在文件夹迭代中实现 RAII

Question

我编写这段代码是为了通过文件夹树递归循环并列出文件及其大小（以字节为单位）。

由于我用的是winapi，有一个Handle要打开关闭，我应该在这段代码上实现RAII，问题是网上论坛给出的例子（更何况我是不是母语为英语的人）和很多书籍，包括 Effective C++，对于找不到任何地方获得经验的人来说都是难以理解的。

至少有好心人指点我吗？

#include <iostream>
#include <string>
#include <windows.h>

void findFiles(std::string & spath) {

  size_t i = 1;
  WIN32_FIND_DATA FindFileData;

  std::string sourcepath = spath + std::string("\*.*");

  HANDLE hFind = FindFirstFile(sourcepath.c_str(), & FindFileData);

  if (hFind != INVALID_HANDLE_VALUE)
    do {
      std::string fullpath = std::string(spath) + std::string("\") + std::string(FindFileData.cFileName);

      if ( * (fullpath.rbegin()) == '.')
        continue;
      else
      if (FindFileData.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)
        findFiles(fullpath);
      else

        std::cout << i++ << "-" << FindFileData.cFileName << " " << (FindFileData.nFileSizeHigh *(MAXWORD + 1)) + FindFileData.nFileSizeLow << std::endl;

    } while (FindNextFile(hFind, & FindFileData));

  FindClose(hFind);

}

int main(int argc, char ** argv) {

  std::string spath(argv[1]);

  findFiles(spath);

}

Answer 1

最简单的模式是：

struct HandleWrapper {
    HANDLE hFind = NULL;
    ~HandleWrapper() { if (hFind) ::FindClose(hFind); }
};

您可以在代码中原封不动地使用该代码：

void findFiles(std::string const spath) {
    size_t i = 1;
    WIN32_FIND_DATA FindFileData;
    std::string sourcepath = spath + std::string("\*.*");

    struct HandleWrapper {
        HANDLE hFind = INVALID_HANDLE_VALUE;
        ~HandleWrapper() { if (hFind != INVALID_HANDLE_VALUE) ::FindClose(hFind); }
    } wrapper;

    wrapper.hFind = FindFirstFile(sourcepath.c_str(), &FindFileData);
    if (wrapper.hFind != INVALID_HANDLE_VALUE) {
        do {
            std::string fullpath = std::string(spath) + std::string("\") +
                std::string(FindFileData.cFileName);
            if (*(fullpath.rbegin()) == '.')
                continue;
            else if (FindFileData.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)
                findFiles(fullpath);
            else
                std::cout << i++ << "-" << FindFileData.cFileName << " "
                    << (FindFileData.nFileSizeHigh * (MAXWORD + 1)) +
                    FindFileData.nFileSizeLow
                    << std::endl;
        } while (FindNextFile(wrapper.hFind, &FindFileData));
    }
    // no more CloseHandle here
}

现在 RAII 确保 FindClose 即使在异常情况下也始终被调用。

成语

一直写 RAII 包装器不是习惯。事实上，对于 WIN32 API 句柄，您通常可以使用现有的构建块：

std::unique_ptr<void, decltype(&::FindClose)>
    hFind(FindFirstFile(sourcepath.c_str(), &FindFileData), ::FindClose);

这是可行的，因为 HANDLE 实际上是一个 VOID 指针。访问句柄时，使用hFind.get().

More encapsulation?

You might actually want to encapsulate more of your finder into a struct. I leave this as an exorcism for the reader

等等。 Filesystem Library

Filesystem Library 已经满足您的需求，并且它是便携的：

Live On Coliru

#include <iostream>
#include <string>
#include <vector>
#include <filesystem>

namespace fs = std::filesystem;

int main(int argc, char** argv) {
    size_t i = 1;
    for (auto arg : std::vector<std::string>(argv + 1, argv + argc))
    {
        auto flags = fs::directory_options::skip_permission_denied;
        for (auto de : fs::recursive_directory_iterator(arg, flags)) {
            if (de.status().type() == fs::file_type::regular)
                std::cout << i++ << "-" << de.path() << " " << de.file_size() << "\n";
        }
    }
}

