Thanks for your response, Albrecht.
The FLTK version that I am using is 1.3.5. For installation, I performed the steps in the following link, meaning that I used the bundled IDE files.
As I mentioned in my first post, the source code runs fine in x86. However, if I switch to build it for x64, I am getting errors. I also found that the preprocessor macro WIN32 is required to build in x64, however, if I include this macro, I am getting another errors in this case, which are linking errors. I've copied these errors here below.
Severity Code Description Project File Line Suppression State
Error LNK2019 unresolved external symbol "public: virtual __cdecl Fl_Widget::~Fl_Widget(void)" (??1Fl_Widget@@UEAA@XZ) referenced in function "public: virtual __cdecl Fl_Box::~Fl_Box(void)" (??1Fl_Box@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2001 unresolved external symbol "public: virtual __cdecl Fl_Widget::~Fl_Widget(void)" (??1Fl_Widget@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Source.obj 1
Error LNK2001 unresolved external symbol "public: virtual __cdecl Fl_Widget::~Fl_Widget(void)" (??1Fl_Widget@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual __cdecl Fl_Widget::~Fl_Widget(void)" (??1Fl_Widget@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\ZoomBox.obj 1
Error LNK2019 unresolved external symbol "public: void __cdecl Fl_Widget::redraw(void)" (?redraw@Fl_Widget@@QEAAXXZ) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2001 unresolved external symbol "public: void __cdecl Fl_Widget::redraw(void)" (?redraw@Fl_Widget@@QEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\ZoomBox.obj 1
Error LNK2019 unresolved external symbol "public: virtual __cdecl Fl_Group::~Fl_Group(void)" (??1Fl_Group@@UEAA@XZ) referenced in function "public: virtual __cdecl Toolbar::~Toolbar(void)" (??1Toolbar@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2001 unresolved external symbol "public: virtual __cdecl Fl_Group::~Fl_Group(void)" (??1Fl_Group@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Source.obj 1
Error LNK2001 unresolved external symbol "public: virtual __cdecl Fl_Group::~Fl_Group(void)" (??1Fl_Group@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "protected: virtual void __cdecl Fl_Window::draw(void)" (?draw@Fl_Window@@MEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2001 unresolved external symbol "protected: virtual void __cdecl Fl_Window::flush(void)" (?flush@Fl_Window@@MEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: __cdecl Fl_Window::Fl_Window(int,int,char const *)" (??0Fl_Window@@QEAA@HHPEBD@Z) referenced in function "public: __cdecl MainWindow::MainWindow(void)" (??0MainWindow@@QEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: virtual __cdecl Fl_Window::~Fl_Window(void)" (??1Fl_Window@@UEAA@XZ) referenced in function "int `public: __cdecl MainWindow::MainWindow(void)'::`1'::dtor$0" (?dtor$0@?0???0MainWindow@@QEAA@XZ@4HA) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2001 unresolved external symbol "public: virtual __cdecl Fl_Window::~Fl_Window(void)" (??1Fl_Window@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Source.obj 1
Error LNK2001 unresolved external symbol "public: virtual int __cdecl Fl_Window::handle(int)" (?handle@Fl_Window@@UEAAHH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Window::resize(int,int,int,int)" (?resize@Fl_Window@@UEAAXHHHH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Window::show(void)" (?show@Fl_Window@@UEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Window::show(void)" (?show@Fl_Window@@UEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Source.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Window::hide(void)" (?hide@Fl_Window@@UEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: virtual __cdecl Fl_Menu_::~Fl_Menu_(void)" (??1Fl_Menu_@@UEAA@XZ) referenced in function "public: virtual __cdecl Fl_Menu_Bar::~Fl_Menu_Bar(void)" (??1Fl_Menu_Bar@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2001 unresolved external symbol "public: virtual __cdecl Fl_Menu_::~Fl_Menu_(void)" (??1Fl_Menu_@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Source.obj 1
