delegate vs function

Lars T. Kyllingstad public at kyllingen.NOSPAMnet
Tue Nov 23 04:25:18 PST 2010

On Tue, 23 Nov 2010 13:15:46 +0100, spir wrote:

> Hello,
> alias void function (int) F;
> alias void delegate (int) D;
> void fnFunc (F f, int i) {f(i);}
> void dgFunc (D d, int i) {d(i);}
> void writeOut (int i) {writeln(i);}
> void test () {
>     void writeIn (int i) {writeln(i);}
>     fnFunc(&writeOut, 1);
>     dgFunc(&writeIn, 1);
> //~     fnFunc(&writeIn, 1);    // error (expected a func, got a
> delegate...) //~     dgFunc(&writeOut, 1);   // error (... and
> conversely) }
> If a function is defined at the module's toplevel and then passed (via a
> pointer) to a higher-order func that expects a function, al works fine.
> But if it is defined inside a function, then the pointer is
> automatically typed as delegate, even if the function does not use any
> variable in scope, and I get an error.

Mark the function as 'static', like this:

  static void writeIn(int i) { ... }

Then the compiler even ensures that it doesn't use any symbols from the 
enclosing scope.

> Conversely, if the higher order
> func is defined to expect a delegate, then it fails if I pass a func
> defined at the top-level. How to solve this?

Use std.functional.toDelegate(), like this:

  dgFunc(toDelegate(&writeOut), 1);

(For some reason the documentation for toDelegate() seems to be missing 
from the D web site, but I don't know why.  I'll look into it.)


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