Show description

However, C++ automatically applies integral promotion to the operands in this case; as a result, the seemingly distinct types are promoted to a single common type before they are compared. The symmetrical quality implies that an overloaded operator == is to be defined as a friend function rather than a member function. So that you can see why, here's a comparison of two different versions of the same overloaded operator ==: class Date { private: int day; int month; int year; public: Date(); bool operator == (const Date & d) const; // 1 asymmetrical friend bool operator ==(const Date& d1, const Date& d2); //2 symmetrical }; bool operator ==(const Date& d1, const Date& d2); The overloaded operator == that is declared as a member function in (1) is inconsistent with the built-in operator == because it takes two arguments of different types.

Void func (Base& b) { if ( Derived *pd = dynamic_cast < Derived* > (&b) ) //declaration // inside an if-condition { //dynamic_cast was successful; use pd here return; }//pd goes out of scope at this point //otherwise dynamic_cast failed; variable pd is not in scope } The advantage of declaring the pointer pd locally is obvious: It is always initialized with an appropriate value, and it isn't visible to other parts of the program that are not to use it (see Chapter 12, "Optimizing Your Code").

For example Monday < Tuesday; //overloaded < Greater_than(Monday, Tuesday); The history of operator overloading can be traced back to the early days of Fortran. Fortran, the first high-level programming language, presented the concept of operator overloading in a way that was revolutionary back in the mid-1950s. For the first time, built-in operators such as + or - could be applied to various data types: integers, real and complex. Until then, assembly languages -- which didn't even support operator notation -- had been the only choice for programmers.