Re: How to use base class & child class as parameter in one function ?
On Friday, 22 May 2020 at 22:44:17 UTC, H. S. Teoh wrote:
On Fri, May 22, 2020 at 09:39:16PM +, Vinod K Chandran via
Digitalmars-d-learn wrote: [...]
So in the same manner, i want
void function(Base) = fnPtr wiil work with
void function(Child)
You cannot, because that's type unsafe:
class Base {}
class Derived : Base {
void derivedFunc() {}
}
class Another : Base {}
void derivedFunc(Derived d) { d.derivedFunc(); }
void function(Base) funPtr;
funPtr = derivedFunc; // suppose this was allowed
Base obj = new Another;
funPtr(obj); // crash: obj does not have derivedFunc()
T
Yeah, I understand that. And i just changed my code. Thanks for
the guidance. :)
Re: How to use base class & child class as parameter in one function ?
On Friday, 22 May 2020 at 22:40:50 UTC, Steven Schveighoffer
wrote:
On 5/22/20 5:39 PM, Vinod K Chandran wrote:
[...]
That is the opposite of what you are thinking. A function
pointer has to be valid based on its parameter types. Covariant
functions are allowed.
This is OK:
void function(Child) fptr;
void foo(Base) {}
fptr = // OK! it's fine to call fptr with a Child,
because it is a Base as well
void function(Base) fptr2;
void foo2(Child) {}
fptr2 = // Error! if you called fptr2 with a Base that
is NOT a Child, bad things will happen.
This is more clear if you actually try calling them:
fptr2(new Base); // the compiler should allow this
foo2(new Base); // but would not allow this
So why should fptr2 be allowed to point at foo2?
-Steve
Thank you for the guidance. I got the point. :)
Re: How to use base class & child class as parameter in one function ?
On 5/22/20 5:39 PM, Vinod K Chandran wrote:
On Friday, 22 May 2020 at 20:51:20 UTC, Steven Schveighoffer wrote:
On 5/22/20 4:04 PM, Vinod K Chandran wrote:
[...]
Yes. What you cannot do is this (which I hope doesn't compile in
VB.net, but I wouldn't be surprised):
Dim sampleList As New List(Of Child)
sampleList.Add(New Base(10))
Which is the equivalent of what you were requesting.
Nope--
List(Of Base) will contain an instance of a Child.
So in the same manner, i want
void function(Base) = fnPtr wiil work with
void function(Child)
That is the opposite of what you are thinking. A function pointer has to
be valid based on its parameter types. Covariant functions are allowed.
This is OK:
void function(Child) fptr;
void foo(Base) {}
fptr = // OK! it's fine to call fptr with a Child, because it is a
Base as well
void function(Base) fptr2;
void foo2(Child) {}
fptr2 = // Error! if you called fptr2 with a Base that is NOT a
Child, bad things will happen.
This is more clear if you actually try calling them:
fptr2(new Base); // the compiler should allow this
foo2(new Base); // but would not allow this
So why should fptr2 be allowed to point at foo2?
-Steve
Re: How to use base class & child class as parameter in one function ?
On Fri, May 22, 2020 at 09:39:16PM +, Vinod K Chandran via
Digitalmars-d-learn wrote:
[...]
> So in the same manner, i want
> void function(Base) = fnPtr wiil work with
> void function(Child)
You cannot, because that's type unsafe:
class Base {}
class Derived : Base {
void derivedFunc() {}
}
class Another : Base {}
void derivedFunc(Derived d) { d.derivedFunc(); }
void function(Base) funPtr;
funPtr = derivedFunc; // suppose this was allowed
Base obj = new Another;
funPtr(obj); // crash: obj does not have derivedFunc()
T
--
Век живи - век учись. А дураком помрёшь.
Re: How to use base class & child class as parameter in one function ?
On Friday, 22 May 2020 at 20:51:20 UTC, Steven Schveighoffer wrote: On 5/22/20 4:04 PM, Vinod K Chandran wrote: [...] Yes. What you cannot do is this (which I hope doesn't compile in VB.net, but I wouldn't be surprised): Dim sampleList As New List(Of Child) sampleList.Add(New Base(10)) Which is the equivalent of what you were requesting. -Steve Nope-- List(Of Base) will contain an instance of a Child. So in the same manner, i want void function(Base) = fnPtr wiil work with void function(Child)
Re: How to use base class & child class as parameter in one function ?
On Fri, May 22, 2020 at 08:55:45PM +, Vinod K Chandran via
Digitalmars-d-learn wrote:
> On Friday, 22 May 2020 at 20:06:20 UTC, Adam D. Ruppe wrote:
> > On Friday, 22 May 2020 at 20:04:24 UTC, Vinod K Chandran wrote:
> > > sampleList.Add(New Child(10.5)) Is this possible in D without
> > > casting ?
