C'tors from templates

[Andrei Alexandrescu]

dsimcha wrote:
> == Quote from dsimcha (dsimcha at yahoo.com)'s article
>> == Quote from grauzone (none at example.net)'s article
>>> dsimcha wrote:
>>>> Is there a way to run a class's c'tor on a block of memory from a template
>>>> function?  For example:
>>>>
>>>> C newClass(C, CtorArgs...)(CtorArgs args) {
>>>>     // Allocate, initialize.
>>>>     // Want to call the c'tor that takes type CtorArgs.
>>>> }
>>> Deeply hidden on the Digitalmars homepage, it hints _ctor() and shows
>>> how to correctly construct an object (if you're using dmd):
>>> http://www.digitalmars.com/techtips/class_objects.html
>> Yeah, I had thought of this, but apparently it doesn't work anymore.  This is
>> called excessive encapsulation--making things so opaque that, when someone really
>> knows what they're doing and has a good reason to mess around with the internals,
>> they still can't.  I'll file an enhancement requesting that this be put back.
> 
> Never mind, I realized I was doing two things wrong:
> 
> 1.  It changed from _ctor to __ctor.
> 2.  The class needs to define a c'tor  If it doesn't, then there's no default
> __ctor() that takes no args.  However, this is easy to check for in a static if
> statement.

Walter and I talked a few times about the necessity of providing useful 
functions a la C++'s placement new in Phobos. std.algorithm offers swap 
and move, but we need some more. Please advise.

Andrei