How arguments are passed into a cppFunction

问题

I'm puzzled as to how arguments are passed into a cppFunction when we use Rcpp. In particular, I wonder if someone can explain the result of the following code.

library(Rcpp)
cppFunction("void test(double &x, NumericVector y) {
    x = 2016;
    y[0] = 2016;
}")
a = 1L
b = 1L
c = 1
d = 1
test(a,b)
test(c,d)
cat(a,b,c,d) #this prints "1 1 1 2016"

回答1:

As stated before in other areas, Rcpp establishes convenient classes around R's SEXP objects.

For the first parameter, the double type does not have a default SEXP object. This is because within R, there is no such thing as a scalar. Thus, new memory is allocate making the & reference incompatible. Hence, the variable scope for the modification is limited to the function and there is never an update to the result. As a result, for both test cases, you will see 1.
For the second case, there is a mismatch between object classes. Within the first call object supplied is of type integer due to the L appended on the end, which conflicts with the C++ function expected type of numeric. The issue is resolved once the L is dropped as the object is instantiated as a numeric. Therefore, an intermediary memory location does not need to be created that is of the correct type to receive the value. Hence, the modification in the second case is able to propagate back to R.

e.g.

a = 1L
class(a)
# "integer"
a = 1
class(a)
# "numeric"

来源：https://stackoverflow.com/questions/41083921/how-arguments-are-passed-into-a-cppfunction

标签

rcpp