SubsetProxy should validate input vector sizes

[kevinushey]

Currently, the get_indices() functions (which are called in the constructor) do not validate the input sizes We should validate early that the right-hand side vector is either length 1, or same length as the left-hand side vector.

https://github.com/RcppCore/Rcpp/blob/master/inst/include/Rcpp/vector/Subsetter.h#L40

Note that we do perform this check when actually performing the subset (e.g. https://github.com/RcppCore/Rcpp/blob/master/inst/include/Rcpp/vector/Subsetter.h#L56-L66) but I think this is happening too late (we should validate before we do any work)

[eddelbuettel]

@thirdwing has something here.

[thirdwing]

I don't think lhs and rhs need to be the same length.

#include <Rcpp.h>

using namespace Rcpp;

//[[Rcpp::export]]
IntegerVector test(IntegerVector x, IntegerVector y) {
  return x[y];
}

/*
> Rcpp::sourceCpp("test.cpp")
> a <- c(1:20L)
> b <- c(2L, 13L)
> test(a, b)
[1]  3 14
*/

[kevinushey]

Whoops. Sorry, that's only true for logical vectors -- of course, we should be able to subset with vectors of various lengths for integers, numerics, etc...