Read access violation after shuffling vector multiple times - c++

Question

For a school project I need to shuffle a vector with unique pointers.
However when I do this I get a read access violation error after some time.
I create the vector and then call the shuffle. This I do multiple times. I just create the vector with the values and shuffle it once and after a couple of times I get the read access violation error.
code
int RandomEngine::generateRandomInt(int minValue, int maxValue)
{
std::uniform_int_distribution<int> distribution(minValue, maxValue);
std::cout << "Random getal van " << minValue << " tot " << maxValue << std::endl;
return distribution(def_rand_engine);
}
void RandomEngine::shuffleCharacterCards(std::vector<std::unique_ptr<CharacterCard>>& cards)
{
// Succeeds once but with multiple swaps read access violation
//auto randomInt = generateRandomInt(0, cards.size() - 1);
//iter_swap(cards.begin() + randomInt, cards.begin());
// Read access violation
//for (int i = 0; i < cards.size(); i++)
//{
// std::swap(cards[i], cards[std::rand() % cards.size()]);
//}
// Read access violation
//std::shuffle(cards.begin(), cards.end(), std::mt19937{ def_rand_engine});
// Read access violation
//std::random_shuffle(cards.begin(), cards.end());
// Read access violation
//std::shuffle(cards.begin(), cards.end(), std::mt19937(std::random_device()()));
}
Other class where I call the shuffle from the static class
void CharacterCardStack::prepare()
{
std::cout << "Preparing CharacterCardStack..." << std::endl;
if(cards_.size() > 2)
{
// Shuffle the cards
RandomEngine::shuffleCharacterCards(cards_);
// Burn the first card
burnedCharacterCard_ = std::move(cards_[0]);
std::cout << "Burned charactercard: " << burnedCharacterCard_->getName() << std::endl;
cards_.erase(cards_.begin());
// Als de open kaart een koning is dan moet deze vervangen worden door een ander.
if(cards_[0]->getName() == "Koning")
{
turnedCharacterCard_ = std::move(cards_[1]);
}
else
{
turnedCharacterCard_ = std::move(cards_[0]);
}
std::cout << "Turned charactercard: " << turnedCharacterCard_->getName() << std::endl;
cards_.erase(cards_.begin());
}
else
{
std::cerr << "CharacterCardStack::prepare cards size is " << cards_.size() << std::endl;
throw std::exception("Error...");
}
for (const auto& c : cards_)
{
std::cout << "Other card: " << c->getName() << std::endl;
}
std::cout << "CharacterCardStack prepared" << std::endl;
}

Answer 1

You have flow in your logic, you move out either first or the second element based on condition:
if(cards_[0]->getName() == "Koning")
{
turnedCharacterCard_ = std::move(cards_[1]);
}
else
{
turnedCharacterCard_ = std::move(cards_[0]);
}
but then unconditionaly erase the first element:
cards_.erase(cards_.begin());
so you may end up with moved out pointers.
Simple fix could be:
if(cards_[0]->getName() == "Koning")
{
std::swap( cards_[0], cards_[1] );
}
turnedCharacterCard_ = std::move(cards_[0]);
std::cout << "Turned charactercard: " << turnedCharacterCard_->getName() << std::endl;
cards_.erase(cards_.begin());
this way it is easier to keep it correct and I would prefer use cards.front() instead of cards_[0], which is I think more readable in this case.