> >
> > Direct translation of this code works just fine in D.
>
> Yeah, my bad. I just checked in D. But think inherited type difference
> is a problem in function pointer's parameters only.
> alias EvtFuncPtr = void function(EventArgs);
> Now, this EvtFuncPtr won't allow any derived classes of EventArgs as
> parameter. That's the problem i am facing.
[...]
So you're basically saying:
void function(DerivedClass)
cannot implicitly convert to:
void function(BaseClass)
right?
This is as it should be:
class Base { ... }
class Derived : Base { ... }
class Another : Base { ... }
void baseFunc(Base) { ... }
void derivedFunc(Derived) { ... }
void function(Base) funcPtr;
funcPtr = baseFunc; // OK
funcPtr = derivedFunc; // Not allowed
Why is it bad to assign derivedFunc to funcPtr? Consider this:
Base obj = new Another;
funcPtr = baseFunc;
funcPtr(obj); // OK, because Another is a subtype of Base
funcPtr = derivedFunc; // suppose this was allowed
funcPtr(obj); // Uh oh, derivedFunc receives an argument of
// type Another which is not a subtype of
// Derived
So, it's not permissible to allow assigning derivedFunc to funcPtr
without a cast.
T
--
Indifference will certainly be the downfall of mankind, but who cares? --
Miquel van Smoorenburg
Re: How to use base class & child class as parameter in one function ?
On Friday, 22 May 2020 at 20:06:20 UTC, Adam D. Ruppe wrote: On Friday, 22 May 2020 at 20:04:24 UTC, Vinod K Chandran wrote: sampleList.Add(New Child(10.5)) Is this possible in D without casting ? Direct translation of this code works just fine in D. Yeah, my bad. I just checked in D. But think inherited type difference is a problem in function pointer's parameters only. alias EvtFuncPtr = void function(EventArgs); Now, this EvtFuncPtr won't allow any derived classes of EventArgs as parameter. That's the problem i am facing. What about a template ? alias EvtFuncPtr = void function(T)(T = EventArgs); This is not compiled.
Re: How to use base class & child class as parameter in one function ?
On 5/22/20 4:04 PM, Vinod K Chandran wrote:
On Friday, 22 May 2020 at 16:12:12 UTC, Steven Schveighoffer wrote:
On 5/22/20 9:10 AM, Vinod K Chandran wrote:
On Friday, 22 May 2020 at 12:21:25 UTC, rikki cattermole wrote:
if (Child child = cast(Child)parent) {
assert(child !is null);
}
Actually, problem occurs in addHandler function. It expects an
argument of type "EventArgs", not MouseEventArgs.
Yes, because what if you did this with your function:
fnp(new EventArgs(...));
It would be called with the type being implicitly cast to the child
type without that being true.
What Rikki was recommending is that you write your handler like this:
void onClick(EventArgs e){
if(auto me = cast(MouseEventArgs)e) {
log("form clicked on x = ", me.x, ", and y = ", me.y);
}
}
Actually, if you are certain it's a programming error for onClick to
be called with a different type of event args, I'd do:
void onClick(EventArgs e){
auto me = cast(MouseEventArgs)e;
assert(me !is null, "Error, onClick called with invalid event type");
log("form clicked on x = ", me.x, ", and y = ", me.y);
}
Thanks for the answer. I understand that, in D, derived class and base
class are not the same as in vb.net or any other language. (Please
correct me if i am wrong).
In vb.net, assume that i have a class setup like this--
Public Class Base
Public Property SampleInt As Integer
End Class
Public Class Child : Inherits Base
Public Property SampleDouble As Double
End Class
//Assume that i have a list of Base class like this--
Dim sampleList As New List(Of Base)
// Now, i can use this list like this--
sampleList.Add(New Child(10.5)) Is this possible in D without casting ?
Yes. What you cannot do is this (which I hope doesn't compile in VB.net,
but I wouldn't be surprised):
Dim sampleList As New List(Of Child)
sampleList.Add(New Base(10))
Which is the equivalent of what you were requesting.
-Steve
Re: How to use base class & child class as parameter in one function ?
On Friday, 22 May 2020 at 20:04:24 UTC, Vinod K Chandran wrote: sampleList.Add(New Child(10.5)) Is this possible in D without casting ? Direct translation of this code works just fine in D.
Re: How to use base class & child class as parameter in one function ?
On Friday, 22 May 2020 at 16:12:12 UTC, Steven Schveighoffer
wrote:
On 5/22/20 9:10 AM, Vinod K Chandran wrote:
On Friday, 22 May 2020 at 12:21:25 UTC, rikki cattermole wrote:
if (Child child = cast(Child)parent) {
assert(child !is null);
}
Actually, problem occurs in addHandler function. It expects an
argument of type "EventArgs", not MouseEventArgs.
