c++,c++11 , Pointer not working anymore after second iteration


Pointer not working anymore after second iteration

Question:

Tag: c++,c++11

I'm doing regular challenges at reddits dailyprogrammer to learn and practice C++. With last week challenge I got stuck on something with pointers.

The challenge is/was to create a todo-list with categories. I already had the todo-list part implemented in the previous challenge and I workend on from there.

This is the code I have for this challenge:

#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;
}

But when I execute it I get bad memory exceptions in the method TodoList::Add.

When I execute these lines, the pointers to the previous todo item becomes invallid.

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

Could someone explain to me why this is happening and how I can fix this?

PS: I'm using visual studio, don't think it matters for this.


Answer:

After the feedback on the internal working of std::vector from Joachim Pileborg, I realised I needed on other container structure. With the structure std::forward_list I don't have the problem I've encounterd before.


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