不再进行奇怪的大小计算（那是什么意思？我们还在 1980 年吗？）并且不再进行手动资源管理。

啊，你实际上免费获得了更多的健壮性，因为我们可以只传递一个标志来忽略遍历过程中的权限错误。如果需要，您还可以指定如何处理目录链接。

这是现场演示的输出（命令行参数是 . /usr/local/include/boost/archive）：

1-"./main.cpp" 545
2-"./a.out" 21672
3-"/usr/local/include/boost/archive/iterators/unescape.hpp" 2343
4-"/usr/local/include/boost/archive/iterators/dataflow.hpp" 2707
5-"/usr/local/include/boost/archive/iterators/xml_unescape.hpp" 3684
6-"/usr/local/include/boost/archive/iterators/xml_escape.hpp" 2903
7-"/usr/local/include/boost/archive/iterators/istream_iterator.hpp" 2525
8-"/usr/local/include/boost/archive/iterators/mb_from_wchar.hpp" 3804
9-"/usr/local/include/boost/archive/iterators/base64_exception.hpp" 1879
10-"/usr/local/include/boost/archive/iterators/wchar_from_mb.hpp" 5645
11-"/usr/local/include/boost/archive/iterators/insert_linebreaks.hpp" 2671
12-"/usr/local/include/boost/archive/iterators/escape.hpp" 3086
13-"/usr/local/include/boost/archive/iterators/xml_unescape_exception.hpp" 1312
14-"/usr/local/include/boost/archive/iterators/transform_width.hpp" 5546
15-"/usr/local/include/boost/archive/iterators/head_iterator.hpp" 2084
16-"/usr/local/include/boost/archive/iterators/dataflow_exception.hpp" 2267
17-"/usr/local/include/boost/archive/iterators/ostream_iterator.hpp" 2396
18-"/usr/local/include/boost/archive/iterators/remove_whitespace.hpp" 4552
19-"/usr/local/include/boost/archive/iterators/base64_from_binary.hpp" 3193
20-"/usr/local/include/boost/archive/iterators/binary_from_base64.hpp" 3842
21-"/usr/local/include/boost/archive/polymorphic_text_wiarchive.hpp" 1615
22-"/usr/local/include/boost/archive/text_iarchive.hpp" 3481
23-"/usr/local/include/boost/archive/binary_iarchive_impl.hpp" 2989
24-"/usr/local/include/boost/archive/wcslen.hpp" 1321
25-"/usr/local/include/boost/archive/polymorphic_xml_iarchive.hpp" 1466
26-"/usr/local/include/boost/archive/archive_exception.hpp" 4137
27-"/usr/local/include/boost/archive/basic_text_oprimitive.hpp" 7277
28-"/usr/local/include/boost/archive/polymorphic_text_iarchive.hpp" 1477
29-"/usr/local/include/boost/archive/binary_oarchive.hpp" 1970
30-"/usr/local/include/boost/archive/basic_binary_iprimitive.hpp" 6445
31-"/usr/local/include/boost/archive/polymorphic_text_oarchive.hpp" 1340
32-"/usr/local/include/boost/archive/polymorphic_oarchive.hpp" 4779
33-"/usr/local/include/boost/archive/text_woarchive.hpp" 4473
34-"/usr/local/include/boost/archive/impl/basic_text_oprimitive.ipp" 2949
35-"/usr/local/include/boost/archive/impl/basic_text_iarchive.ipp" 2516
36-"/usr/local/include/boost/archive/impl/xml_wiarchive_impl.ipp" 5256
37-"/usr/local/include/boost/archive/impl/xml_iarchive_impl.ipp" 5703
38-"/usr/local/include/boost/archive/impl/text_oarchive_impl.ipp" 3086
39-"/usr/local/include/boost/archive/impl/basic_binary_oprimitive.ipp" 3735
40-"/usr/local/include/boost/archive/impl/text_woarchive_impl.ipp" 2155
41-"/usr/local/include/boost/archive/impl/basic_binary_iarchive.ipp" 4325
42-"/usr/local/include/boost/archive/impl/basic_xml_oarchive.ipp" 7673
43-"/usr/local/include/boost/archive/impl/basic_binary_iprimitive.ipp" 5097
44-"/usr/local/include/boost/archive/impl/basic_xml_iarchive.ipp" 3565
45-"/usr/local/include/boost/archive/impl/basic_text_iprimitive.ipp" 3510
46-"/usr/local/include/boost/archive/impl/xml_oarchive_impl.ipp" 4082
47-"/usr/local/include/boost/archive/impl/text_iarchive_impl.ipp" 3298
48-"/usr/local/include/boost/archive/impl/basic_binary_oarchive.ipp" 1282
49-"/usr/local/include/boost/archive/impl/basic_xml_grammar.hpp" 5159
50-"/usr/local/include/boost/archive/impl/archive_serializer_map.ipp" 2347
51-"/usr/local/include/boost/archive/impl/xml_woarchive_impl.ipp" 4663
52-"/usr/local/include/boost/archive/impl/text_wiarchive_impl.ipp" 2881
53-"/usr/local/include/boost/archive/impl/basic_text_oarchive.ipp" 1653
54-"/usr/local/include/boost/archive/tmpdir.hpp" 1227
55-"/usr/local/include/boost/archive/basic_archive.hpp" 9404
