You set up a new C++\/WinRT project and build it, and everything looks great.<\/p>\n
#include <winrt\/Windows.Gaming.Input.h>\r\n\r\nvoid CheckGamepads()\r\n{\r\n auto gamepads =\r\n winrt::Windows::Gaming::Input::Gamepad::Gamepads();\r\n for (auto&& gamepad : gamepads)\r\n {\r\n check(gamepad);\r\n }\r\n}\r\n<\/pre>\nThe code builds just fine except that you get a linker error that makes no sense. (Let’s face it, most linker errors make no sense until you put on your linker-colored glasses.)<\/p>\n
error LNK2019: unresolved external symbol \"public: struct winrt::What the heck is going on here?<\/p>\n
Take away all the decorations and it boils down to this:<\/p>\n
unresolved external symbol \"winrt::The linker couldn’t find a definition for the First<\/code> method.<\/p>\nThe answer from the linker’s point of view is obvious: You called this consume_BlahBlah<\/code> method but never defined it.<\/p>\nYeah, so tell me something I don’t know.<\/p>\n
Each C++\/WinRT header file contains the information needed to call methods on the classes in that namespace. In our case, we included Windows.Gaming.Input.h<\/code>, which tells us how to call methods on winrt::<\/code>Windows::<\/code>Gaming::<\/code>Input::Gamepad<\/code> objects. That made it possible to call Gamepad::<\/code>Gamepads()<\/code>. The resulting gamepads<\/code> variable is a winrt::<\/code>Windows::<\/code>Foundation::<\/code>Collections::<\/code>IVectorView<Gamepad><\/code>. We then use a ranged for<\/code> statement to enumerate them, and that means that we’re calling methods on the gamepads<\/code> object, which means that we’re calling methods on a winrt::<\/code>Windows::<\/code>Foundation::<\/code>Collections::<\/code>IVectorView<\/code> object.<\/p>\nAh, but we never told the compiler how to call the methods of IVectorView<\/code>. The Windows.Gaming.Input.h<\/code> header file included only the information to allow the methods of Gamepad<\/code> to be called. “Okay, I got you all set up for Gamepad<\/code>.” Any types required from other interfaces were left as forward declarations. “If you need them, you can get the definitions yourself.”\u00b9<\/p>\nWe used those forward declarations without ever defining them, hence the linker error.<\/p>\n
The solution is to include the required header file for the namespace.<\/p>\n
#include <winrt\/Windows.Foundation.Collections.h>\r\n<\/pre>\nThis is one of those rookie mistakes that make you scratch your head the first time you encounter it. The need to include the header file is mentioned in a big green box in the documentation<\/a>, but that’s not much consolation after you lost a few hours trying to figure it out.<\/p>\nThere’s some good news and bad news about this error message.<\/p>\n
The good news is that this error message is going away. The bad news is that it’s being replaced with a different error message. (But hopefully the new one is easier to understand.) More details next time<\/a>.<\/p>\n\u00b9 The idea is that you pay only for the namespaces you use. If every header file included its transitive closure of dependencies, (1)\u00a0you would create circular dependencies, and (2)\u00a0including a single header file would end up including all the other header files when you chased through all the dependencies.<\/p>\n