{"id":1893,"date":"2014-02-03T07:00:00","date_gmt":"2014-02-03T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2014\/02\/03\/how-can-i-make-a-wndproc-or-dlgproc-a-member-of-my-c-class\/"},"modified":"2014-02-03T07:00:00","modified_gmt":"2014-02-03T07:00:00","slug":"how-can-i-make-a-wndproc-or-dlgproc-a-member-of-my-c-class","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20140203-00\/?p=1893","title":{"rendered":"How can I make a WNDPROC or DLGPROC a member of my C++ class?"},"content":{"rendered":"

\nContinuing my discussion of\n\nHow can I make a callback function a member of my C++ class?<\/i><\/a>\n<\/p>\n

\nCommon special cases for wanting to use a member function as a\ncallback function are the window procedure and its cousin the\ndialog procedure.\nThe question, then, is where to put the reference data.\n<\/p>\n

\nLet’s start with window procedures.\nThe Create­Window<\/code> function and its close friend\nCreate­Window­Ex<\/code> let you pass your reference\ndata as the final parameter, prototyped as\nLPVOID lpParam<\/code>.\nAs noted in the documentation,\nthat value is passed back to the window procedure by the\nWM_NC­CREATE<\/code> and\nWM_CREATE<\/code> messages\nas part of the CREATE­STRUCT<\/code> structure.\nOne of the first messages passed to a window is\nWM_NC­CREATE<\/code>,\nso that’s where we’ll grab the reference data and save it for later.\n<\/p>\n

\nYou can follow along\n\nin this simple C++ program<\/a>:\nThe static window procedure handles the\nWM_NC­CREATE<\/code> message by\nextracting the lpCreate­Params<\/code> from the\nCREATE­STRUCT<\/code> and saving it in the\nGWLP_USER­DATA<\/code> window bytes.\nThat value is a special per-window storage location provided\nfor the benefit of the window procedure,\nand most people use it to store their context parameter for\nsafekeeping.\n<\/p>\n

\nIf the message is not\nWM_NC­CREATE<\/code>,\nthen we retrieve the context parameter from\nwhere we had stashed it.\n<\/p>\n

\nEither way, we end up with a copy of the context parameter.\nIf you want your window procedure to be a member function,\nthe natural choice for the context parameter is the this<\/code>\npointer for the instance.\nThe static window procedure therefore tends to look like this:\n<\/p>\n

\nLRESULT CALLBACK MyWindowClass::s_WndProc(\n    HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)\n{\n MyWindowClass *pThis; \/\/ our \"this\" pointer will go here\n if (uMsg == WM_NCCREATE) {\n  \/\/ Recover the \"this\" pointer which was passed as a parameter\n  \/\/ to CreateWindow(Ex).\n  LPCREATESTRUCT lpcs = reinterpret_cast<LPCREATESTRUCT>(lParam);\n  pThis = static_cast<MyWindowClass*>(lpcs->lpCreateParams);\n  \/\/ Put the value in a safe place for future use\n  SetWindowLongPtr(hwnd, GWLP_USERDATA,\n                   reinterpret_cast<LONG_PTR>(pThis));\n } else {\n  \/\/ Recover the \"this\" pointer from where our WM_NCCREATE handler\n  \/\/ stashed it.\n  pThis = reinterpret_cast<MyWindowClass*>(\n              GetWindowLongPtr(hwnd, GWLP_USERDATA));\n }\n if (pThis) {\n  \/\/ Now that we have recovered our \"this\" pointer, let the\n  \/\/ member function finish the job.\n  return pThis->WndProc(hwnd, uMsg, wParam, lParam);\n }\n \/\/ We don't know what our \"this\" pointer is, so just do the default\n \/\/ thing. Hopefully, we didn't need to customize the behavior yet.\n return DefWindowProc(hwnd, uMsg, wParam, lParam);\n}\n<\/pre>\n
\nYou pass the the this<\/code> pointer to Create­Window<\/code>\nas the last parameter, so that the window procedure can pick it up.\n<\/p>\n
\nhwnd = CreateWindow(... other parameters..., this<\/u>);\n<\/pre>\n
\nFor dialog boxes, you can do basically the same thing.\nIt’s just that the bookkeeping is slightly different.\n<\/p>\n