How do I return a const data table from an R function?

How to return const data.table

from a function R

?

A data.table

from the package data.table

is passed and returned from functions by reference.

Hence, the function returning data.table

allows the calling function to modify the returned one data.table

.

So the question is: how to return a constant data.table

from a function R

?

There are a few obvious solutions that don't work for us:

• Call copy()
  
  in data.table
  
  on return.

  constByCopy <- function() {
    dt <- ... # dt is a data.table not created in the current environment
    ...
    return(copy(dt)) 
  }
  
        
  
          
          
          
        
  
      

  It works; however ours data.table
  
  are large enough that this solution is not viable.

• Use lockBinding()
  
  and unlockBinding()
  
  . However, this does not work with data.table
  
  as it is still possible to change the locked one data.table
  
  . (This is similar to the difference between int * const
  
  and int const * const
  
  in C ++.)