Reference to a vector element become invalid after n iterations - c++

Question

Program stop occur in this line
guess = secret;
From that, I guess that reference is broken, because if I change reference to simple value
const string secret = word_list[idx_word];
the program finishes correctly. So, my question is why this happen. The word_list is not changed/resided in loop.
Erorr occur on 392 iteration.
#include <QCoreApplication>
#include <QFile>
#include <QDir>
#include <QVector>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <map>
#include <set>
#include <thread>
#include <mutex>
#include <iomanip>
#include <string>
using namespace std;
bool debug = true;
const int COUNT = 5;
string MASK_FULL_MATCH(COUNT, 'o');
const string getMask(const string& word, const string& answer) {
if (word.size() != COUNT || answer.size() != COUNT) {
cout << word.size() << " " << answer.size() << endl;
}
char mask[5];
bool visited[5];
for (int i = 0; i < COUNT; i++) {
mask[i] = 'x';
visited[i] = false;
}
// find correct letters
for(int i = 0; i < COUNT; i++){
if (word[i] == answer[i]){
mask[i] = 'o';
visited[i] = true;
}
}
// find present letters
for (int i = 0; i < COUNT; i++){
if (mask[i] != 'o'){
for (int j = 0; j < COUNT; j++) {
if (answer[j] == word[i] && !visited[j]) {
mask[i] = '-';
visited[j] = true;
break;
}
}
}
}
return string(mask, COUNT);
}
int main(int argc, char *argv[])
{
QString pathToFile = QString("C:/Users/Ivan/Desktop/w_assets/w") + QString::number(COUNT) + QString("_entropy.txt");
QFile file(pathToFile);
if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
return -2;
QTextStream in(&file);
QVector<string> word_list;
while (!in.atEnd()) {
QString line = in.readLine();
word_list.append(line.split(QChar(' '))[0].toStdString());
}
file.close();
for (int idx_word = 0; idx_word < word_list.size(); idx_word++) {
const string &secret = word_list[idx_word];
cout << secret << '\t';
}
int total = 0;
for (int idx_word = 0; idx_word < word_list.size(); idx_word++) {
const string &secret = word_list[idx_word];
cout << "NEW SECRET " << secret << endl;
QVector<string> possible_answers = word_list;
for (int row = 0; row < 6; row++) {
string guess;
if (row == 0) {
guess = word_list[0];
}
else {
cout << "before broken secret\n";
guess = secret;
cout << "after broken secret\n";
cout << "row " << row << "; GUESS " << guess << endl;
}
debug = true;
string mask = getMask(guess, secret);
debug = false;
cout << "MASK: " << mask << endl;
if (mask == MASK_FULL_MATCH) {
break;
}
QVector<string> new_possible_answers;
for (const auto& pa : possible_answers) {
if (getMask(guess, pa) == mask) {
new_possible_answers.append(pa);
}
}
possible_answers = new_possible_answers;
cout << "NEW POSSIBLE WORDS SIZE " << possible_answers.size() << endl;
}
}
return 0;
}

Answer 1

word_list[0]; - this is a non-const operation in a QVector (see documentation, there is even a note about the possible detach) and since the reference count of your word_list is two due to the copy to possible_answers some lines above, the container has to do a detach and therefore your reference goes out of scope.
If you work with references on Qt containers you have to make sure to either have a reference count of 1 or only use const-access to the container (e.g. by creating a const ref to the container -> const auto &const_word_list = word_list; guess = const_word_list [0])