www.digitalmars.com         C & C++   DMDScript  

D - Method pointers

reply "Dario" <supdar yahoo.com> writes:
D delegates are a structure of an object (or a stack frame pointer) and a
pointer to a method (or to a nested function).
Anyway D has no way to declare a pointer to a method: Walter probably thinks
it is useless, or not worthy the effort of implementing it.
I find it a very useful feature instead (since they help me to write less or
simpler code).

Is that so difficult to implement?
What's wrong about a sintax like:
.   class C
.   {    void f1() { fp = &f2; }
.         void f2() { fp = &f1; }
.         void C.function() fp;
.         void f() { this.*fp(); } // we can reuse C++ sintax here
.    }
NOTE: actually you have to use delegates and waste 4 bytes for the reference
and some clock cycles.

I think that D should provide all the basic concepts and let the users code
with them: it is more important that D supports function and method pointers
than delegates (even if they can be useful sometimes, and would be even more
useful if a function pointer could be casted to a delegate!).

(Please forgive me for my English! I understand English well but I find it
hard to speak it correctly. ;-)
Oct 18 2003
parent reply "Walter" <walter digitalmars.com> writes:
I recognize that pointers to members are useful, but I think that delegates
are conceptually more powerful and cover the uses for pointers to members.
The more I use delegates the more useful they get! Anyhow, I ask that you
give delegates another chance.

"Dario" <supdar yahoo.com> wrote in message
news:bmrc5d$16c3$1 digitaldaemon.com...
 D delegates are a structure of an object (or a stack frame pointer) and a
 pointer to a method (or to a nested function).
 Anyway D has no way to declare a pointer to a method: Walter probably

 it is useless, or not worthy the effort of implementing it.
 I find it a very useful feature instead (since they help me to write less

 simpler code).

 Is that so difficult to implement?
 What's wrong about a sintax like:
 .   class C
 .   {    void f1() { fp = &f2; }
 .         void f2() { fp = &f1; }
 .         void C.function() fp;
 .         void f() { this.*fp(); } // we can reuse C++ sintax here
 .    }
 NOTE: actually you have to use delegates and waste 4 bytes for the

 and some clock cycles.

 I think that D should provide all the basic concepts and let the users

 with them: it is more important that D supports function and method

 than delegates (even if they can be useful sometimes, and would be even

 useful if a function pointer could be casted to a delegate!).

 (Please forgive me for my English! I understand English well but I find it
 hard to speak it correctly. ;-)

Nov 03 2003
next sibling parent reply "Charles Sanders" <sanders-consulting comcast.net> writes:
delegates rock!

consider the following two snippets of code:

// C++

class MsgSlot {
public:
 virtual bool proccess( LRESULT &rv, WPARAM wparam, LPARAM lparam ) { return
false; }

};


typedef LRESULT (MsgSlot::*win32Func) (LRESULT&, WPARAM, LPARAM);
typedef LRESULT (MsgSlot::*mouseFunc) (int,int,int);


class Win32MsgSlot : public MsgSlot {
public:
 Win32MsgSlot(win32Func func) : _func(func) { }

 virtual bool proccess(LRESULT &rv, WPARAM wparam, LPARAM lparam ) {
  (*this.*_func) (rv,wparam,lparam );
  return true;
 }

protected:

 win32Func _func;
};


// D


class MsgSlot
{
public:
 bit processMessage( out LRESULT rv, WPARAM wparam, LPARAM lparam )
 {
  return false;
 }
}

alias LRESULT delegate(WPARAM,LPARAM) win32Func;
alias LRESULT delegate(int,int,int) mouseFunc;

class Win32MsgSlot : MsgSlot
{
protected:
 win32Func _func;
public:
 this( win32Func func )
 {
  _func = func;
 }

 bit processMessage( out LRESULT rv, WPARAM wparam, LPARAM lparam )
 {
  rv = _func( wparam, lparam );
  return true;
 }
}


Much cleaner!

C


"Walter" <walter digitalmars.com> wrote in message
news:bo703s$2uji$1 digitaldaemon.com...
 I recognize that pointers to members are useful, but I think that

 are conceptually more powerful and cover the uses for pointers to members.
 The more I use delegates the more useful they get! Anyhow, I ask that you
 give delegates another chance.

