Editing Immutable object (section)

=== C++ ===
In C++, a [[const-correctness|const-correct]] implementation of <code>Cart</code> would allow the user to create instances of the class and then use them as either <code>const</code> (immutable) or mutable, as desired, by providing two different versions of the <code>items()</code> method. (Notice that in C++ it is not necessary — and in fact impossible — to provide a specialized constructor for <code>const</code> instances.)

<syntaxhighlight lang="cpp">
class Cart {
 public:
  Cart(std::vector<Item> items): items_(items) {}

  std::vector<Item>& items() { return items_; }
  const std::vector<Item>& items() const { return items_; }

  int ComputeTotalCost() const { /* return sum of the prices */ }

 private:
  std::vector<Item> items_;
};
</syntaxhighlight>

Note that, when there is a data member that is a pointer or reference to another object, then it is possible to mutate the object pointed to or referenced only within a non-const method.

C++ also provides abstract (as opposed to bitwise) immutability via the <code>mutable</code> keyword, which lets a [[member variable]] be changed from within a <code>const</code> method.

<syntaxhighlight lang="cpp">
class Cart {
 public:
  Cart(std::vector<Item> items): items_(items) {}

  const std::vector<Item>& items() const { return items_; }

  int ComputeTotalCost() const {
    if (total_cost_) {
      return *total_cost_;
    }

    int total_cost = 0;
    for (const auto& item : items_) {
      total_cost += item.Cost();
    }
    total_cost_ = total_cost;
    return total_cost;
  }

 private:
  std::vector<Item> items_;
  mutable std::optional<int> total_cost_;
};
</syntaxhighlight>