pass lambda as Windows CALLBACK function failed - c++11

Question

static std::function<LRESULT(int nCode, WPARAM wParam, LPARAM lParam)> keyProc =
[](int nCode, WPARAM wParam, LPARAM lParam)->LRESULT {
return 1;
};
keyHook = SetWindowsHookEx(WH_KEYBOARD_LL, keyProc, theApp.m_hInstance, NULL);
Error C2664 'HHOOK SetWindowsHookExW(int,HOOKPROC,HINSTANCE,DWORD)':
cannot convert argument 2 from 'std::function' to
'HOOKPROC' MFCTest d:\test\mfctest2\mfctest\keymaphooker.cpp 62
PS: Why did I use such a long expression rather than auto? Because I want to it be static.
I changed it to
auto keyProc =
[this](int nCode, WPARAM wParam, LPARAM lParam)->LRESULT {
return 1;//return KeyboardProc(keyHook, keyMap, nCode, wParam, lParam);
};
It doesn't work again.
error C2664: 'HHOOK SetWindowsHookExW(int,HOOKPROC,HINSTANCE,DWORD)':
cannot convert argument 2 from
'KeyMapHooker::StartHook::'
to 'HOOKPROC'

Answer 1

A stateless lambda (such as the one you are using) can indeed be converted to a pointer to function, and thus can be used as CALLBACK. However, std::function cannot be converted in this manner.
Note that the type of a lambda expression is not std::function, it's an unnamed class type (the closure type of the lambda expression). You can capture the correct type using auto (which has no effect on storage duration):
static auto keyProc =
[](int nCode, WPARAM wParam, LPARAM lParam)->LRESULT {
return 1;
};
keyHook = SetWindowsHookEx(WH_KEYBOARD_LL, keyProc, theApp.m_hInstance, NULL);