To overload operator we have three options .
1) Member Functions : when left hand Operand is of Class type
obj + 2 ; obj1 + obj2 .
This will not work for 3 + obj1
as it will become 3.operator+(obj1) and + is not overloaded for int type.
Any operator function intended to accept first operand as basic type CAN'T be Member function.
Imp : operator = , [] , -> , () ,->* all these must be Member
2) Global Functions : If we want left hand operand to be of other class type .
Example: >> , << . To support 4 + obj1
3) Friend functions : friend is just provided for global functions to get access
of Private and protected members .
But if we dont have Constructor number(int I = 0) which is imlicitly casting
1 ----> number And letting a + 1; to work.
compilation will fail .
BJARNE recommendation:
1.Operator that inherently modify value of first Argument should be member fuction. like +=,-= ,*= and all .
complex & operator+=(complex a) ;
2. Operator that simply produce a new value based on its Arguments as + ,- ,*, / should defined outside of class.
complex operator + (const complex & a , const complex & b){ complex r = a;
return r+=b;
}
Q Why cant we overload :: (Scope ) . (dot ) and .* (member selection throgh ptr to functoion) ?
Ans: All of these operator take second argument as name not as value .
They are used to refer members not to operate on them.
Q Why -> is overloadable and .(dot is not )?
--------------overloading array ---------
86 // overloaded subscript operator for non-const Arrays; 87 // reference return creates a modifiable lvalue 88 int &Array::operator[]( int subscript ) 89 { 90 // check for subscript out-of-range error 91 if ( subscript < 0 || subscript >= size ) 92 { 93 cerr << "\nError: Subscript " << subscript 94 << " out of range" << endl; 95 exit( 1 ); // terminate program; subscript out of range 96 } // end if 97 98 return ptr[ subscript ]; // reference return 99 } // end function operator[] 100 101 // overloaded subscript operator for const Arrays 102 // const reference return creates an rvalue 103 int Array::operator[]( int subscript ) const 104 { 105 // check for subscript out-of-range error 106 if ( subscript < 0 || subscript >= size ) 107 { 108 cerr << "\nError: Subscript " << subscript 109 << " out of range" << endl; 110 exit( 1 ); // terminate program; subscript out of range 111 } // end if 112 113 return ptr[ subscript ]; // returns copy of this element 114 } // end function operator[]
LINK :http://www.deitel.com/articles/cplusplus_tutorials/20060204/cplusplus_operatoroverloading_arrayclass_Page3.html
