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digitalmars.D.learn - An issue with setting delegates via templates

reply Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
class Foo
{
    void func(double) { }
    void func(int) { }

    void set(T)(T handler) { dg = handler; }
    void delegate(int) dg;
}

void main()
{
    auto foo = new Foo;
    foo.set(&foo.func);  // NG, func(double) is picked up first
}

Error: cannot implicitly convert expression (handler) of type void
delegate(double _param_0) to void delegate(int)

The problem here is that `void func(double)` is declared first, and
that's what the address-of operator picks up. If you swap the two
overloads this sample will compile.

How could this be worked around /while still using templates/? There
can only be one topmost function overload, so with two overloads this
can be worked around, but with more overloads this workaround can't be
used.

I've almost found a workaround via the getOverloads trait:

import std.traits;

class Foo
{
    void func(double) { }
    void func(int) { }

    void set(T)(T handler)
    {
        dg = handler;
    }

    void delegate(int) dg;
}

void main()
{
    auto foo = new Foo;
    foo.set(&__traits(getOverloads, foo, "func")[1]);
}

This works, but I can't use this from within a template. IOW, I can't
pass a function pointer to a template and then figure out if that
function is actually an overload of some class. I'd need more powerful
compile-time features for that. If I were able to do that then I could
enumerate all the overloads and pick one if it matches the type of
'dg'. It's kind of overkill but it could work, at least theoretically.

Otherwise I'd really need a way to explicitly specify which function
overload to pass when I use the address-of operator, but that would
probably have to be a language feature.
Oct 25 2011
next sibling parent reply travert phare.normalesup.org (Christophe) writes:
Andrej Mitrovic , dans le message (digitalmars.D.learn:30286), a écrit :

 class Foo
 {
     void func(double) { }
     void func(int) { }
 
     void set(T)(T handler) { dg = handler; }
     void delegate(int) dg;
 }
 
 void main()
 {
     auto foo = new Foo;
     foo.set(&foo.func);  // NG, func(double) is picked up first
 }
 
 Error: cannot implicitly convert expression (handler) of type void
 delegate(double _param_0) to void delegate(int)
 
 The problem here is that `void func(double)` is declared first, and
 that's what the address-of operator picks up. If you swap the two
 overloads this sample will compile.
 
 How could this be worked around /while still using templates/? There
 can only be one topmost function overload, so with two overloads this
 can be worked around, but with more overloads this workaround can't be
 used.
 
 I've almost found a workaround via the getOverloads trait:
 
 import std.traits;
 
 class Foo
 {
     void func(double) { }
     void func(int) { }
 
     void set(T)(T handler)
     {
         dg = handler;
     }
 
     void delegate(int) dg;
 }
 
 void main()
 {
     auto foo = new Foo;
     foo.set(&__traits(getOverloads, foo, "func")[1]);
 }
 
 This works, but I can't use this from within a template. IOW, I can't
 pass a function pointer to a template and then figure out if that
 function is actually an overload of some class. I'd need more powerful
 compile-time features for that. If I were able to do that then I could
 enumerate all the overloads and pick one if it matches the type of
 'dg'. It's kind of overkill but it could work, at least theoretically.
 
 Otherwise I'd really need a way to explicitly specify which function
 overload to pass when I use the address-of operator, but that would
 probably have to be a language feature.




Once you are inside set, handler is just a function and context pointer 
pair. Finding out what are the overloads of handler just with it's 
adress will be very complicated for a very limitted usage. Maybe an 
approach would be to make handler an alias rather than a delegate (but 
this won't work here because you have to pass foo along with 
&Foo.func...).

Why does Foo.set has to be a template ?
The obvious workarround is to make set expect a void delegate(int), 
instead of an undefined type, since this is the only type you can assign 
to dg.

I think I need a better example to understand why set is a template, and 
how to work arround that.
Oct 27 2011
parent travert phare.normalesup.org (Christophe Travert) writes:
Andrej Mitrovic , dans le message (digitalmars.D.learn:30315), a écrit :

 The new one: https://gist.github.com/1194497
 
 I'll investigate this further then.



OK, so there is indeed a filter to find a suitable delegate to load:

 trusted T connect(T)(T handler) if(isHandler!(T, Types));

But only the first Bar.x function is picked and given to isHandler!(T, 
int) to check it can be called. Bar.x(double) do not pass this filter.

I guess if the signature were:

T connect(void handler(Types));
T connect(bool handler(Types));
etc...

or maybe
T connect(T : void handler(Types)) (T handler);
T connect(T : bool handler(Types)) (T handler);
etc...

Then the compiler should be able to select the right Bar.x method. But 
with the isHandler filter,it is not able to find it. I don't see a way 
to improve isHandler to make it able to find the right Bar.x method, 
without modification of the compiler*, but I am not an expert here.

* the compiler should allow all overloads of a function given as a 
template argument to be tested against the filter to choose the right 
one, and complain if several overloads match the filter.

-- 
Christophe
Nov 02 2011
prev sibling parent Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
So it seems this is an issue with a newer signals implementation.

The old one works:

import std.signals;

struct Foo
{
    mixin Signal!(int) sig;
}

class Bar
{
   void x(string) { }
   void x(int) { }
}

void main()
{
    Foo foo;
    auto bar = new Bar;

    foo.sig.connect(&bar.x);
    foo.sig.emit(1);
}

But the new reimplementation (not in Phobos) doesn't:

import signalsnew;

struct Foo
{
    Signal!(int) sig;  // no need for mixin in new signals
}

class Bar
{
   void x(string) { }
   void x(int) { }
}

void main()
{
    Foo foo;
    auto bar = new Bar;

    foo.sig.connect(&bar.x);
    foo.sig.emit(1);
}

The new one: https://gist.github.com/1194497

I'll investigate this further then.
Oct 27 2011