Error LNK2019 unresolved external symbol "public: int __cdecl Fl_Menu_::add(char const *,int,void (__cdecl*)(class Fl_Widget *,void *),void *,int)" (?add@Fl_Menu_@@QEAAHPEBDHP6AXPEAVFl_Widget@@PEAX@Z2H@Z) referenced in function "private: void __cdecl MainWindow::initialize_menu_bar(void)" (?initialize_menu_bar@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: int __cdecl Fl_Menu_::add(char const *)" (?add@Fl_Menu_@@QEAAHPEBD@Z) referenced in function "private: void __cdecl MainWindow::initialize_menu_bar(void)" (?initialize_menu_bar@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: __cdecl Fl_Menu_Bar::Fl_Menu_Bar(int,int,int,int,char const *)" (??0Fl_Menu_Bar@@QEAA@HHHHPEBD@Z) referenced in function "public: __cdecl MainWindow::MainWindow(void)" (??0MainWindow@@QEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: __cdecl Fl_Native_File_Chooser::Fl_Native_File_Chooser(int)" (??0Fl_Native_File_Chooser@@QEAA@H@Z) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: __cdecl Fl_Native_File_Chooser::~Fl_Native_File_Chooser(void)" (??1Fl_Native_File_Chooser@@QEAA@XZ) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: void __cdecl Fl_Native_File_Chooser::type(int)" (?type@Fl_Native_File_Chooser@@QEAAXH@Z) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: char const * __cdecl Fl_Native_File_Chooser::filename(void)const " (?filename@Fl_Native_File_Chooser@@QEBAPEBDXZ) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: void __cdecl Fl_Native_File_Chooser::directory(char const *)" (?directory@Fl_Native_File_Chooser@@QEAAXPEBD@Z) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: void __cdecl Fl_Native_File_Chooser::title(char const *)" (?title@Fl_Native_File_Chooser@@QEAAXPEBD@Z) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: void __cdecl Fl_Native_File_Chooser::filter(char const *)" (?filter@Fl_Native_File_Chooser@@QEAAXPEBD@Z) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: char const * __cdecl Fl_Native_File_Chooser::errmsg(void)const " (?errmsg@Fl_Native_File_Chooser@@QEBAPEBDXZ) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: int __cdecl Fl_Native_File_Chooser::show(void)" (?show@Fl_Native_File_Chooser@@QEAAHXZ) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "void __cdecl fl_register_images(void)" (?fl_register_images@@YAXXZ) referenced in function "public: __cdecl MainWindow::MainWindow(void)" (??0MainWindow@@QEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: __cdecl Fl_PNG_Image::Fl_PNG_Image(char const *)" (??0Fl_PNG_Image@@QEAA@PEBD@Z) referenced in function "private: void __cdecl MainWindow::menu_open_action(void)" (?menu_open_action@MainWindow@@AEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2019 unresolved external symbol "public: virtual __cdecl Fl_Pixmap::~Fl_Pixmap(void)" (??1Fl_Pixmap@@UEAA@XZ) referenced in function "public: virtual __cdecl Toolbar::~Toolbar(void)" (??1Toolbar@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\MainWindow.obj 1
Error LNK2001 unresolved external symbol "public: virtual __cdecl Fl_Pixmap::~Fl_Pixmap(void)" (??1Fl_Pixmap@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Source.obj 1
Error LNK2001 unresolved external symbol "public: virtual __cdecl Fl_Pixmap::~Fl_Pixmap(void)" (??1Fl_Pixmap@@UEAA@XZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2019 unresolved external symbol "public: static int __cdecl Fl::run(void)" (?run@Fl@@SAHXZ) referenced in function main VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Source.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Widget::show(void)" (?show@Fl_Widget@@UEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Widget::show(void)" (?show@Fl_Widget@@UEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\ZoomBox.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Widget::hide(void)" (?hide@Fl_Widget@@UEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Widget::hide(void)" (?hide@Fl_Widget@@UEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\ZoomBox.obj 1