56-"/usr/local/include/boost/archive/xml_iarchive.hpp" 3644
57-"/usr/local/include/boost/archive/polymorphic_iarchive.hpp" 5137
58-"/usr/local/include/boost/archive/basic_xml_oarchive.hpp" 4319
59-"/usr/local/include/boost/archive/binary_oarchive_impl.hpp" 3018
60-"/usr/local/include/boost/archive/xml_oarchive.hpp" 3652
61-"/usr/local/include/boost/archive/polymorphic_xml_woarchive.hpp" 1467
62-"/usr/local/include/boost/archive/dinkumware.hpp" 5395
63-"/usr/local/include/boost/archive/detail/basic_iarchive.hpp" 3576
64-"/usr/local/include/boost/archive/detail/basic_pointer_oserializer.hpp" 1942
65-"/usr/local/include/boost/archive/detail/auto_link_warchive.hpp" 1552
66-"/usr/local/include/boost/archive/detail/utf8_codecvt_facet.hpp" 966
67-"/usr/local/include/boost/archive/detail/common_iarchive.hpp" 2469
68-"/usr/local/include/boost/archive/detail/oserializer.hpp" 17569
69-"/usr/local/include/boost/archive/detail/basic_oarchive.hpp" 3269
70-"/usr/local/include/boost/archive/detail/polymorphic_oarchive_route.hpp" 6318
71-"/usr/local/include/boost/archive/detail/abi_suffix.hpp" 545
72-"/usr/local/include/boost/archive/detail/polymorphic_iarchive_route.hpp" 6647
73-"/usr/local/include/boost/archive/detail/register_archive.hpp" 3695
74-"/usr/local/include/boost/archive/detail/common_oarchive.hpp" 2436
75-"/usr/local/include/boost/archive/detail/basic_oserializer.hpp" 2595
76-"/usr/local/include/boost/archive/detail/basic_serializer_map.hpp" 1925
77-"/usr/local/include/boost/archive/detail/basic_serializer.hpp" 2159
78-"/usr/local/include/boost/archive/detail/abi_prefix.hpp" 593
79-"/usr/local/include/boost/archive/detail/basic_pointer_iserializer.hpp" 2049
80-"/usr/local/include/boost/archive/detail/iserializer.hpp" 20947
81-"/usr/local/include/boost/archive/detail/archive_serializer_map.hpp" 1714
82-"/usr/local/include/boost/archive/detail/basic_iserializer.hpp" 2704
83-"/usr/local/include/boost/archive/detail/check.hpp" 5397
84-"/usr/local/include/boost/archive/detail/auto_link_archive.hpp" 1689
85-"/usr/local/include/boost/archive/detail/decl.hpp" 1744
86-"/usr/local/include/boost/archive/detail/interface_oarchive.hpp" 2504
87-"/usr/local/include/boost/archive/detail/helper_collection.hpp" 2844
88-"/usr/local/include/boost/archive/detail/interface_iarchive.hpp" 2534
89-"/usr/local/include/boost/archive/polymorphic_xml_wiarchive.hpp" 1469
90-"/usr/local/include/boost/archive/polymorphic_binary_iarchive.hpp" 1499
91-"/usr/local/include/boost/archive/basic_text_oarchive.hpp" 3400
92-"/usr/local/include/boost/archive/xml_archive_exception.hpp" 1786
93-"/usr/local/include/boost/archive/basic_binary_iarchive.hpp" 7563
94-"/usr/local/include/boost/archive/binary_wiarchive.hpp" 1704
95-"/usr/local/include/boost/archive/binary_iarchive.hpp" 1973
96-"/usr/local/include/boost/archive/text_wiarchive.hpp" 3630
97-"/usr/local/include/boost/archive/basic_text_iprimitive.hpp" 4015
98-"/usr/local/include/boost/archive/basic_binary_oarchive.hpp" 6245
99-"/usr/local/include/boost/archive/basic_binary_oprimitive.hpp" 6233
100-"/usr/local/include/boost/archive/basic_streambuf_locale_saver.hpp" 2853
101-"/usr/local/include/boost/archive/polymorphic_text_woarchive.hpp" 1478
102-"/usr/local/include/boost/archive/text_oarchive.hpp" 3511
103-"/usr/local/include/boost/archive/basic_text_iarchive.hpp" 3001
104-"/usr/local/include/boost/archive/basic_xml_iarchive.hpp" 4003
105-"/usr/local/include/boost/archive/xml_woarchive.hpp" 3780
106-"/usr/local/include/boost/archive/codecvt_null.hpp" 3100
107-"/usr/local/include/boost/archive/xml_wiarchive.hpp" 3879
108-"/usr/local/include/boost/archive/binary_woarchive.hpp" 1890
109-"/usr/local/include/boost/archive/polymorphic_binary_oarchive.hpp" 1362
110-"/usr/local/include/boost/archive/polymorphic_xml_oarchive.hpp" 1328
111-"/usr/local/include/boost/archive/basic_xml_archive.hpp" 1644
112-"/usr/local/include/boost/archive/add_facet.hpp" 1733

