Thread: Static function pointer linker error

Hello all,

I'm writing a class with a static function pointer in it, however I'm getting a linker error saying that it is an unresolved external symbol. Here's my code:

BindHook.h

Code:

#ifndef BINDHOOK
#define BINDHOOK

#include <Windows.h>

class BindHook
{
public:
	static HHOOK kbHook, msHook;

	static bool (*onKeyEvent)(int vkey, WPARAM msg);
	
	static void InitHooks();
	static void StopHooks();

	static LRESULT CALLBACK KeyboardProc(int nCode, WPARAM wParam, LPARAM lParam);
	static LRESULT CALLBACK MouseProc(int nCode, WPARAM wParam, LPARAM lParam);
};

#endif

BindHook.cpp

Code:

#include "BindHook.h"

bool (BindHook::*onKeyEvent)(int vkey, WPARAM msg) = NULL;

HHOOK BindHook::kbHook = 0;
HHOOK BindHook::msHook = 0;

LRESULT CALLBACK BindHook::KeyboardProc(int nCode, WPARAM wParam, LPARAM lParam)
{
	if (nCode != HC_ACTION)
		return CallNextHookEx(kbHook, nCode, wParam, lParam);
	
	//get the struct
	PKBDLLHOOKSTRUCT kb = (PKBDLLHOOKSTRUCT)lParam;

	//injected key?
	bool inj = ((kb->flags & 0x00000010) == 16);

	//call fn's
	if ((wParam == WM_KEYDOWN || wParam == WM_KEYUP) && !inj)
		if (onKeyEvent(kb->vkCode, wParam))
			return true;

	return CallNextHookEx(kbHook, nCode, wParam, lParam);
}

LRESULT CALLBACK BindHook::MouseProc(int nCode, WPARAM wParam, LPARAM lParam)
{
	if (nCode != HC_ACTION)
		return CallNextHookEx(msHook, nCode, wParam, lParam);

	PMSLLHOOKSTRUCT ms = (PMSLLHOOKSTRUCT)lParam;
	bool inj = ((ms->flags & 0x00000001) == 1);
	int vk = -1;

	//determine VKEY
	switch (wParam)
	{
	case WM_LBUTTONUP: case WM_LBUTTONDOWN:
		vk = VK_LBUTTON;
		break;
	case WM_RBUTTONUP: case WM_RBUTTONDOWN:
		vk = VK_LBUTTON;
		break;
	case WM_MBUTTONUP: case WM_MBUTTONDOWN:
		vk = VK_LBUTTON;
		break;
	case WM_XBUTTONUP: case WM_XBUTTONDOWN:
		vk = ((ms->mouseData >> 16 ) & 0xFFFF) == 1 ? VK_XBUTTON1 : VK_XBUTTON2;
		break;
	}

	switch (wParam)
	{
	case WM_LBUTTONUP:
	case WM_RBUTTONUP:
	case WM_MBUTTONUP:
	case WM_XBUTTONUP:
		if (!inj)
			if(onKeyEvent(vk, WM_KEYUP))
				return true;
		break;
	case WM_LBUTTONDOWN:
	case WM_RBUTTONDOWN:
	case WM_MBUTTONDOWN:
	case WM_XBUTTONDOWN:
		if (!inj)
			if(onKeyEvent(vk, WM_KEYDOWN))
				return true;
		break;
	}

	return CallNextHookEx(msHook, nCode, wParam, lParam);
}

void BindHook::InitHooks()
{
	kbHook = SetWindowsHookEx(WH_KEYBOARD_LL, (HOOKPROC)KeyboardProc, NULL, NULL);
}

void BindHook::StopHooks()
{
	UnhookWindowsHookEx(kbHook);
}

The specific Error:

Code:

Error	1	error LNK2001: unresolved external symbol "public: static bool (__cdecl* BindHook::onKeyEvent)(int,unsigned int)" (?onKeyEvent@BindHook@@2P6A_NHI@ZA)	C:\[...]\BindHook.obj	BindSystem_Console

Any help would be greatly appreciated.

Thanks,

-Matt

Code:

bool (BindHook::*onKeyEvent)(int vkey, WPARAM msg) = NULL;

This should be

Code:

bool (*(BindHook::onKeyEvent))(int vkey, WPARAM msg) = NULL;

I don't know how to explain this particular function pointer rule. I think that perhaps BindHook::*onKeyEvent is the type, and *(BindHook:: onKeyEvent) is the variable itself (that's just speculation though).

The situation becomes much clearer if you use typedefs (I'd recommend the typedefs route any time you need function pointers):

Code:

//in the header
class BindHook
{
    ...
    typedef bool (*KeyEventFunc)(int vkey, WPARAM msg);
    static KeyEventFunc onKeyEvent;
    ...
};

//and in the code
BindHook::KeyEventFunc BindHook::onKeyEvent = NULL;

Even better:

Code:

typedef bool (KeyEventFunc_t)(int vkey, WPARAM msg);
KeyEventFunc_t* KeyEventFunc;

As with any static variable, you need to define them in a source file (and only once):

Code:

KeyEventFunc_t* MyClass::KeyEventFunc;