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:
The language permits an overloaded assignment operator to have an arbitrary return type (including [[void type|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:
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:
void swap(My_Array& other) // the swap member function (should never fail!)
My_Array& operator = (My_Array other) // note: argument passed by value!
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 Descendant : public Ancestor ;
int mainCopying from ancestor to descendant objects, which could leave descendant's fields uninitialized, is not permitted.
. Bjarne Stroustrup . The C++ Programming Language . 3 . Addison-Wesley . 2000 . 978-0-201-70073-2 . stroustrup . 244 .