在文件夹迭代上实现 RAII
Implementing RAII on a folder iteration
我编写此代码是为了递归循环浏览文件夹树并列出文件的大小(以字节为单位(。
由于我使用的是 winapi，并且有一个应该打开和关闭的Handle，我应该在此代码上实现 RAII，问题是在线论坛中给出的示例(更不用说我不是母语为英语的人(和许多书籍，包括有效的C++都超出了一个找不到任何地方获得经验的人的头。
至少有人愿意指出我吗？
#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);
}
最简单的模式是：
struct HandleWrapper {
HANDLE hFind = NULL;
~HandleWrapper() { if (hFind) ::FindClose(hFind); }
};
您可以在代码中逐字使用该代码，无需修改：
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 确保即使在异常的情况下也始终调用FindClose。
成语
不习惯一直编写 RAII 包装器。事实上，对于 WIN32 API 句柄，通常可以使用现有的构建基块：
std::unique_ptr<void, decltype(&::FindClose)>
hFind(FindFirstFile(sourcepath.c_str(), &FindFileData), ::FindClose);
这是因为 HANDLE 实际上是一个 VOID 指针。访问句柄时，请使用hFind.get()。
更多封装？
您可能实际上希望将更多的查找器封装到结构中。我把它留给读者作为驱魔
但是等等。文件系统库
文件系统库已经涵盖了您，并且它是可移植的：
住在科里鲁
#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";
}
}
}
不再有奇怪的大小计算(这是怎么回事？我们还在1980年吗？(，也不再有手动资源管理。
啊，你实际上可以免费获得更多的健壮性，因为我们只需传递一个标志即可在遍历期间忽略权限错误。如果需要，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. /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
在winapi标签和 C API 的上下文中，我发现一种非侵入性但 RAII 安全的方法是在样板包装器中屏蔽资源的获取和释放，其中操作的实际定义在实例化点推迟到 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(); }
};
然后，原始代码可以编写如下，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));
}
相同的模式可用于任何具有获取/释放语义的对象(句柄、指针等(。
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