on.exit records the expression given as its argument as needing
to be executed when the current function exits (either naturally or as
the result of an error). This is useful for resetting graphical
parameters or performing other cleanup actions.
If no expression is provided, i.e., the call is on.exit(), then
the current on.exit code is removed.
on.exit(expr = NULL, add = FALSE, after = TRUE)an expression to be executed.
if TRUE, add expr to be executed after any previously
set expressions (or before if after is FALSE); otherwise (the
default) expr will overwrite any previously set expressions.
if add is TRUE and after is FALSE, then
expr will be added on top of the expressions that were already
registered. The resulting last in first out order is useful for freeing
or closing resources in reverse order.
Invisible NULL.
The expr argument passed to on.exit is recorded without
evaluation. If it is not subsequently removed/replaced by another
on.exit call in the same function, it is evaluated in the
evaluation frame of the function when it exits (including during
standard error handling). Thus any functions or variables in the
expression will be looked for in the function and its environment at
the time of exit: to capture the current value in expr use
substitute or similar.
If multiple on.exit expressions are set using add = TRUE
then all expressions will be run even if one signals an error.
This is a ‘special’ primitive function: it only
evaluates the argument add.
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
sys.on.exit which returns the expression stored for use
by on.exit() in the function in which sys.on.exit() is
evaluated.
# NOT RUN {
require(graphics)
opar <- par(mai = c(1,1,1,1))
on.exit(par(opar))
# }
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