Assignment operator (C++)

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 

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]

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 My_Array{     int* array;     int count; public:     My_Array& operator=(const My_Array& 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:^[3]

public:      void swap(My_Array& other) // the swap member function (should never fail!)     {         // 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);     }      My_Array& operator = (My_Array other) // note: argument passed by value!     {         // 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.

• Operator overloading
• Move assignment operator
• Rule of three (C++ programming)
• Operators in C and C++

• The Anatomy of the Assignment Operator, by Richard Gillam