Yes, because what if you did this with your function:
fnp(new EventArgs(...));
It would be called with the type being implicitly cast to the
child type without that being true.
What Rikki was recommending is that you write your handler like
this:
void onClick(EventArgs e){
if(auto me = cast(MouseEventArgs)e) {
log("form clicked on x = ", me.x, ", and y = ", me.y);
}
}
Actually, if you are certain it's a programming error for
onClick to be called with a different type of event args, I'd
do:
void onClick(EventArgs e){
auto me = cast(MouseEventArgs)e;
assert(me !is null, "Error, onClick called with invalid
event type");
log("form clicked on x = ", me.x, ", and y = ", me.y);
}
Thanks for the answer. I understand that, in D, derived class and
base class are not the same as in vb.net or any other language.
(Please correct me if i am wrong).
In vb.net, assume that i have a class setup like this--
Public Class Base
Public Property SampleInt As Integer
End Class
Public Class Child : Inherits Base
Public Property SampleDouble As Double
End Class
//Assume that i have a list of Base class like this--
Dim sampleList As New List(Of Base)
// Now, i can use this list like this--
sampleList.Add(New Child(10.5)) Is this possible in D without
casting ?
Re: How to use base class & child class as parameter in one function ?
On 5/22/20 9:10 AM, Vinod K Chandran wrote:
On Friday, 22 May 2020 at 12:21:25 UTC, rikki cattermole wrote:
if (Child child = cast(Child)parent) {
assert(child !is null);
}
Actually, problem occurs in addHandler function. It expects an argument
of type "EventArgs", not MouseEventArgs.
Yes, because what if you did this with your function:
fnp(new EventArgs(...));
It would be called with the type being implicitly cast to the child type
without that being true.
What Rikki was recommending is that you write your handler like this:
void onClick(EventArgs e){
if(auto me = cast(MouseEventArgs)e) {
log("form clicked on x = ", me.x, ", and y = ", me.y);
}
}
Actually, if you are certain it's a programming error for onClick to be
called with a different type of event args, I'd do:
void onClick(EventArgs e){
auto me = cast(MouseEventArgs)e;
assert(me !is null, "Error, onClick called with invalid event type");
log("form clicked on x = ", me.x, ", and y = ", me.y);
}
Re: How to use base class & child class as parameter in one function ?
On Friday, 22 May 2020 at 12:21:25 UTC, rikki cattermole wrote:
if (Child child = cast(Child)parent) {
assert(child !is null);
}
Actually, problem occurs in addHandler function. It expects an
argument of type "EventArgs", not MouseEventArgs.
Re: How to use base class & child class as parameter in one function ?
if (Child child = cast(Child)parent) {
assert(child !is null);
}
How to use base class & child class as parameter in one function ?
Hi all,
I have a windows gui setup like this;
class EventArgs {} \\ Base class for all messages
class MouseEventArgs : EventArgs { // child class for handling
mouse messages
...
int x;
int y;
this(WPARAM wpm, LPARAM lpm){
this.x = xFromLparam(lpm);
this.y = yFromLparam(lpm);
}
}
alias EvtFuncPtr = void function(EventArgs); // function pointer
struct MsgHandler {
uint message ;
HWND handle ;
bool isActive ;
EvtFuncPtr fnPtr;
}
// Then in my window class...
void addHandler(uint we, EvtFuncPtr fnp){ // This is error point.
auto mh = MsgHandler();
mh.handle = this.mHandle;
mh.message = we;
mh.fnPtr = fnp;
mh.isActive = true;
this.msgHandlerList ~= mh; // This is a list in
window class.
}
// And in the WndProc...
auto thisWin = findWindowClass(hWnd); // get the window class
with hWnd.
auto mh = thisWin.findHandler(hWnd, message);// get the event
handler for this message & hWnd
if(mh.isActive) { // if there is an event handler fixed,
switch (message){
case 512 : .. case 526 : // if it's a Mouse messages
auto ea = new MouseEventArgs(wParam, lParam);
mh.fnPtr(ea); // execute that event handler function
break;
default : break;
}
}
// And this is the window creation site...
auto app = new Application() ;
auto frm = new Window(app) ;
frm.createWindow() ;
frm.addHandler(frm.load, );
frm.addHandler(frm.Click, );
void onLoad(EventArgs e){
log("form is loaded...");
}
void onClick(MouseEventArgs e){ // Compiler wants to change the
MouseEventArgs with EventArgs.
log("form clicked on x = ", e.x, ", and y = ", e.y);
}
I wrote EventArgs as the parameter type in function pointer but
i want to use it's child classes also. But i can't.