 "Dario" <supdar yahoo.com> wrote in message
 news:bmrc5d$16c3$1 digitaldaemon.com...
 D delegates are a structure of an object (or a stack frame pointer) and


 pointer to a method (or to a nested function).
 Anyway D has no way to declare a pointer to a method: Walter probably

 it is useless, or not worthy the effort of implementing it.
 I find it a very useful feature instead (since they help me to write


 or
 simpler code).

 Is that so difficult to implement?
 What's wrong about a sintax like:
 .   class C
 .   {    void f1() { fp = &f2; }
 .         void f2() { fp = &f1; }
 .         void C.function() fp;
 .         void f() { this.*fp(); } // we can reuse C++ sintax here
 .    }
 NOTE: actually you have to use delegates and waste 4 bytes for the

 and some clock cycles.

 I think that D should provide all the basic concepts and let the users

 with them: it is more important that D supports function and method

 than delegates (even if they can be useful sometimes, and would be even

 useful if a function pointer could be casted to a delegate!).

 (Please forgive me for my English! I understand English well but I find


 hard to speak it correctly. ;-)


Nov 04 2003
parent Dario <Dario_member pathlink.com> writes:
Charles Sanders:
delegates rock!
consider the following two snippets of code:
[...]
Much cleaner!

Yes, I agree this is a case where delegates are the best option. I like that D has them, but I regret that a more basic feature like method pointers is missing. :(
Nov 05 2003
prev sibling parent reply Dario <Dario_member pathlink.com> writes:
Walter:
I recognize that pointers to members are useful, but I think that delegates
are conceptually more powerful and cover the uses for pointers to members.
The more I use delegates the more useful they get! Anyhow, I ask that you
give delegates another chance.

I'm not saying delegates are bad. But method pointers are useful and not always can they be replaced by delegates. Anyway, what's wrong with method pointers? Are they just hard to implement? (I just want to remember that D is meant not to be difficult for C++ coders to switch to. Is this unimportant now? Don't you think that their absence would cause problems when converting C++ code into D?) P.S.= Perhaps I'd better to explain what I use method pointers for. Consider that I want to change a class behaviour according to its state. I'd like write the following code: Class C { void C.function() fp; void func1() {...} void func2() {...} void func() { ... ... this.fp(); ... } void change() { fp = (fp!=&func1)?&func1:&func2; } } If I used delegates I would be wasting 4 bytes for the pointer to this (which is part of the delegate). In some cases I don't even know which object I will use the method with, so I can't use delegates at all. Note that if we want method pointers to be polymorphic, they have to be index for the vtables... But this is not as important. Thanks for your attention. Dario
Nov 05 2003
parent "Walter" <walter digitalmars.com> writes:
"Dario" <Dario_member pathlink.com> wrote in message
news:bob2k0$2lbv$1 digitaldaemon.com...
 Walter:
I recognize that pointers to members are useful, but I think that


are conceptually more powerful and cover the uses for pointers to


The more I use delegates the more useful they get! Anyhow, I ask that you
give delegates another chance.

I'm not saying delegates are bad. But method pointers are useful and not always can they be replaced by delegates. Anyway, what's wrong with method pointers? Are they just hard to implement?

They are hard to implement, and hard to understand.
 (I just want to remember that D is meant not to
 be difficult for C++ coders to switch to.
 Is this unimportant now? Don't you think that
 their absence would cause problems when converting
 C++ code into D?)

 P.S.= Perhaps I'd better to explain what I use
 method pointers for. Consider that I want to
 change a class behaviour according to its state.
 I'd like write the following code:
   Class C
   {
       void C.function() fp;
       void func1() {...}
       void func2() {...}
       void func()
       {   ...
           ...
           this.fp();
           ...
       }
       void change() { fp = (fp!=&func1)?&func1:&func2; }
   }
 If I used delegates I would be wasting 4 bytes
 for the pointer to this (which is part of the
 delegate).
 In some cases I don't even know which object I
 will use the method with, so I can't use delegates
 at all.

What you can do is add a method that combines the object with the method into a delegate, and returns that when the code does know what the object is.
 Note that if we want method pointers to be
 polymorphic, they have to be index for the vtables...
 But this is not as important.

 Thanks for your attention.
 Dario

Jan 26 2004