How to use destructor in nim ?

[kcvinu]

Hi all, I am trying this piece of code 
    
    
    proc `=destroy`*(x : var CustomButtonClass) =
        RemoveWindowSubclass(x.handle, Btn_WndProc, 1)
    
    
    Run

All i want to remove the subclassing when CustomButtonClass destroyed. But the 
compiler says that "signature for '=destroy' must be proc[T: object](x: var T)" 
I am confused. An example will be enough for me to understand the concept. 
Thanks in advance.