Assignment operator (C++)

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.

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{ int* array; int count; 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; } // ... };

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:

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 }

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.