Pass function (not alias) to template and/or delegate-ize a template argument

[ag0aep6g]

On 06/11/2018 08:46 AM, cc wrote:
> struct MYUDA {}
> class Foo {
>      @(MYUDA) int bar(int x) { return x*2; }
> }
> auto foo = new Foo();
[...]
> I did figure out I can do it with mixins like so:
> 
> void MCALL(string func, V...)(V args) {
>      mixin(`assert(hasUDA!(`~func~`, MYUDA));`); // Ok
>      mixin(`assert(`~func~`(args) == 16);`); // Ok
> }
> MCALL!"foo.bar"(8);
> 
> Which works.  Is this the "best" way to do this?

That only works when foo is visible to MCALL. That's a serious limitation.

> Or is there another pattern that looks cleaner?

I can't think of a really nice solution where you just pass `foo.bar` or 
`&foo.bar`. The problem is that an alias of `foo.bar` is really an alias 
of `Foo.bar`, i.e. there is no connection to the object `foo`. And 
`&foo.bar` is just a generic delegate. There is no (compile-time) type 
information that links it to the type `Foo`.

The best I could come up with is passing the method's name separately 
from the object:

----
void main()
{
     auto foo = new Foo();
     foo.nCall!"bar"(9);
}

void nCall(string methodName, O, V...)(O obj, V args)
     /* 'n' is for "name" */
{
     import std.traits: hasUDA;
     auto methodDelegate = mixin("&obj." ~ methodName);
     mixin("alias methodAlias = obj." ~ methodName ~ ";");
     assert(hasUDA!(methodAlias, MYUDA)); // Ok
     assert(methodDelegate(args) == 18); // Ok
}

struct MYUDA {}

class Foo
{
     @(MYUDA) int bar(int x) { return x*2; }
}
----

If it's feasible to hard-code a list of all possible object types into 
the "call" function, then you can pass `&foo.bar` and find the correct 
object type by trying them all:

----
void main()
{
     auto foo = new Foo();
     bCall(&foo.bar, 9);
}

void bCall(D, V...)(D methodDelegate, V args)
     /* 'b' is for "bloat" */
{
     import std.traits: hasUDA;
     import std.meta: AliasSeq;

     assert(methodDelegate(args) == 18); // Ok

     auto obj = cast(Object) methodDelegate.ptr;
     if (obj is null) return; /* Delegate is not of a method. */

     static foreach (O; AliasSeq!(Foo, /* Bar, Baz, etc */))
     {
         static foreach (mName; __traits(allMembers, O))
         {{
             alias m = AliasSeq!(__traits(getMember, O, mName))[0];
             static if (is(typeof(&m)))
             {
                 if (typeid(O) == typeid(obj) &&
                     &m is cast(void*) methodDelegate.funcptr)
                 {
                     assert(hasUDA!(m, MYUDA)); // Ok
                 }
             }
         }}
     }
}

struct MYUDA {}

class Foo
{
     @(MYUDA) int bar(int x) { return x*2; }
}
----

But that's probably not feasible.