The ARG_PRESENT function is useful in user-written procedures that need to know if the lifetime of a value they are creating extends beyond the current routine's lifetime. This can be important for two reasons:
Result = ARG_PRESENT(Variable)
Returns a nonzero value if the following conditions are met:
In other words, ARG_PRESENT returns TRUE if the value of the specified variable will be passed back to the caller.
The variable to be tested.
Suppose that you are writing an IDL procedure that has the following procedure definition line:
PRO myproc, RET_PTR = ret_ptr
The intent of the RET_PTR keyword is to pass back a pointer to a new pointer heap variable. The following command could be used to avoid creating (and possibly losing) a pointer if no named variable is provided by the caller:
IF ARG_PRESENT(ret_ptr) THEN BEGIN
The commands that follow would only be executed if
ret_ptr is supplied and will be copied into a variable in the scope of the calling routine.
KEYWORD_SET, N_ELEMENTS, N_PARAMS