Skip to main content

Item 12: Copy all parts of an object

Concept​

When you write your own copying functions (copy constructor and copy assignment operator), the compiler will not warn you if you forget to copy a data member. This is especially problematic when you add new members to a class or when you have a derived class that fails to copy its base class parts. Every copying function must copy all local data members and invoke the appropriate copying function in all base classes.

Code Example​

class Customer {
public:
    Customer(const Customer& rhs)
        : name(rhs.name), lastTransaction(rhs.lastTransaction) {}  // copy ALL members

    Customer& operator=(const Customer& rhs) {
        name = rhs.name;
        lastTransaction = rhs.lastTransaction;  // don't forget new members!
        return *this;
    }
private:
    std::string name;
    Date lastTransaction;
};

class PriorityCustomer : public Customer {
public:
    PriorityCustomer(const PriorityCustomer& rhs)
        : Customer(rhs),         // invoke base class copy ctor!
          priority(rhs.priority) {}

    PriorityCustomer& operator=(const PriorityCustomer& rhs) {
        Customer::operator=(rhs);  // assign base class parts!
        priority = rhs.priority;
        return *this;
    }
private:
    int priority;
};

Full source code

Things to Remember​

  • Copying functions should be sure to copy all of an object's data members and all of its base class parts.
  • Don't try to implement one of the copying functions in terms of the other. Instead, put common functionality in a third function that both call.
  • Item 5 — Compiler-generated copy operations
  • Item 10 — Returning *this from assignment operators
  • Item 11 — Self-assignment safety