Pointers to non-static member functions!

[Steven Schveighoffer]

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'm mixed on whether you should be able to get a member "function  
pointer".  It would be nice to simply make it a normal function pointer  
that you can call with the appropriate type.

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.

-Steve