Pointers to non-static member functions!

[Michel Fortin]

On 2011-06-08 10:54:54 -0400, "Steven Schveighoffer" 
<schveiguy at yahoo.com> said:

> On Wed, 08 Jun 2011 10:40:48 -0400, Daniel Murphy  
> <yebblies at nospamgmail.com> wrote:
> 
>> "Steven Schveighoffer" <schveiguy at yahoo.com> wrote in message
>> news:op.vwrgycgjeav7ka at localhost.localdomain...
>>> 
>>> The reason is to allow composition of delegates (not that I think this  is
>>> a worthy reason):
>>> 
>>> class A
>>> {
>>>    void func();
>>>    void func2();
>>> }
>>> 
>>> void main()
>>> {
>>>    auto f = &A.func2;
>>>    auto a = new A();
>>>    auto g = &a.func; // delegate
>>>    g.funcptr = f;
>>>    g(); // calls a.func2()
>>> }
>>> 
>>> I'd argue that a much more useful return type would be a delegate with  the
>>> this pointer set to null, but then I don't know what funcptr would
>>> return.  I almost think you need a separate type for it, which seems  like
>>> a lot of effort for something that's hardly used.
>>> 
>>> -Steve
>> 
>> I can see this being useful, but I think it should be done by having
>> delegate.funcptr be a void* and &ClassType.nonstaticmemberfunction return
>> void*.
>> The type safety provided by the current way it works is an illusion, as  the
>> following compiles:
>> 
>> class A
>> {
>>   void func() { do something using member variables or the vtable etc }
>> }
>> class B
>> {
>>   void func() {}
>> }
>> void main()
>> {
>>   auto b = new B();
>>   auto dg = &b.func;
>>   dg.funcptr = &A.func;
>>   dg(); // Probably a segfault
>> }
> 
> Yes, but removing type safety does not prevent that from happening, 
> plus  it allows something like this:
> 
> class A
> {
>    void func();
>    int func2();
> }
> 
> void main()
> {
>     auto a = new A;
>     auto dg = &a.func2();
>     dg.funcptr = &A.func; // works if dg.funcptr and &A.func are void*, 
> but  doesn't work today
>     int x = dg(); // runs and returns garbage
> }
> 
> I almost would prefer that hacking delegates would be illegal.  Yes, 
> you  can get the function pointer and data pointer out of a delegate, 
> but  cannot set them.  I can't think of any good use cases for them.

I was "hacking delegates" a lot in the D/Objective-C bridge (before I 
decided to bring things directly in the compiler). To make it short, I 
needed to do that to call the appropriate D function from Objective-C 
stub method implementation.


> BTW, if the calling convention is different, we may be able to allow it 
>  via creating a "this" function pointer:
> 
> assert(is(typeof(&A.func2) == int function(A this)));
> 
> Since this is a keyword, it can be interpreted as a different calling  
> convention.

I was thinking about that too. Note that it forces int function(A this) 
to be implicitly castable to int function(void* this) when assigning to 
a delegate.


-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/