merge with function aggregation with Rcpp v2

library(data.table)
library(plyr)
library(dplyr)
library(purrr)
library(magrittr)
library(Rcpp)

sourceCpp(code = '
// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadillo.h>
// #include <Rcpp.h>
#include <string>
#include <vector>
#include <algorithm>
using namespace Rcpp;
using namespace std;
using namespace arma;

// [[Rcpp::export]]
NumericVector add_narm(SEXP xs, SEXP ys){
  NumericVector xr(xs);
  NumericVector yr(ys);
  colvec x(xr.begin(), xr.size(), false);
  colvec y(yr.begin(), yr.size(), false);
  uvec loc_na_x = find_nonfinite(x);
  uvec loc_na_y = find_nonfinite(y);
  colvec z = x + y;
  vector<uword> loc_na_std;
  set_intersection(loc_na_x.begin(), loc_na_x.end(),
    loc_na_y.begin(), loc_na_y.end(),
    back_inserter(loc_na_std));
  uvec loc_na = conv_to<uvec>::from(loc_na_std);
  x.elem(loc_na_x).zeros();
  y.elem(loc_na_y).zeros();
  z = x + y;
  z.elem(loc_na).fill(NA_REAL);
  return wrap(z);
}

// [[Rcpp::export]]
DataFrame merge_all_cpp(List dfs){
  DataFrame out(dfs[0]);
  vector<string> df_names = out.names();
  for (int i = 1; i < dfs.length(); i++)
  {
    DataFrame tmp(dfs[i]);
    vector<string> tmp_names = tmp.names();
    for (int j = 0; j < tmp.length(); j++)
    {
      if (find(df_names.begin(), df_names.end(), tmp_names[j]) != df_names.end())
        out[tmp_names[j]] = add_narm(out[tmp_names[j]], tmp[tmp_names[j]]);
      else
      {
        out.push_back(tmp[tmp_names[j]], tmp_names[j]);
        df_names.push_back(tmp_names[j]);
      }
    }
  }
  return out;
}');

t = proc.time()
dfs = list(w,x,y,z)
by = "SP"
overall_keys = dfs %>% map(~select_(., .dots = by)) %>% rbindlist %>% distinct
dfs %<>% map(~merge(., overall_keys, by = by, all = TRUE))
## not use purrr
# overall_keys = dfs %>% lapply(function(x) select_(x, .dots = by)) %>% rbindlist %>% distinct
# dfs %<>% lapply(function(x) merge(x, overall_keys, by = by, all = TRUE))
f = function(x) x / length(dfs)
f = function(x) x / length(dfs)
out8 = merge_all_cpp(dfs) %>% tbl_dt(FALSE) %>% mutate_each_(funs(f), by)
proc.time() - t
#   user  system elapsed
#   0.04    0.03    0.12
out8 %<>% select_(.dots = names(out4)) %>%
  arrange_(.dots = by)  %>% mutate_each(funs(as.integer))
all.equal(out4, out8) # TRUE