CODEWITHSUNDEEP
c++constants
Nick
Nick

Reputation: 10539

Class with pointers and const method

Suppose we have class for linked list of T. (I don't know if code actually works)

template <typename T>
struct List{

   struct Node{
      T data;
      Node *next;
   };

   // add, remove etc...

   Node *locate(const T &a) const{
      for(Node *node = head; node; node = node->next)
         if (node->data == a)
            return node;

      return nullptr;
   }

private;
   Node *head;
};

If you inspect method locate, it basically can harm and ruin the linked list, even the method is marked as const.

I just noticed something else. Since Node is not const, Node::data can be changed as well.

Supposing this was programmer mistake, is there a C++ way this to be avoid?

I know the method can be written as this.

    const Node *locateConst(const T &a) const{
      for(const Node *node = head; node; node = node->next)
         if (node->data == a)
            return node;

      return nullptr;
   }

Upvotes: 1

Views: 57

Answers (3)

Red.Wave
Red.Wave

Reputation: 4201

You are seeking an experimental feature, yet unavailable on many platforms. If your std libray supports, you can use std::experimental::propagate_const<node*> instead of naked raw pointers. https://en.cppreference.com/w/cpp/experimental/propagate_const In the mean time, you can implement your - maybe less generic - own version.

Upvotes: 1

user9876
user9876

Reputation: 11102

In C++, "const" means "can't change this object", it does NOT mean "can't change things this object points to".

I.e. with const List *const_list you can't change const_list->head, but you can change const_list->head->next or const_list->head->data.

If you're writing a List class, then you probably want const List to mean "can't change things this object points to", and C++ provides the tools you need to implement List so it works that way. But it's your job, as the author of the List class, to do that - the compiler can't just magically know that's what you intended.

Upvotes: 0

catnip
catnip

Reputation: 25388

is there a C++ way this to be avoid?

No, of course not. Programmers do have responsibilities you know. The way the method is declared promises not to change head, not *head.

Upvotes: 0

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