是什么导致存储的std::list::iterator失效？

Question

edit: 下面的代码可以是运行 on https://wandbox.org/permlink/1Qry83quzoDveYDi
我实施了评论中的各种建议，但不幸的是，我仍然不清楚为什么要在 这个特定的 std::list::iterator 处删除该项目（第 86 行）在运行时崩溃。下面给出的所有解释似乎都在肯定迭代器此时应该仍然有效。

我的印象是 std::list 中的项目的迭代器在将项目插入列表时不会失效（请参阅 this excellent post）。

但是，在下面的代码中，行
items.at(noOfitems-2)->erase(iter++);（第 86 行）
使程序崩溃
malloc: *** error for object 0x100778b28: pointer being freed was not allocated.

你能帮我理解为什么（在哪里）这个迭代器 std::list<std::string>::iterator 变得无效，我如何才能让它工作而不用再次迭代找到它？

我是不是理解错了？

#include <iostream>
#include <iostream>
#include <vector>
#include <random>
#include <unordered_set>
#include <set>
#include <unordered_map>
#include <map>
#include <list>
#include <utility>
#include <chrono>
#include <sstream>
#include <tr1/memory>

class Increment;
struct Item;

struct Pairhash {
public:
  template <typename T>
  std::size_t operator()(const T &x) const
  {
      return std::hash<T>()(x) ^ std::hash<T>()(x);
  }
};

struct Item {
    Item() = default;
    std::string name;
    int counter;
    Item(std::string name) : counter(0)
    {
        this->name = name;
    }
    
    bool operator==(const Item& p) const
    {
        return (this->name == p.name);
    }
    
    bool operator<(const Item& p) const
    {
        return (this->counter < p.counter);
    }
};

class Increment {
private:
    std::unordered_map<std::string, std::pair<std::list<std::string>::iterator , std::shared_ptr<Item> >, Pairhash >  itemMap;
    std::vector<std::shared_ptr<std::list<std::string>>> items;
public:
    Increment() = default;
    std::list<std::string>::iterator insertItem(std::string & name , int noOfitems)
    {
        if (noOfitems > items.size())
        {
            items.emplace_back(std::make_shared<std::list<std::string>>(std::initializer_list<std::string>{ name }));
            return items.back()->begin();
        }
        else
        {
            items.at(noOfitems-1)->emplace_back(name);
            return items.at(noOfitems-1)->rbegin().base(); //Last position in list
        }
    }
    
    std::list<std::string>::iterator adjustItemPosition(std::string & name, int noOfitems, std::list<std::string>::iterator & iter)
    {
        if (noOfitems > items.size())
        {
            std::list<std::string> temp{name};
            items.push_back(std::make_shared<std::list<std::string>>(temp));
        }
        else
        {
            items.at(noOfitems-1)->emplace_back(name);
        }
        /* // Works as expected
        auto itr = std::find(items.at(noOfitems-2)->begin(), items.at(noOfitems-2)->end(), name);
        if (itr != items.at(noOfitems-2)->end())
        {
            items.at(noOfitems-2)->erase(itr);
        }
        */
        items.at(noOfitems-2)->erase(iter++); //TODO Crashes
        return items.at(noOfitems-1)->rbegin().base(); //Last position in list
    }
    
    void incrementByOne(std::string name)
    {
        auto it = itemMap.find(name);
        if (it != itemMap.end()) //Item already in map
        {
            it->second.second->counter++;
            it->second.first = adjustItemPosition(name, it->second.second->counter,
                                                    it->second.first);
        }
        else  //New item to be inserted
        {
            std::shared_ptr<Item> temp = std::make_shared<Item>(Item(name));
            temp->counter = 1;
            std::list<std::string>::iterator listIter = insertItem(name, 1);
            itemMap.emplace(name, std::make_pair( listIter, temp));
        }
    }
    
    std::string printTop10() const
    {
        std::stringstream ss;
        auto count(0);
        for (auto it = items.rbegin(); it != items.rend(); ++it)
        {
            for (auto item : **it)
            {
                if (count == 10)
                {
                    break;
                }
                ss << "We have " << itemMap.at(item).second->counter << " " << item << std::endl;
                ++count;
            }
        }
        return ss.str();
    }
};

int main(int argc, const char * argv[]) {
    Increment incrementer;
    std::vector<std::string> names{ "Bananas", "Apples", "Peaches", "Durians", "Hazelnuts", "Avocados", "Pineapples", "Cherries", "Almonds", "Olives", "Eggs", "Yoghurts", "Peas", "Blueberries" };

    for (int i = 0; i < 100; ++i) {
        incrementer.incrementByOne(names.at(i%10));
    }
    std::cout << incrementer.printTop10() << std::endl;
    return 0;
}

Answer 1

What causes the stored std::list::iterator to become invalid?

