Assignment operator (C++)

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In the C++ programming language, the assignment operator, =, is the operator used for assignment. Like most other operators in C++, it can be overloaded.

The copy assignment operator, often just called the "assignment operator", is a special case of assignment operator where the source (right-hand side) and destination (left-hand side) are of the same class type. It is one of the special member functions, which means that a default version of it is generated automatically by the compiler if the programmer does not declare one. The default version performs a memberwise copy, where each member is copied by its own copy assignment operator (which may also be programmer-declared or compiler-generated).

The copy assignment operator differs from the copy constructor in that it must clean up the data members of the assignment's target (and correctly handle self-assignment) whereas the copy constructor assigns values to uninitialized data members.[1] For example:

My_Array first;           // initialization by default constructor
My_Array second(first);   // initialization by copy constructor
My_Array third = first;   // Also initialization by copy constructor
second = third;           // assignment by copy assignment operator

Return value of overloaded assignment operator[edit]

The language permits an overloaded assignment operator to have an arbitrary return type (including void). However, the operator is usually defined to return a reference to the assignee. This is consistent with the behavior of assignment operator for built-in types (returning the assigned value) and allows for using the operator invocation as an expression, for instance in control statements or in chained assignment. Also, the C++ Standard Library requires this behavior for some user-supplied types.[2]

Overloading copy assignment operator[edit]

When deep copies of objects have to be made, exception safety should be taken into consideration. One way to achieve this when resource deallocation never fails is:

  1. Acquire new resources
  2. Release old resources
  3. Assign the new resources' handles to the object
class MyArray {
 public:
  MyArray& operator=(const MyArray& other) {
    if (this != &other) {  // protect against invalid self-assignment
      // 1: allocate new memory and copy the elements
      int* new_array = new int[other.count];
      std::copy(other.array, other.array + other.count, new_array);

      // 2: deallocate old memory
      delete[] array;

      // 3: assign the new memory to the object
      array = new_array;
      count = other.count;
    }
    // by convention, always return *this
    return *this;
  }

 private:
  int* array;
  int count;

  // ...
};

However, if a no-fail (no-throw) swap function is available for all the member subobjects and the class provides a copy constructor and destructor (which it should do according to the rule of three), the most straightforward way to implement copy assignment is as follows:[3]

 public:
  // The swap member function (should never fail!).
  void swap(MyArray& other) {
    // swap all the members (and base subobject, if applicable) with other
    using std::swap;  // because of ADL the compiler will use
    // custom swap for members if it exists
    // falling back to std::swap
    swap(array, other.array);
    swap(count, other.count);
  }

  // Note: argument passed by value!
  MyArray& operator=(MyArray other) {
    // swap this with other
    swap(other);

    // by convention, always return *this
    return *this;

    // other is destroyed, releasing the memory
  }

Assignment between different classes[edit]

C++ supports assignment between different classes, both via implicit copy constructor and assignment operator, if the destination instance class is the ancestor of the source instance class:

class Ancestor {
 public:
  int a;
};

class Descendant : public Ancestor {
 public:
  int b;
};

int main() {
  Descendant d;
  Ancestor a(d);
  Ancestor b(d);
  a = d;
}

Copying from ancestor to descendant objects, which could leave descendant's fields uninitialized, is not permitted.

See also[edit]

References[edit]

  1. ^ Stroustrup, Bjarne (2000). The C++ Programming Language (3 ed.). Addison-Wesley. p. 244. ISBN 978-0-201-70073-2.
  2. ^ Working Draft, Standard for Programming Language C++, Section 17.6.3.1, Table 23; http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf
  3. ^ Sutter, H.; Alexandrescu, A. (October 2004), C++ Coding Standards, Addison-Wesley, ISBN 0-321-11358-6

External links[edit]