can't initialize .outer in inner class

[Andrei Alexandrescu via Digitalmars-d]

On 5/6/15 3:31 AM, Timon Gehr wrote:
> On 05/06/2015 10:01 AM, Andrei Alexandrescu wrote:
>> Consider:
>>
>> void fun()
>> {
>>      int x;
>>      class C
>>      {
>>          ...
>>      }
>> }
>>
>> Objects of type C have access to x because they have an .outer
>> pseudo-member.
>>
>> Problem is, emplace() and any other in-situ initialization techniques
>> fail (e.g. emplace() will fail with inner classes).
>>
>> This seems to be a compiler issue - there's no way to initialize outer
>> without calling new.
>
>
> The following workaround seems to do it (I didn't test it thoroughly
> though, in particular, I didn't test whether escape analysis always
> works correctly for this implementation):
>
> T nestedEmplace(T,alias x,S...)(void[] mem,S args){
>      auto res=cast(T)mem.ptr;
>      enum siz=__traits(classInstanceSize, T);
>      (cast(byte[])mem)[0..siz]=typeid(T).init[];
>      auto dg=(){ return x; };
>      res.outer=dg.ptr;
>      static if(is(typeof(res.__ctor(args)))) res.__ctor(args);
>      else assert(!is(typeof(&res.__ctor))&&args.length==0);
>      return res;
> }
>
> void main(){
>      int x=12345;
>      class C{
>          this(){}
>          int foo(){
>              return x;
>          }
>      }
>      void[__traits(classInstanceSize,C)] mem=void;
>      auto c=nestedEmplace!(C,x)(mem);
>      assert(c.foo()==12345);
>      x=3;
>      assert(c.foo()==3);
> }

Thanks, that's quite the tour de force. I tried to massage things in 
various places to avoid changing emplace()'s signature, no avail. I 
think we have a problem here. -- Andrei