Question

I’m using a proprietary library that has an “openConnection” function that I use as such:

conn <- openConnection("user", "pass")
# do some stuff with 'conn' that may return early or throw exceptions
closeConnection(conn)

What’s the R idiom for making sure that the connection gets closed no matter how the current method gets exited. In C++ it would be RAII, in Java it probably would be a “finally” block. What is it in R?

Answer 1

Typically, just a call to on.exit is used, but you need to do it inside a function.

f <- function() {
  conn <- openConnection("user", "pass")
  on.exit(close(conn))

  # use conn...
  readLines(conn)

} # on.exit is run here...

A common case is when you get passed a connection or file name, and you should only create (and close) the connection if you’re given a file name:

myRead <- function(file) {
  conn <- file
  if (!inherits(file, "connection")) {
      conn <- file(file, "r")
      on.exit(close(conn))
  } # else just use the connection...

  readLines(conn)

} # on.exit runs here...

# Try it out:
cat("hello\nworld\n", file="foo.txt")

myRead("foo.txt") # file
myRead(stdin())   # connection