Re: Function Pointer Query

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"Igor Tandetnik" wrote:

"Rich S." <RichS@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:CAAA1E12-6432-4B32-9053-8047799C13E0@xxxxxxxxxxxxx
Is it possible to have a function pointer that points to a function
in a class that is NON-static?

Here's a pointer to member function:

class C {
public:
void f(int);
};

typedef void (C::*PF)(int);
PF pf = &C::f;

C c;
(c.*pf)(1); // calls c.f(1)

Note that pointer to member function is incompatible with regular
function pointer. You can't pass the former where the latter is
expected. If you hope to pass a member function pointer to some API that
needs a callback, you are sadly out of luck. Most APIs taking a callback
require a non-member or a static member function.

Since static member functions are so inconvenient, I am hoping that
there is a way to pass a pointer to a function that is non-static.

Most APIs that take a callback also take a context (aka closure)
parameter that they pass right back to the callback. Let's take
EnumWindows:

class C {
public:
void EnumWindows();
BOOL EnumProc(HWND);
static BOOL CALLBACK EnumProcStatic(HWND, LPARAM);
};

void C::EnumWindows() {
::EnumWindows(EnumProcStatic, reinterpret_cast<LPARAM>(this));
}

BOOL CALLBACK C::EnumProcStatic(HWND hwnd, LPARAM lparam) {
C* pThis = reinterpret_cast<C*>(lparam);
return pThis->EnumProc(hwnd);
}

BOOL C::EnumProc(HWND hwnd) {
// Do actual work here
}

--
With best wishes,
Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925




Thank you Igor. I was vaguely aware of things called "pointer to member",
but I did not know how they fit into the scheme of things before your answer.

I guess I will have to implement one of the workarounds that you and Carl
mentioned.

Rich
.

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