Error LNK2001 unresolved external symbol "protected: virtual void __cdecl Fl_Group::draw(void)" (?draw@Fl_Group@@MEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual int __cdecl Fl_Group::handle(int)" (?handle@Fl_Group@@UEAAHH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Group::resize(int,int,int,int)" (?resize@Fl_Group@@UEAAXHHHH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2019 unresolved external symbol "public: __cdecl Fl_Group::Fl_Group(int,int,int,int,char const *)" (??0Fl_Group@@QEAA@HHHHPEBD@Z) referenced in function "public: __cdecl Toolbar::Toolbar(int,int,int,int)" (??0Toolbar@@QEAA@HHHH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2019 unresolved external symbol "public: __cdecl Fl_Button::Fl_Button(int,int,int,int,char const *)" (??0Fl_Button@@QEAA@HHHHPEBD@Z) referenced in function "public: __cdecl Toolbar::Toolbar(int,int,int,int)" (??0Toolbar@@QEAA@HHHH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2019 unresolved external symbol "public: __cdecl Fl_Image::Fl_Image(int,int,int)" (??0Fl_Image@@QEAA@HHH@Z) referenced in function "public: __cdecl Fl_Pixmap::Fl_Pixmap(char const * const *)" (??0Fl_Pixmap@@QEAA@PEBQEBD@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2019 unresolved external symbol "public: virtual __cdecl Fl_Image::~Fl_Image(void)" (??1Fl_Image@@UEAA@XZ) referenced in function "int `public: __cdecl Fl_Pixmap::Fl_Pixmap(char const * const *)'::`1'::dtor$0" (?dtor$0@?0???0Fl_Pixmap@@QEAA@PEBQEBD@Z@4HA) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2019 unresolved external symbol "private: void __cdecl Fl_Pixmap::set_data(char const * const *)" (?set_data@Fl_Pixmap@@AEAAXPEBQEBD@Z) referenced in function "public: __cdecl Fl_Pixmap::Fl_Pixmap(char const * const *)" (??0Fl_Pixmap@@QEAA@PEBQEBD@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2019 unresolved external symbol "protected: void __cdecl Fl_Pixmap::measure(void)" (?measure@Fl_Pixmap@@IEAAXXZ) referenced in function "public: __cdecl Fl_Pixmap::Fl_Pixmap(char const * const *)" (??0Fl_Pixmap@@QEAA@PEBQEBD@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual class Fl_Image * __cdecl Fl_Pixmap::copy(int,int)" (?copy@Fl_Pixmap@@UEAAPEAVFl_Image@@HH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Pixmap::color_average(unsigned int,float)" (?color_average@Fl_Pixmap@@UEAAXIM@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Pixmap::desaturate(void)" (?desaturate@Fl_Pixmap@@UEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Pixmap::draw(int,int,int,int,int,int)" (?draw@Fl_Pixmap@@UEAAXHHHHHH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Pixmap::label(class Fl_Widget *)" (?label@Fl_Pixmap@@UEAAXPEAVFl_Widget@@@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Pixmap::label(struct Fl_Menu_Item *)" (?label@Fl_Pixmap@@UEAAXPEAUFl_Menu_Item@@@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Pixmap::uncache(void)" (?uncache@Fl_Pixmap@@UEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\Toolbar.obj 1
Error LNK2019 unresolved external symbol "public: virtual int __cdecl Fl_Widget::handle(int)" (?handle@Fl_Widget@@UEAAHH@Z) referenced in function "public: virtual int __cdecl ZoomBox::handle(int)" (?handle@ZoomBox@@UEAAHH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\ZoomBox.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl Fl_Widget::resize(int,int,int,int)" (?resize@Fl_Widget@@UEAAXHHHH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\ZoomBox.obj 1
Error LNK2001 unresolved external symbol "protected: virtual void __cdecl Fl_Box::draw(void)" (?draw@Fl_Box@@MEAAXXZ) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\ZoomBox.obj 1
Error LNK2019 unresolved external symbol "public: __cdecl Fl_Box::Fl_Box(int,int,int,int,char const *)" (??0Fl_Box@@QEAA@HHHHPEBD@Z) referenced in function "public: __cdecl ZoomBox::ZoomBox(int,int,int,int)" (??0ZoomBox@@QEAA@HHHH@Z) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\ZoomBox.obj 1
Error LNK2001 unresolved external symbol "public: static int Fl::e_dy" (?e_dy@Fl@@2HA) VirtualAdvertisement C:\Users\Tom\source\repos\VirtualAdvertisement\VirtualAdvertisement\ZoomBox.obj 1
I am also getting some warnings, which are the following.