Answer 2

在 winapi 标记和一般 C API 的上下文中，我发现一种非常不具侵入性但 RAII 安全的方法是在样板包装器中隔离资源的获取和释放，其中实际动作的定义在实例化时推迟到 lambda。

#include <functional>

class XWrap
{
  std::function<void()> fnOut;

public:
  XWrap(const std::function<void()> &fnIn,
        const std::function<void()> &fnOut) : fnOut(fnOut)
  { if(fnIn) fnIn(); }

  ~XWrap()
  { if(fnOut) fnOut(); }
};

那么，原来的代码可以这样写，其中使用hFind和FindFileData的代码主体基本保持不变。

void findFiles(const std::string &spath)
{
  size_t i = 1;
  std::string sourcepath = spath + std::string("\*.*");

  HANDLE hFind;
  WIN32_FIND_DATA FindFileData;
  XWrap wrap(
    [&]() { hFind = ::FindFirstFile(sourcepath.c_str(), &FindFileData); },
    [&]() { if(hFind != INVALID_HANDLE_VALUE) { ::FindClose(hFind); /* plus maybe hFind = INVALID_HANDLE_VALUE; */ } }
  );
  if(hFind == INVALID_HANDLE_VALUE) return;

  do {
    std::string fullpath = std::string(spath) + std::string("\") + std::string(FindFileData.cFileName);

    if (fullpath.back() == '.')
      continue;

    if (FindFileData.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)
      findFiles(fullpath);
    else
      std::cout << i++ << "-" << FindFileData.cFileName << " " << (FindFileData.nFileSizeHigh *(MAXWORD + 1)) + FindFileData.nFileSizeLow << std::endl;

  } while (FindNextFile(hFind, &FindFileData));
}

相同的模式可用于具有 acquire/release 语义（句柄、指针等）的任何对象。

在文件夹迭代中实现 RAII

Implementing RAII on a folder iteration

c++

winapi

design-patterns

raii

成语

More encapsulation?

等等。 Filesystem Library