12 Overloading [over]

12.6 Overloaded operators [over.oper]

12.6.3 Assignment [over.ass]

An assignment operator shall be implemented by a non-static member function with exactly one parameter.
Because a copy assignment operator operator= is implicitly declared for a class if not declared by the user ([class.copy.assign]), a base class assignment operator is always hidden by the copy assignment operator of the derived class.
Any assignment operator, even the copy and move assignment operators, can be virtual.
For a derived class D with a base class B for which a virtual copy/move assignment has been declared, the copy/move assignment operator in D does not override B's virtual copy/move assignment operator.
struct B {
  virtual int operator= (int);
  virtual B& operator= (const B&);
struct D : B {
  virtual int operator= (int);
  virtual D& operator= (const B&);

D dobj1;
D dobj2;
B* bptr = &dobj1;
void f() {
  bptr->operator=(99);          // calls D​::​operator=(int)
  *bptr = 99;                   // ditto
  bptr->operator=(dobj2);       // calls D​::​operator=(const B&)
  *bptr = dobj2;                // ditto
  dobj1 = dobj2;                // calls implicitly-declared D​::​operator=(const D&)
— end example
— end note