How Can I Confirm that Two R objects have the same structure?

My test below doesn't work. Can anyone suggest a different approach?

===different contents, same structure, want "true" for comparison

> x<-c(1,2,3)
> y<-x
> identical(str(x),str(y))
 num [1:3] 1 2 3
 num [1:3] 1 2 3
[1] TRUE
> y[3]<-999
> identical(str(x),str(y))
 num [1:3] 1 2 3
 num [1:3] 1 2 999
[1] TRUE
> str(x)
 num [1:3] 1 2 3
> str(y)
 num [1:3] 1 2 999
> 

but this approach is wrong because this says x and z have the same structure!

> z<-list("a","b")
> identical(str(x),str(z))
 num [1:3] 1 2 3
List of 2
 $ : chr "a"
 $ : chr "b"
[1] TRUE

I'm trying this because I need a way to confirm that an R object I construct has exactly the same type as what is provided in R package examples.