Generic functions to convert to void* and from void*

Daniel Keep daniel.keep.lists at
Tue Feb 24 16:03:32 PST 2009

TSalm wrote:
> In my case, there's also no possibility to get the wrong type, because
> it is managed by the type of the ColumnMem.

You still have to get the code right.  There's a surprising number of
corner cases trying to store arbitrary types.

> And about Object, if I want to store base type like int,double,etc...,
> if I do something like :
>  Object o;
>  int a = 30 ;
>  o = cast(Object) &a ;
> is this syntax is GC safe ?

It's not safe, period.  If the compiler lets you do that, I'd be
tremendously surprised; even more surprised if it doesn't cause major
problems later.  This is what I'm worried about; you're doing dangerous
things with a type system you don't understand.  Don't do this.

Here's the problem: void* isn't going to work for everything.  It'll
work for Object references, other pointers, and that's it.  You can't
store arrays, and you can't store value types like structs or primitive
types.  For that, you need to allocate heap storage, copy the value and
then store the pointer to that.  Oh, and don't forget that fixed-length
arrays have value semantics whereas dynamic arrays and slices have
reference semantics; although you generally solve that issue by having a
special template for your type which rewrites T[n] as T[].  Also,
delegates won't fit, but function pointers will.

This is why I was pointing you at Variant because I already went through
the trouble to solve all this once.  :P

If you still want to do this with void*, build that code in isolation
and test the heck out of it.

  -- Daniel

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