c++ - 第二次迭代后指针不再工作

Question

我在 reddits dailyprogrammer 进行定期挑战学习和实践 C++。与上周challenge我被指针卡住了。

挑战是/曾经是创建一个带有类别的待办事项列表。我已经在 previous challenge 中实现了待办事项列表部分我从那里开始工作。

这是我为这个挑战准备的代码:

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

class TodoItem;
class Category{
public:
    Category(std::string name)
        : name(name) {

    }

    void addToCategory(TodoItem* item){
        items.push_back(item);
    }

    void removeFromCategory(const TodoItem* item){
        auto it = std::find(items.begin(), items.end(), item);
        if (it != items.end())
        {
            items.erase(it);
        }
    }

    static Category combine(const Category &left, const Category &right){
        Category combined(left.getName() + ", " + right.getName());

        std::vector<TodoItem*> sortedLeft(left.items);
        std::vector<TodoItem*> sortedRight(right.items);

        std::sort(sortedLeft.begin(), sortedLeft.end());
        std::sort(sortedRight.begin(), sortedRight.end());

        std::set_intersection(sortedLeft.begin(), sortedLeft.end(), sortedRight.begin(), sortedRight.end(), std::back_inserter(combined.items));
        return combined;
    }

    std::string getName() const{
        return name;
    }

    bool operator==(const Category& a) const {
        return a.getName() == getName();
    }

    bool operator!=(const Category& a) const {
        return a != *this;
    }

    bool operator<(const Category& a) const {
        return this->getName() < a.getName();
    }

    friend std::ostream& operator<<(std::ostream& os, const Category& category);

private:
    std::string name;
    std::vector<TodoItem*> items;
};

class TodoItem
{
public:
    TodoItem(const std::string &item)
        : task(item) {
    }

    void update(const std::string &item){
        task = item;
    }

    std::string getTask() const{
        return task;
    }

    bool operator==(const TodoItem& a) const {
        return a.getTask() == getTask();
    }

    bool operator!=(const TodoItem& a) const {
        return a != *this;
    }

    bool operator<(const TodoItem& a) const {
        return this->getTask() < a.getTask();
    }

    friend std::ostream& operator<<(std::ostream& os, const TodoItem& item) {
        os << item.getTask() << std::endl;
        return os;
    }

private:
    std::string task;
};


inline std::ostream& operator<<(std::ostream& os, const Category& category) {
    os << "----" << category.getName() << "----" << std::endl;
    for (auto &item : category.items){
        os << *item << std::endl;
    }
    return os;
}

class TodoList
{
public:

    void add(const std::string &task, const std::string &category = "No category"){

        Category* catPtr = nullptr;

        for (size_t j = 0; j < categories.size(); j++)
        {
            if (categories[j] == Category(category))
            {
                catPtr = &categories[j];
                break;
            }

        }

        if (catPtr == nullptr)
        {
            categories.push_back(Category(category));
            catPtr = &categories[categories.size() - 1];
        }


        for (size_t i = 0; i < items.size(); i++)
        {
            if (items[i] == TodoItem(task))
            {
                catPtr->addToCategory(&items[i]);
                return;
            }
        }

        items.emplace_back(TodoItem(task));
        catPtr->addToCategory(&items[items.size() - 1]);
    }

    template<typename ... args>
    void add(const std::string &task, const std::string &category, args... otherCategories){
        add(task, category);
        add(task, otherCategories...);
    }

    void update(const std::string &oldTask, const std::string &newTask){
        auto itemIt = std::find(items.begin(), items.end(), TodoItem(oldTask));
        if (itemIt != items.end())
        {
            itemIt->update(newTask);
        }
    }


    template<typename ... args>
    void viewList(std::ostream& os, args ... categoriesArgs) const{
        os << getCategory(categoriesArgs...);
    }

private:
    Category getCategory(const std::string category) const{
        return *std::find(categories.begin(), categories.end(), Category(category));
    }

    template<typename ... args>
    Category getCategory(const std::string category, args ... categoriesArgs) const{
        return  Category::combine(*std::find(categories.begin(), categories.end(), Category(category)), getCategory(categoriesArgs...));
    }


    std::vector<TodoItem> items;
    std::vector<Category> categories;
};


int main(){
    TodoList list;

    list.add("A pixel is not a pixel is not a pixel", "Programming");
    list.add("The Scheme Programming Language", "Programming");
    list.add("Memory in C", "Programming");
    list.add("Haskell's School of Music", "Programming", "Music");
    list.add("Algorithmic Symphonies from one line of code", "Programming", "Music");

    list.add("Modes in Folk Music", "Music");
    list.add("The use of the Melodic Minor Scale", "Music");

    list.viewList(std::cout, "Music");
    list.viewList(std::cout, "Programming");

    list.update("Algorithmic Symphonies from one line of code", "Algorithmic Symphonies from one line of code in C");
    list.viewList(std::cout, "Programming", "Music");

    return 0;
}

但是当我执行它时，我在方法 TodoList::Add 中遇到错误的内存异常。

当我执行这些行时，指向前一个待办事项的指针变得无效。

        items.emplace_back(TodoItem(task));
        catPtr->addToCategory(&items[items.size() - 1]);

有人可以向我解释为什么会发生这种情况以及我该如何解决这个问题吗？

PS:我用的是visual studio，觉得这无关紧要。

Answer 1

在 Joachim Pileborg 对 std::vector 的内部工作的反馈之后，我意识到我需要其他容器结构。有了结构std::forward_list，我就没有以前遇到的问题了。