using D without GC

[Oleg B via Digitalmars-d-learn]

> No just reserve some memory and preallocate the buffer you want 
> before using it. It'll be a lot faster and cheaper.
I know, it's only for test.

> You shouldn't be using delete or new for that matter.
> You should be using malloc + free. And emplace.

auto myalloc(T)( size_t count )
{
     struct Impl{ size_t len; void* ptr; }
     union Wrap { Impl impl; T[] arr; }
     auto ret = Wrap( Impl( count, calloc( count, T.sizeof ) ) 
).arr;
     enforce( ret.ptr !is null );
     return ret;
}

class Bar
{
     float[] buf;
     this() { buf = myalloc!float( 65536 ); }
     ~this() { free(buf.ptr); }
}

For simple types arrays all clear. Emplace realization not 
trivial for me yet. As I understand correctly it concerns placing 
initial values of struct for example or values before calling 
ctor in place to array segment without copying?

But what I need use for class objects?

class Foo
{
     Bar[] buf;

     this()
     {
         buf = myalloc!Bar( 100 ); // I think it's ok
         foreach( ref b; buf )
             b = new Bar; // but calloc can't initialize class 
correctly
     }

     ~this()
     {
         static if( gc_disable )
         {
             foreach( ref b; buf ) delete b;
             free(buf.ptr);
         }
     }
}

Operator new automatically allocate range in GC-manage memory? If 
it is whether is possible to change this behavior? It's question 
relates to emplace too?