I can has @nogc and throw Exceptions?

[Tobias Pankrath via Digitalmars-d-learn]

On Friday, 13 February 2015 at 19:03:10 UTC, Jonathan Marler 
wrote:
> This question comes from wanting to be able to throw an 
> exception in code that is @nogc.
>
> I don't know if it's possible but I'd like to be able to throw 
> an exception without allocating memory for the garbage 
> collector?  You can do it in C++ so I think you should be able 
> to in D.  One idea I had was to allocate the memory for the 
> Exception beforehand and create the Exception class with the 
> pre-allocated memory.  I came up with the following code:
>
> T construct(T,A...)(void* buffer, A args)
> {
>   return (cast(T)buffer).__ctor(args);
> }
>
> Now to test it:
>
> void main()
> {
>   ubyte[ __traits(classInstanceSize, Exception)] 
> exceptionBuffer;
>   throw construct!(Exception)(exceptionBuffer.ptr, "My 
> Exception Allocated on the STACK!");
> }
>
> I got an assertion error. I'm not sure why, but when I print 
> out the contents of the buffer of my stack exception it differs 
> from an exception created for the garbage collector with "new".
>  It looks like it has some accounting information embedded in 
> the class instance. I figured as much but I didn't think the 
> code that performs the "throw" would be dependent on this.
>
> Also, this doesn't look like a very safe option because the 
> initial values for the class members don't get set using this 
> "construct" template.
>
> If anyone has any other ideas or a way to fix mine let me know, 
> thanks.

1. Throw preallocated exceptions is the way to go
2. Allocating them on the stackframe that will cease to exist by 
throwing is a bad idea
3. use emplace
To construct a type in preallocated memory