Implicit casting for method handler?

Question

Some code

typedef void (*EventHandler) (EventObject* sender, EventArgs* args, void* closure);

class Control
{
void AddHandler(int eventId, EventHandler handler, void* data)
}

class SubControl
{
static void mousemove_cb(EventObject* sender, MouseEventArgs* calldata, void* closure);
}

The error

error C2664: 'Control::AddHandler' : cannot convert parameter 2 from 'void (__cdecl *)(EventObject *,MouseEventArgs *,void *)' to 'EventHandler'

Here is the line that produce the error:

control.AddHandler(MouseMoveEvent, mousemove_cb, 0);

Description

The problem is that MouseEventArgs is a sub-class of EventArgs ! So, is there a way to have an automatic casting and register my method with the precise 'Event args'?

Presumably `MouseEventArgs` derives from `EventArgs` and your headers are set up correctly so that the code can determine this? — Antony Woods, Dec 10 '10 at 10:41

score 1 · Answer 1 · answered Dec 10 '10 at 10:47

1

You get the error because it's actually forbidden by the language. If it had been possible this would open a hole in the type system. Consider this code:

struct EventArgs {};

void f(EventHandler handler)
{
    EventArgs args;
    handler(0, &args, 0);
}

struct MouseEventArgs : EventArgs { void GetMousePosition(); };

void g(EventObject* sender, MouseEventArgs* args, void* closure)
{
    args->GetMousePosition(); 
}

f(g); // oops... g calls GetMousePosition on EventArgs

Fortunately the compiler catches this bug.

answered Dec 10 '10 at 10:47

Yakov Galka

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Does this answer the question? Is the issue not that `MouseEventArgs` and `EventArgs` are fundamentally different types? – Antony Woods Dec 10 '10 at 10:48
@acron: No, it's impossible even if one is the base of the other. See the edit. – Yakov Galka Dec 10 '10 at 10:52

Nawaz · Accepted Answer · 2010-12-10T11:46:28.277

C++ template could solve this problem. Use this:

struct Control
{
    //Note this change!
    template<typename TEventHandler>
    void AddHandler(int eventId, TEventHandler handler, void* data);
};

struct SubControl
{
    static void mousemove_cb(EventObject* sender, MouseEventArgs* calldata, void* closure);
    SubControl()
    {
        Control control;
        control.AddHandler(0, mousemove_cb, 0);
    }
};

score 0 · Answer 3 · answered Dec 10 '10 at 11:11

No, there is no automatic cast. Types have to coincide. You can change the signature to the AddHandler in two different ways:

Set it to accept a void* and then do force the conversion to the exact pointer to function type.
Convert the AddHandler into a template that accepts a type T so that it can be called with the correct parameters, t(sender, args,...), where the second parameter is the arguments. However, the call has to convert first the args to the correct type (for example, if it is a mouse event, convert it to MouseEventArgs manually before calling t.

Implicit casting for method handler?

3 Answers3