In the C++ programming language, the assignment operator, '=', is the operator used for assignment. Like most other operators in C++, it can be overloaded.
Contents
- Return value of overloaded assignment operator
- Overloading copy assignment operator
- Assignment between different classes
- References
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. For example:
Return value of overloaded 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.
Overloading copy assignment operator
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:
- Acquire new resources
- Release old resources
- Assign the new resources' handles to the object
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:
Assignment between different classes
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:
Copying from ancestor to descendant objects, which could leave descendant's fields uninitialized, is not permitted.