How to assign a delegate to a var ?

[Ali Çehreli via Digitalmars-d-learn]

On 06/03/2014 05:57 AM, bioinfornatics wrote:

 > On Tuesday, 3 June 2014 at 12:27:45 UTC, bioinfornatics wrote:

 >>>> I would like store the delegate to another var but when i try i
 >>>> get:
 >>>>
 >>>>
 >>>> testTraitsWithDelegate.d(13): Error: expression template
 >>>> __lambda2 is void and has no value
 >>>>
 >>>> I do not want to run it only to save the «function» somewhere.

 > ------------- code -----------
 > import std.stdio;
 > import std.typecons : Tuple;
 > //~ import std.functional : toDelegate;
 > //~ struct attribute( alias Pred )
 > //~ {
 > //~     public:
 > //~     alias Predicate = Pred;
 > //~ }
 >
 > struct attribute
 > {
 >      public bool delegate( int ) predicate;
 >      public this(  bool delegate( int ) pred )
 >      {
 >          predicate = pred;
 >      }
 > }
 >
 > struct Data
 > {
 >      @attribute( (int a) => a == 42 )
 >      int x;
 > }
 >
 >
 >
 > void main()
 > {
 >
 >      bool delegate( int ) tmp;
 >
 >      pragma( msg, __traits(getAttributes, Data.x)[0] );
 >      pragma( msg, __traits(getAttributes, Data.x)[0].predicate( 42 ) );
 >
 >      pragma( msg, __traits(getAttributes, 
Data.x)[0].predicate.stringof );
 >      tmp = __traits(getAttributes, Data.x)[0].predicate;
 >      writeln( tmp(42 ) );
 >      //~tmp = toDelegate(__traits(getAttributes, Data.x)[0].Predicate);
 > }
 > ----------------------------------
 >
 >
 >
 >
 > give at compile time this:
 >
 > attribute(function (int a) => a == 42)
 > true
 > (attribute __ctmp1474;
 >   , __ctmp1474).this(function (int a) => a == 42).predicate
 >
 >
 > And segfault at run-time

A delegate has a context pointer that it uses when executing at 
run-time. However, there can't be a run-time context of a delegate that 
is created at compile-time. I think that is why the segfault.

Is there a reason why it needs to be a delegate?

Replacing every 'delegate' with 'function' makes your code work.

Ali