\nreparse points<\/A>, which can induce infinite loops if you’re not\ncareful.\n<\/P>\n\nWell, that wasn’t so hard to write. But that’s because we made\nlife hard for the consumer. The consumer needs to maintain state\nacross each callback. For example, suppose you wanted to build\na list of directories and their sizes (both including and\nexcluding subdirectories).\n<\/P>\n
\nclass EnumState {\npublic:\n EnumState()\n : m_pdirCur(new Directory(NULL)) { }\n ~EnumState() { Dispose(); }\n FERESULT Callback(FEOPERATION feo,\n LPCTSTR pszDir, LPCTSTR pszPath,\n const WIN32_FIND_DATA* pwfd);\n void FinishDir(LPCTSTR pszDir);<\/p>\nprivate:<\/p>\n
struct Directory {\n Directory(Directory* pdirParent)\n : m_pdirParent(pdirParent)\n , m_ullSizeSelf(0)\n , m_ullSizeAll(0) { }\n Directory* m_pdirParent;\n ULONGLONG m_ullSizeSelf;\n ULONGLONG m_ullSizeAll;\n };\n Directory* Push();\n void Pop();\n void Dispose();<\/p>\n
Directory* m_pdirCur;\n};<\/p>\n
EnumState::Directory* EnumState::Push()\n{\n Directory* pdir = new Directory(m_pdirCur);\n if (pdir) {\n m_pdirCur = pdir;\n }\n return pdir;\n}<\/p>\n
void EnumState::Pop()\n{\n Directory* pdir = m_pdirCur->m_pdirParent;\n delete m_pdirCur;\n m_pdirCur = pdir;\n}<\/p>\n
void EnumState::Dispose()\n{\n while (m_pdirCur) {\n Pop();\n }\n}<\/p>\n
void EnumState::FinishDir(LPCTSTR pszDir)\n{\n m_pdirCur->m_ullSizeAll +=\n m_pdirCur->m_ullSizeSelf;\n printf(“Size of %s is %I64d (%I64d)\\n”,\n pszDir, m_pdirCur->m_ullSizeSelf,\n m_pdirCur->m_ullSizeAll);\n}<\/p>\n
ULONGLONG FileSize(const WIN32_FIND_DATA *pwfd)\n{\n return \n ((ULONGLONG)pwfd->nFileSizeHigh << 32) +\n pwfd->nFileSizeLow;\n}<\/p>\n
FERESULT EnumState::Callback(FEOPERATION feo,\n LPCTSTR pszDir, LPCTSTR pszPath,\n const WIN32_FIND_DATA* pwfd)\n{\n if (!m_pdirCur) return FER_STOP;<\/p>\n
switch (feo) {\n case FEO_FILE:\n m_pdirCur->m_ullSizeSelf += FileSize(pwfd);\n return FER_CONTINUE;<\/p>\n
case FEO_DIR:\n if (Push()) {\n return FER_CONTINUE;\n } else {\n return FER_SKIP;\n }<\/p>\n
case FEO_LEAVEDIR:\n FinishDir(pszPath);<\/p>\n
\/* Propagate size into parent *\/\n m_pdirCur->m_pdirParent->m_ullSizeAll +=\n m_pdirCur->m_ullSizeAll;\n Pop();\n return FER_CONTINUE;<\/p>\n
default:\n return FER_CONTINUE;\n }\n \/* notreached *\/\n}<\/p>\n
FERESULT CALLBACK EnumState_Callback(\n FEOPERATION feo,\n LPCTSTR pszDir, LPCTSTR pszPath,\n const WIN32_FIND_DATA* pwfd,\n void* pvContext)\n{\n EnumState* pstate =\n reinterpret_cast<EnumState*>(pvContext);\n return pstate->Callback(feo, pszDir,\n pszPath, pwfd);\n}<\/p>\n
int __cdecl main(int argc, char **argv)\n{\n EnumState state;\n if (EnumDirectoryTree(TEXT(“.”),\n EnumState_Callback,\n &state) == FER_CONTINUE) {\n state.FinishDir(TEXT(“.”));\n }\n return 0;\n}\n<\/PRE>\n
\nBoy that sure was an awful lot of typing, and what’s\nworse, the whole structure of the program has\nbeen obscured by the explicit state management.\nIt sure is hard to tell at a glance what this\nchunk of code is trying to do.\nInstead, you have to stare at the EnumState<\/CODE> class\nand reverse-engineer what’s going on.\n<\/P>\n\n(Yes, I could have simplified this code a little by\nusing a built-in stack class, but as I have already noted\nin the context of smart pointers,\nI try to present these articles in\n“pure” C++ so people won’t get into arguments about which\nclass library is best.)\n<\/P>\n
\nTomorrow, we’ll look at how the world would be\nif the function EnumDirectoryTree<\/CODE> were spec’d out\nby the caller rather than the enumerator!\n<\/P><\/p>\n","protected":false},"excerpt":{"rendered":"The COM model for enumeration (enumeration objects) is biased towards making life easy for the consumer and hard for the producer. The enumeration object (producer) needs to be structured as a state machine, which can be quite onerous for complicated enumerators, for example, tree walking or composite enumeration. On the other hand, the callback model […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-36853","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
The COM model for enumeration (enumeration objects) is biased towards making life easy for the consumer and hard for the producer. The enumeration object (producer) needs to be structured as a state machine, which can be quite onerous for complicated enumerators, for example, tree walking or composite enumeration. On the other hand, the callback model […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/36853","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=36853"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/36853\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=36853"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=36853"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=36853"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}