我不明白为什么我对字符串的排序会破坏一切

Question

I have the following code:

#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <unordered_map>

using namespace std;

vector<vector<string>> findAnagrams(vector<string> wordlist) {
  vector<vector<string>> result;
  unordered_map<string, vector<string>*> indexes;

  for (const string& word : wordlist) {
    string wordSorted = word;
    sort(wordSorted.begin(), wordSorted.end()); // <= This line break everything

    auto index = indexes.find(wordSorted);
    if (index == indexes.end()) {
      vector<string> vec = { word };
      result.push_back(vec);
      indexes[wordSorted] = &vec;
    } else {
      index->second->push_back(word);
    }
  }

  return result;
}

int main()
{
    vector<string> wordlist = {"eat", "tea", "tan", "ate", "nat", "bat", "test", "estt"};
    auto result = findAnagrams(wordlist);

    for (const auto& vec : result) {
      for (const auto& word : vec) {
        cout << word << " ";
      }
      cout << endl;
    }

    return 0;
}

This code detects all anagrams in a list of given words.

As my comment says, when I sort wordSorted using std::sort , it breaks everything and my code ends with a bad_alloc. As if the std::sort manipulates the memory outside of wordSorted . If I remove this specific line, the code "works" (the result is obviously wrong, but it does what it should do).

How it is possible? What am I missing?

Answer 1

I'm guessing these lines are the main cause of your problem:

{
    vector<string> vec = { word };
    result.push_back(vec);
    indexes[wordSorted] = &vec;
}

Here you store a pointer to the local variable vec in the indexes map. When the block ends at } the life-time of vec also ends, and the pointer you just stored will become invalid.

Any use of this pointer will lead to undefined behavior .

It seems to me that the solution is to simply not store pointers to the vector (pointers to containers are seldom, if ever, needed), and instead store a copy.