• H. S. Teoh via Digitalmars-d (33/33) Sep 18 2014 I'm trying to write a struct with delegate member variables that I want
• anonymous (17/24) Sep 18 2014 [...]

"H. S. Teoh via Digitalmars-d" <digitalmars-d puremagic.com> writes:

I'm trying to write a struct with delegate member variables that I want
to default to do-nothing stubs (rather than null function pointers that
will crash when called), but it appears that such a thing is not
supported. I've tried all sorts of things, but none of them worked:

	struct S {
		void delegate(dchar) dg = (dchar) {};
		// test.d(2): Error: delegate test.S.__lambda2 cannot be class members
	}

	struct S {
		void delegate(dchar) dg = {};
		// test.d(2): Error: delegate test.S.__dgliteral2 cannot be class members
	}

	void delegate(dchar) stub = (dchar) {};
	struct S {
		void delegate(dchar) dg = stub;
		// test.d(3): Error: static variable stub cannot be read at compile time
	}

	immutable(void delegate(dchar)) stub = (dchar) {};
	// test.d(1): Error: non-constant nested delegate literal expression __lambda4
	// test.d(1): Error: non-constant nested delegate literal expression __lambda4
	// [Why the repeated message, and what on earth does this mean
	// anyway?!]
	struct S {
		void delegate(dchar) dg = stub;
	}

Is there a way to achieve this? If not, why not? Would it be worth
filing an enhancement request for this?

Note that the lack of argumentless struct ctors exacerbates this
problem, since there is no way to work around the lack of language
support here.


T

-- 
Береги платье снову, а здоровье смолоду. 

"anonymous" <anonymous example.com> writes:

On Thursday, 18 September 2014 at 17:49:34 UTC, H. S. Teoh via
Digitalmars-d wrote:
 I'm trying to write a struct with delegate member variables 
 that I want
 to default to do-nothing stubs (rather than null function 
 pointers that
 will crash when called), but it appears that such a thing is not
 supported.

[...]
 Is there a way to achieve this?

Here's a hack:

struct S
{
      union
      {
          struct
          {
              void* contextPtr;
              void function(dchar, void* contextPtr) funcPtr = (a,
ignored) {writeln(a);};
          }
          void delegate(dchar) dg;
      }
}