cannot assign to array of pointers in c++

Question

I need to keep an array of pointers in c++. I have a Set which keeps objects of my class. Then I need to iterate over the set and keep the pointer of each item in array. array is 2D by the way. so What I need in summary is : pointers[1][4] = PinterToAnItem; the reason is I need to delete the item later, So I want to keep the pointer and directly go to that and delete it, instead of iterate again and find and then delete. But I have an error in assigning the array. I dont know how this idea will work, but so far it gives me error. Please let me know if it is not clear

the error is : " a value of type CONST Movie cannot be assigned to type of Movie"

struct Movie
        {
            int movieId;
            int shopId;
            int priceId;
    
        };
    
    Movie *pointers[100][100];
    
    set<Movie> setMovie;
    
    void main()
    {
    
    //reading and initialize the set with movies
    // for example movie1 = {0,10,3} so I want to keep in my array a pointer to this object as:
    // pointer [0][10] = ObjectPointer
    // I have error in loop body. Also (*id), id and &id does not work. 
                for (auto id = setMovie.begin(); id != setMovie.end(); id++)
                    pointers[(*id).movieId][(*id).shopId] = &(*id);
    
    }

2- by the way, do you guys think it is a fine Idea? can I delete pointer[0][10] which points to an object in my set? so by this way I don't need to iterate again through the set and delete (0,10). I have a pointer to (0,10) and I erase it from the set. Thanks

EDITED

The answer from @Jeffry is right but it does not work for rest of my problem which is erasing the specified item. i intended to keep a pointer to an item and then erase it directly. But we know set.erase(ITERATOR) or (actual_value). so pointer here does not work for me. I had to change array of iterators, then it works. So I completed here maybe works for some later.

    set<Movie> ::iterator pointers[100][100];
    for (auto id = setMovie.begin(); id != setMovie.end(); id++)
                    pointers[(*id).movieId][(*id).shopId] = id; // I changed here as well
    setMovie[1].erase(pointers[1][0]); // works well 
// pointers[1][0] is an iterator to an item in set with movieid=1,shopid=0

Why do you need to manually delete the contents of pointers at all? set<Movie> will call all the Movie destructors when it gets destroyed, when the program ends. — Nathan Pierson
– Nathan Pierson, Commented Jul 26, 2021 at 3:25
Anyway, set has the requirement that its members are constant, to guarantee that once inserted it won't have to change the order later. The compiler is telling you that because &(*id) is a const Movie*, you can't assign it to a Movie*. If you absolutely insist on doing it this way--which, to be clear, you shouldn't, because it's a ton of effort and confusion that only seems to exist to allow you to try to double-delete things--you could change Movie *pointers[100][100]; to const Movie *pointers[100][100]; — Nathan Pierson
– Nathan Pierson, Commented Jul 26, 2021 at 3:28

Jeffrey · Accepted Answer · 2021-07-26 03:31:10Z

2

pointers[(*id).movieId][(*id).shopId] = &(*id);

tries to store in pointers a pointer to a Movie, pointing into the setMovie.

Now consider what happens, after you do this. What if you did:

pointers[42][43]->priceId = 44;

That would possibly invalidate the set (you could have twice the same Movie). That is why the set doesn't let you do it.

One way around is:

const Movie *pointers[100][100];

Then, you could store the pointer because you wouldn't legally be allowed to modify movies, and mess up the ordering. I'm not sure that makes it a good idea, but it would solve your immediate problem.

For 2, no, it would not be a good idea to call delete on a pointer pointing to a movie stored in your set. You did not call new on this object, so you should not delete it. If you try, you'll get a crash immediately, or much later.

answered Jul 26, 2021 at 3:31

Jeffrey

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2 Comments

user9137963 Over a year ago

Thanks but let me explain a bit. the problem solved by adding const. But There is erase function on set. So we can delete an item. maybe I need to use Heap, priority_queue. but I need to delete an item and maybe again add it. So i dont want to iterate every time to find it and delete it. that's my main trick and idea. if it works programmatically, it would be enough for me. But again I have to delete movie(0,10) and again add, add delete multiple times. I use Set or any LOG(N) data structure to for some searching functions which is not related to this problem but I have to. thanks again

Jeffrey Over a year ago

Ok, that clarifies it, but don't use the delete word unless you mean delete. It has a very specific meaning in C++. If you plan on calling set.erase with a reference to the object you want to delete it might work. Still not saying it's a nice design, but I'm not saying it will crash, if you meant erase instead of delete

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