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digitalmars.D.learn - Is there a way to create compile-time delegates?

reply "Simen kjaeraas" <simen.kjaras gmail.com> writes:
Yeah, what the subject says.

I want to have a default delegate for a struct, and without a default
constructor, this has to be a compile-time constant. Now, logically,
there should be nothing wrong with storing the function pointer and a
null context pointer at compile-time, but it seems there is. Any ideas?

struct foo {
     void delegate( ) dg = () {}; // Error: non-constant expression
                                  // __dgliteral1
}

-- 
Simen
Jul 19 2010
parent reply torhu <no spam.invalid> writes:
On 19.07.2010 21:06, Simen kjaeraas wrote:

 Yeah, what the subject says.

 I want to have a default delegate for a struct, and without a default
 constructor, this has to be a compile-time constant. Now, logically,
 there should be nothing wrong with storing the function pointer and a
 null context pointer at compile-time, but it seems there is. Any ideas?

 struct foo {
       void delegate( ) dg = () {}; // Error: non-constant expression
                                    // __dgliteral1
 }


I wasn't able to make it work.  The compiler probably sees delegates as 
something that just can't be created at compile time, since no runtime 
contexts exist then.  Which is reasonable.

Maybe one of those templates that turn functions into delegates will 
work?  Otherwise I guess you're back to using a factory function for 
initializing instances.

Maybe just checking for null pointers before calling those delegates 
ends up being the easiest solution.
Jul 19 2010
parent reply Philippe Sigaud <philippe.sigaud gmail.com> writes:
On Mon, Jul 19, 2010 at 22:01, torhu <no spam.invalid> wrote:


 I wasn't able to make it work.



Me too :(



 The compiler probably sees delegates as something that just can't be
 created at compile time, since no runtime contexts exist then.  Which is
 reasonable.


Can you initialize pointers in general, at compile-time?




 Maybe one of those templates that turn functions into delegates will work?



I had the same idea and tried to use std.functional.toDelegate, but to no
avail.

enum moo = ()  {return 1;};

struct foo {
   int delegate( ) dg = toDelegate(moo);
}

Error: forward reference to inferred return type of function call
toDelegate(delegate int()|

int moo() { return 1;}

struct foo {
   int delegate( ) dg = toDelegate(&moo);
}

Error: forward reference to inferred return type of function call
toDelegate((& moo))|

'auto' strikes again :(
I came to hate these forward reference errors. That and the fact that auto
functions do not appear in the docs, that's enough for me to avoid auto as
much as possible for functions.




 Otherwise I guess you're back to using a factory function for initializing
 instances.

 Maybe just checking for null pointers before calling those delegates ends
 up being the easiest solution.



Philippe
Jul 19 2010
parent reply Don <nospam nospam.com> writes:
Philippe Sigaud wrote:

 On Mon, Jul 19, 2010 at 22:01, torhu <no spam.invalid> wrote:
 
 
 
     I wasn't able to make it work.  
 
 
 Me too :(
  
 
     The compiler probably sees delegates as something that just can't be
     created at compile time, since no runtime contexts exist then.
      Which is reasonable.
 
 
 Can you initialize pointers in general, at compile-time?


You cannot initialize a pointer to runtime-allocated data at 
compile-time, and will never be able to. This particular example cannot 
ever work.
You should be able to initialize pointers to static data at 
compile-time, but currently you can't.


 
  
 
     Maybe one of those templates that turn functions into delegates will
     work?  
 
 
 I had the same idea and tried to use std.functional.toDelegate, but to 
 no avail.
 
 enum moo = ()  {return 1;};
 
 struct foo {
    int delegate( ) dg = toDelegate(moo);
 }
 
 Error: forward reference to inferred return type of function call 
 toDelegate(delegate int()|
 
 int moo() { return 1;}
 
 struct foo {
    int delegate( ) dg = toDelegate(&moo);
 }
 
 Error: forward reference to inferred return type of function call 
 toDelegate((& moo))|
 
 'auto' strikes again :(
 I came to hate these forward reference errors. That and the fact that 
 auto functions do not appear in the docs, that's enough for me to avoid 
 auto as much as possible for functions.
 
  
 
     Otherwise I guess you're back to using a factory function for
     initializing instances.
 
     Maybe just checking for null pointers before calling those delegates
     ends up being the easiest solution.
 
 
 
 Philippe
Jul 19 2010
parent reply Sean Kelly <sean invisibleduck.org> writes:
Don Wrote:


 Philippe Sigaud wrote:

 On Mon, Jul 19, 2010 at 22:01, torhu <no spam.invalid> wrote:
 
 
 
     I wasn't able to make it work.  
 
 
 Me too :(
  
 
     The compiler probably sees delegates as something that just can't be
     created at compile time, since no runtime contexts exist then.
      Which is reasonable.
 
 
 Can you initialize pointers in general, at compile-time?


 
 You cannot initialize a pointer to runtime-allocated data at 
 compile-time, and will never be able to. This particular example cannot 
 ever work.


You can get pretty close by using structs with opCall defined though.
Jul 21 2010
parent reply "Simen kjaeraas" <simen.kjaras gmail.com> writes:
Sean Kelly <sean invisibleduck.org> wrote:


 Don Wrote:


 Philippe Sigaud wrote:

 On Mon, Jul 19, 2010 at 22:01, torhu <no spam.invalid> wrote:



     I wasn't able to make it work.


 Me too :(


     The compiler probably sees delegates as something that just can't  


 be

     created at compile time, since no runtime contexts exist then.
      Which is reasonable.


 Can you initialize pointers in general, at compile-time?


 You cannot initialize a pointer to runtime-allocated data at
 compile-time, and will never be able to. This particular example cannot
 ever work.


 You can get pretty close by using structs with opCall defined though.


Not if the function I got only takes delegates, and needs to only take
delegates.

-- 
Simen
Jul 21 2010
parent Don <nospam nospam.com> writes:
Simen kjaeraas wrote:

 Sean Kelly <sean invisibleduck.org> wrote:
 

 Don Wrote:


 Philippe Sigaud wrote:

 On Mon, Jul 19, 2010 at 22:01, torhu <no spam.invalid> wrote:



     I wasn't able to make it work.


 Me too :(


     The compiler probably sees delegates as something that just 


 can't be

     created at compile time, since no runtime contexts exist then.
      Which is reasonable.


 Can you initialize pointers in general, at compile-time?


 You cannot initialize a pointer to runtime-allocated data at
 compile-time, and will never be able to. This particular example cannot
 ever work.


 You can get pretty close by using structs with opCall defined though.


 
 Not if the function I got only takes delegates, and needs to only take
 delegates.
 


Eventually, it should be possible to have a compile-time delegate which 
points to an immutable static variable. Doesn't work at the moment, though.
Jul 21 2010