C++ Trie implementation including ready-to-go autocomplete

output.txt

word
word2
word3
word4

test.cpp

#include <iostream>
#include "trie.h"

int main()
{
  Trie trie;
  trie.insert("word");
  trie.insert("word2");
  trie.insert("word3");
  trie.insert("word4");
  trie.insert("fugg");
  auto&& ac = trie.autocomplete("wo");
  for (auto&& str : ac) {
    printf("%s\n", str.c_str());
  }
  return 0;
}

trie.hpp

#ifndef __TRIE_HPP
#define __TRIE_HPP
#include <map>
#include <string>
#include <vector>

class trie_node
{
public:
  trie_node(const std::string& val = "") : value(val), is_end(false) {}
  bool is_end;
  std::string value;
  std::map<char, trie_node> child;

  void add(char c)
  {
    if (value == "")
      child[c] = trie_node(std::string(1, c));
    else
      child[c] = trie_node(value + c);
  }

  std::vector<std::string> all_prefixes()
  {
    std::vector<std::string> ret;
    if (is_end) ret.push_back(value);
    if (!child.empty()) {
      for (auto&& tuple : child) {
        auto&& nodes = tuple.second.all_prefixes();
        ret.insert(ret.end(), nodes.begin(), nodes.end());
      }
    }

    return ret;
  }
};

/*
* based off https://github.com/vivekn/autocomplete/blob/master/trie.h
*/
class Trie
{
public:
  Trie() : head() {}
  std::string find(const std::string& word)
  {
    trie_node* node = &head;
    for (auto&& ch : word) {
      if (node->child.find(ch) == node->child.end()) return "";
      node = &node->child[ch];
    }
    return node->value;
  }

  void insert(const std::string& word)
  {
    trie_node* node = &head;
    for (auto&& ch : word) {
      if (node->child.find(ch) == node->child.end()) node->add(ch);
      node = &node->child[ch];
    }
    node->is_end = true;
  }

  std::vector<std::string> autocomplete(const std::string& prefix)
  {
    trie_node* node = &head;
    for (auto&& ch : prefix) {
      if (node->child.find(ch) == node->child.end()) return std::move(std::vector<std::string>());
      node = &node->child[ch];
    }
    return node->all_prefixes();
  }

private:
  trie_node head;
};

#endif

for some reason it insists that output.txt is the filename when it really isn't