

Question

我正在使用 boost 的文件系统和 std::max_element() 来查找给定目录中名称最长的文件:

#include <iostream>
#include <iterator>
#include <algorithm>
#include <string>
#include <boost/filesystem.hpp>
using namespace std;
using namespace boost::filesystem;

bool size_comp( directory_entry de1, directory_entry de2  )
{
    return de1.path().string().size() < de2.path().string().size(); 
}

int main(int argc, char* argv[])
{
    path p (argv[1]);   // p is a path to a directory

    directory_iterator itr (p); 
    directory_iterator itr_end; 
    directory_iterator itr_max=::max_element(itr,itr_end,size_comp);
    int max_size = itr_max->path().string().size();
    cout << "Longest file name: " << itr_max->path() << " has " 
         << max_size << " characters" << endl; 
    return 0;
}

对于包含文件 cat.dat、mouse.dat、elephant.dat 的目录 Animals，输出为:

Longest file name: Animals/mouse.dat has 17 characters

这既不是最长的也不是最短的文件名。上面的代码有什么问题？

Answer 1

boost::filesystem::directory_iterator 股份状态。实现声明实现由 boost::shared_ptr 管理允许 InputIterators 所需的浅拷贝语义。因此，当一个算法，例如 std::max_element , 遍历 [first,last) ，结果迭代器随着 first 的每次增量而间接修改，因为结果迭代器将与 first 共享状态.

要解决此问题，请考虑存储 boost::filesystem::directory_entry 贯穿整个算法。例如，可以构造一个 std::vector<directory_entry>来自directory_iterator范围，然后将 vector 传递给 std::max_element .或者，手动编写算法可能更容易。

这是一个完整的示例，展示了两种方法，在当前目录上运行。

#include <algorithm> // std::copy, std::max_element
#include <iterator>  // std::back_inserter
#include <iostream>  // std::cout, std::endl
#include <vector>
#include <utility>   // std::make_pair

#include <boost/filesystem.hpp>
#include <boost/foreach.hpp>

namespace fs = boost::filesystem;

bool size_comp(const fs::directory_entry& lhs,
               const fs::directory_entry& rhs)
{
  return lhs.path().string().size() < rhs.path().string().size();
}

/// @brief Finds max by copying all directory entries.
fs::directory_entry max_full_copy(
  fs::directory_iterator first,
  fs::directory_iterator last)
{
  // Extract directory_entries from directory_iteartor.
  std::vector<fs::directory_entry> entries;
  std::copy(first, last, std::back_inserter(entries));
  // Find max element.
  return *std::max_element(entries.begin(), entries.end(), &size_comp);
}

/// @brief Finds max by only storing a copy of the max entry.
fs::directory_entry max_single_copy(
  fs::directory_iterator first,
  fs::directory_iterator last)
{
  fs::directory_entry result;
  BOOST_FOREACH(fs::directory_entry& current, std::make_pair(first, last))
  {
    if (size_comp(result, current))
      result = current;
  }
  return result;
}

int main()
{
  std::cout << max_full_copy(fs::directory_iterator("."),
                             fs::directory_iterator()) << "\n"
            << max_single_copy(fs::directory_iterator("."),
                               fs::directory_iterator()) << std::endl;
}

一个示例运行输出:

[tsansbury@localhost tmp]$ lsfile_four  file_one  file_three  file_two[tsansbury@localhost tmp]$ ../a.out "./file_three""./file_three"