Function to read a string as the variables as opposed to taking the string name as the variable

I am writing a custom function that uses an R-function from the
reshape package: cast. ?However, my question could be applicable to
any R function.

Normally one writes the arguments directly into a function, e.g.:

result=cast(table1, column1 + column2 + column3 ? ~ ? ?column4,
mean) ? ? ?(1)

I need to be able to write this statement as follows:

result=cast(table1, string_with_columns ? ~ ? ?column4, mean) ? ?(2)
string_with_columns = group of functions that ultimately outputs:
"column1 + column2 + column3"