nana: undefined behaviour - many violations of strict aliasing

All of these reinterpret_casts are undefined behaviour:

https://github.com/cnjinhao/nana/blob/ffa170b5f560dbf899030bdeb0e539aa95b03649/source/gui/programming_interface.cpp#L297-L310

owner and parent arguments are of type window which is:

https://github.com/cnjinhao/nana/blob/ffa170b5f560dbf899030bdeb0e539aa95b03649/include/nana/gui/basis.hpp#L90-L93

which is:

https://github.com/cnjinhao/nana/blob/ffa170b5f560dbf899030bdeb0e539aa95b03649/include/nana/gui/basis.hpp#L25-L31

The other side of casts is basic_window which is

https://github.com/cnjinhao/nana/blob/ffa170b5f560dbf899030bdeb0e539aa95b03649/include/nana/gui/detail/basic_window.hpp#L83-L84

which is a separate class that does not share any ancestors in it’s inheritance tree with detail::window_handle_impl

Therefore, the dereference of pointers coming from casts between detail::window_handle_impl* and basic_window* is undefined behaviour.

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This issue is probably related to #445 because:

• the intialization path practically always goes through this code (in order to create a window)
• the code does not crash without link-time optimization (used -flto -fno-fat-lto-objects) (this suggests that crucial violations are probably across source-file/function boundary)
• some minimal examples do not crash when compiled with -fno-strict-aliasing

---

Other notes:

• there are many more suspicious reinterpret_casts in the repository, all of them should be checked; total 684 casts in the project as reported by grep -r reinterpret_cast
• compilers can not validate strict aliasing at compile time, in majority of non-trivial cases there are no warnings
• GCC 8.3 with -Wstrict-aliasing=2 or -Wstict-aliasing=1 on this project catches only a small number of only possible strict aliasing violations

nana\source\system\dataexch.cpp:73:72: warning: dereferencing type-punned pointer might break strict-aliasing rules [-Wstrict-aliasing]
    if (::GetDIBits(hDC, (HBITMAP)g.pixmap(), 0, 1, NULL, (BITMAPINFO *)&bmi, DIB_RGB_COLORS) == 0) {
                                                                        ^~~~
nana\source\paint\graphics.cpp: In member function 'void nana::paint::graphics::make(const nana::size&)':
nana\source\paint\graphics.cpp:349:90: warning: dereferencing type-punned pointer might break strict-aliasing rules [-Wstrict-aliasing]
     HBITMAP bmp = ::CreateDIBSection(cdc, &bmi, DIB_RGB_COLORS, reinterpret_cast<void**>(&(dw->pixbuf_ptr)), 0, 0);
                                                                                          ^~~~~~~~~~~~~~~~~
nana\source\paint\graphics.cpp: In member function 'void nana::paint::graphics::save_as_file(const char*) const':
nana\source\paint\graphics.cpp:1079:98: warning: dereferencing type-punned pointer might break strict-aliasing rules [-Wstrict-aliasing]
     HBITMAP hBmp = ::CreateDIBSection(hdcMem, &bmpInfo, DIB_RGB_COLORS, reinterpret_cast<void**>(&pData), 0, 0);
                                                                                                  ^~~~~~

This is guuaranteed to crash given powerful enough optimizer (the last line violates both type aliasing rules and type alignment requirements):

https://github.com/cnjinhao/nana/blob/ffa170b5f560dbf899030bdeb0e539aa95b03649/source/paint/detail/image_bmp.hpp#L79-L85