• Jerry Quinn <jlquinn optonline.net> Jun 09 2009
• grauzone <none example.net> Jun 09 2009
• BCS <none anon.com> Jun 09 2009
• Don <nospam nospam.com> Jun 09 2009

Jerry Quinn <jlquinn optonline.net> writes:

OK, having thought a bit more about delegates, I now have another question.

The ABI shows a delegate as consisting of a context ptr and a function ptr. 
The context ptr can be a class reference, ptr to struct, ptr to closure or ptr
to stack frame.

If you're passing one of these delegates into another function, how does the
receiving function figure out what kind of context it's looking at?  Each of
these things will look different in memory.  Also, structs at least don't have
a header that can be used to disambiguate the situation.

Thanks,
Jerry

grauzone <none example.net> writes:

Jerry Quinn wrote:
 OK, having thought a bit more about delegates, I now have another question.
 
 The ABI shows a delegate as consisting of a context ptr and a function ptr. 
The context ptr can be a class reference, ptr to struct, ptr to closure or ptr
to stack frame.
 
 If you're passing one of these delegates into another function, how does the
receiving function figure out what kind of context it's looking at?  Each of
these things will look different in memory.  Also, structs at least don't have
a header that can be used to disambiguate the situation.


The caller just passes the context pointer as a hidden argument when 
calling the function ptr and doesn't care about the rest. It's up to the 
code on the callee side to actually use the context pointer.

But there's never an ambiguity what the callee expects as the context 
pointer. It's simple: if you take a delegate of an object method, the 
context pointer has to be a class reference. If you take a delegate of a 
nested function, the context pointer must point to the stack frame. And 
so on.

If you want to say so, the code referenced by the function pointer 
decides how the context pointer is interpreted.

Note that this can never go wrong: it's not possible to take a delegate 
of a class method and (somehow) to use a frame pointer as the context 
pointer.

 Thanks,
 Jerry
 

BCS <none anon.com> writes:

Hello Jerry,

 OK, having thought a bit more about delegates, I now have another
 question.
 
 The ABI shows a delegate as consisting of a context ptr and a function
 ptr.  The context ptr can be a class reference, ptr to struct, ptr to
 closure or ptr to stack frame.
 
 If you're passing one of these delegates into another function, how
 does the receiving function figure out what kind of context it's
 looking at?  Each of these things will look different in memory.
 Also, structs at least don't have a header that can be used to
 disambiguate the situation.
 
 Thanks,
 Jerry


ok here is a brain twister: the context can be anything, it's just a size_t 
full of bits.

**** THE FOLLOWING IS NOT RECOMMENDED *****

http://codepad.org/8nnoIKNQ

import std.stdio;
struct S
{
    int go()
    {
        U u;
        u.s = this;
        return u.a + u.b;
    }
}

union U
{
    S* s;
    struct {  short a; short b; }
}

void main()
{
     U u;
     u.a= 54;
     u.b = 42;

     int delegate() dg = &u.s.go;


     writef("%s\n", dg()); 
}

Don <nospam nospam.com> writes:

Jerry Quinn wrote:
 OK, having thought a bit more about delegates, I now have another question.


These type of questions belong in digitalmars.D.learn.