移除迭代器指向的元素，无论是通过clear、pop_X、erase还是销毁列表。 assign 使该列表所有元素的迭代器无效。

Answer 2

您的代码中至少有一个问题：

std::list<std::string> temp;
temp.push_back(name);
items.emplace_back(std::make_shared<std::list<std::string>>(temp));
return temp.begin();

在这里，您将迭代器返回到已删除列表中的元素。您可能还有其他问题。

Answer 3

正在替换
return items.at(noOfitems-1)->rbegin().base();
与
return std::next(items.at(noOfitems-1)->end(), -1);

第 63 和 86 行修复了崩溃问题。
工作解决方案：https://wandbox.org/permlink/I68Szb0XMRKsPZqp

#include <iostream>
#include <iostream>
#include <vector>
#include <random>
#include <unordered_set>
#include <set>
#include <unordered_map>
#include <map>
#include <list>
#include <utility>
#include <chrono>
#include <sstream>
#include <memory>

class Increment;
struct Item;

struct Pairhash {
public:
  template <typename T>
  std::size_t operator()(const T &x) const
  {
      return std::hash<T>()(x) ^ std::hash<T>()(x);
  }
};

struct Item {
    Item() = default;
    std::string name;
    int counter;
    Item(std::string name) : counter(0)
    {
        this->name = name;
    }

    bool operator==(const Item& p) const
    {
        return (this->name == p.name);
    }

    bool operator<(const Item& p) const
    {
        return (this->counter < p.counter);
    }
};

class Increment {
private:
    std::unordered_map<std::string, std::pair<std::list<std::string>::iterator , std::shared_ptr<Item> >, Pairhash >  itemMap;
    std::vector<std::shared_ptr<std::list<std::string>>> items;
public:
    Increment() = default;
    std::list<std::string>::iterator insertItem(std::string & name , int noOfitems)
    {
        if (noOfitems > items.size())
        {
            items.emplace_back(std::make_shared<std::list<std::string>>(std::initializer_list<std::string>{ name }));
            return items.back()->begin();
        }
        else
        {
            items.at(noOfitems-1)->emplace_back(name);
            return std::next(items.at(noOfitems-1)->end(), -1);  // versus return items.at(noOfitems-1)->rbegin().base(); //Crashes
        }
    }

    std::list<std::string>::iterator adjustItemPosition(std::string & name, int noOfitems, std::list<std::string>::iterator & iter)
    {
        if (noOfitems > items.size())
        {
            std::list<std::string> temp{name};
            items.push_back(std::make_shared<std::list<std::string>>(temp));
        }
        else
        {
            items.at(noOfitems-1)->emplace_back(name);
        }
        /* // Works as expected
        auto itr = std::find(items.at(noOfitems-2)->begin(), items.at(noOfitems-2)->end(), name);
        if (itr != items.at(noOfitems-2)->end())
        {
            items.at(noOfitems-2)->erase(itr);
        }
        */
        items.at(noOfitems-2)->erase(iter++); //TODO Crashes
        return std::next(items.at(noOfitems-1)->end(), -1); //versus return items.at(noOfitems-1)->rbegin().base(); //Crashes
    }

    void incrementByOne(std::string name)
    {
        auto it = itemMap.find(name);
        if (it != itemMap.end()) //Item already in map
        {
            it->second.second->counter++;
            it->second.first = adjustItemPosition(name, it->second.second->counter,
                                                    it->second.first);
        }
        else  //New item to be inserted
        {
            std::shared_ptr<Item> temp = std::make_shared<Item>(Item(name));
            temp->counter = 1;
            std::list<std::string>::iterator listIter = insertItem(name, 1);
            itemMap.emplace(name, std::make_pair( listIter, temp));
        }
    }

    std::string printTop10() const
    {
        std::stringstream ss;
        auto count(0);
        for (auto it = items.rbegin(); it != items.rend(); ++it)
        {
            for (auto item : **it)
            {
                if (count == 10)
                {
                    break;
                }
                ss << "We have " << itemMap.at(item).second->counter << " " << item << std::endl;
                ++count;
            }
        }
        return ss.str();
    }
};

int main(int argc, const char * argv[]) {
    Increment incrementer;
    std::vector<std::string> names{ "Bananas", "Apples", "Peaches", "Durians", "Hazelnuts", "Avocados", "Pineapples", "Cherries", "Almonds", "Olives", "Eggs", "Yoghurts", "Peas", "Blueberries" };

    for (int i = 0; i < 100; ++i) {
        incrementer.incrementByOne(names.at(i%10));
    }
    std::cout << incrementer.printTop10() << std::endl;
    return 0;
}

我相信在中给出了为什么会这样的答案，即指向反向迭代器的指针实际上是对它的原始引用的副本，当它运行时被删除超出范围。 http://cplusplus.github.io/LWG/lwg-defects.html#2360

是什么导致存储的std::list::iterator失效？

What causes the stored std::list::iterator to become invalid?

c++

iterator

crash

invalidation