{"id":22503,"date":"2008-05-01T10:00:00","date_gmt":"2008-05-01T10:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2008\/05\/01\/strange-uses-for-window-class-atoms\/"},"modified":"2008-05-01T10:00:00","modified_gmt":"2008-05-01T10:00:00","slug":"strange-uses-for-window-class-atoms","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20080501-00\/?p=22503","title":{"rendered":"Strange uses for window class atoms"},"content":{"rendered":"

\nWhen you register a window class (with the\nRegisterClass<\/code> or\nRegisterClassEx<\/code> function),\nyou get an ATOM<\/code> back.\nWhat use is this atom?\n<\/p>\n

\n\nNot much<\/a>.\n<\/p>\n

\nYou can use this atom in many places where a window class name\ncan be used; just convert it to a string with the\nMAKEINTATOM<\/code> macro.\nLet’s change our\n\nscratch program<\/a> to illustrate:\n<\/p>\n

\nATOM g_atmClass;<\/font>\nBOOL\nInitApp(void)\n{\n    ...\n    g_atmClass = RegisterClass(&wc);\n    if (!g_atmClass) return FALSE;<\/font>\n    ...\n}\nint WINAPI WinMain(HINSTANCE hinst, HINSTANCE hinstPrev,\n                   LPSTR lpCmdLine, int nShowCmd)\n{\n    ...\n        hwnd = CreateWindow(\n            MAKEINTATOM(g_atmClass)<\/font>,        \/* Class Name *\/\n            \"Scratch\",                      \/* Title *\/\n            WS_OVERLAPPEDWINDOW,            \/* Style *\/\n            CW_USEDEFAULT, CW_USEDEFAULT,   \/* Position *\/\n            CW_USEDEFAULT, CW_USEDEFAULT,   \/* Size *\/\n            NULL,                           \/* Parent *\/\n            NULL,                           \/* No menu *\/\n            hinst,                          \/* Instance *\/\n            0);                             \/* No special parameters *\/\n    ...\n}\n<\/pre>\n
\nWe save the atom returned by the RegisterClass<\/code>\nfunction and use it (in the form of a MAKEINTATOM<\/code>)\nin place of the class name.\nif you run this program, you’ll see that it works exactly the same\nas the old version that used the class name.\nThe class atom is valid as long as the class remains registered.\n<\/p>\n
\nFunctions that accept a MAKEINTATOM<\/code> as the class name\ninclude\nCreateWindow<\/code>,\nFindWindow<\/code>,\nGetClassInfo<\/code>,\nand UnregisterClass<\/code>\n(and the Ex<\/code> versions of them).\n<\/p>\n
\nWhy would you do this?\n<\/p>\n
\nWell, there really isn’t much reason.\nThe string name works just as well as the atom,\nso the atom is just one more thing to keep track of.\nHowever, even though you don’t use it,\nyou have to be aware that other people might.\nFor example, the lpszClass<\/code> member of the\nCREATESTRUCT<\/code> structure is usually a pointer\nto a string, but it could be a MAKEINTATOM<\/code> if\nsomebody decided to pass an atom instead of a string to\nCreateWindow<\/code>.\nThose of you who’ve read\n\nthe first Bonus Chapter of my book<\/a>\nare already familiar with the program that crashed when\nsomebody created a window via an atom.\n<\/p>\n
\nThere is one interesting thing you can do with the atom:\nIf you have a valid class atom, you can quickly tell whether\na window belongs to that class by checking the window word\nfor the atom:\n<\/p>\n
\nif (GetWindowWord(hwnd, GWW_ATOM) == atom) ...\n<\/pre>\n
\nThis technique saves you the trouble of calling\nGetClassName<\/code> and then doing a string comparison,\nreducing it instead to an integer comparison.\nThis technique makes it very simple to\n\nwrite a TestIfWndIsDialog<\/code> function<\/a>:\n<\/p>\n
\nBOOL TestIfWndIsDialog(HWND hwnd)\n{\n  return GetWindowWord(hwnd, GWW_ATOM) == (ULONG_PTR)WC_DIALOG<\/a>;\n}\n<\/pre>\n
\nExercise<\/b>: Discuss the limitations of the above\nTestIfWndIsDialog<\/code> function.<\/p>\n","protected":false},"excerpt":{"rendered":"
When you register a window class (with the RegisterClass or RegisterClassEx function), you get an ATOM back. What use is this atom? Not much. You can use this atom in many places where a window class name can be used; just convert it to a string with the MAKEINTATOM macro. Let’s change our scratch program […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-22503","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
When you register a window class (with the RegisterClass or RegisterClassEx function), you get an ATOM back. What use is this atom? Not much. You can use this atom in many places where a window class name can be used; just convert it to a string with the MAKEINTATOM macro. Let’s change our scratch program […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/22503","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=22503"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/22503\/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=22503"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=22503"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=22503"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}