Question and Answer Database
FAQ407C.txt Operator Overloading
Category :C/C++ Language Issues
Platform :All
Product :BC++ 5.x
Question:
What is operator overloading?
Answer:
Basics of operator overloading: using an operator like "a + b" is like
calling a function with 2 parameters "operator+(a,b)" or calling a member
function "a.operator+(b)". Note: the former 2 expressions are valid C++
expressions and can be used in programs.
overloading operator "+" between type A and B is in fact overloading the
function "operator+(A,B)" or the member function "operator+(B)" of the A
class (A must be a class in this case of course).
So defining a "+" and "+=" operation for a complex classes could be:
class complex
{
public:
double re, im;
complex() {}
complex(double r, double i = 0) : re(r), im(i) {}
complex& operator+=(const complex& a)
{
re += a.re;
im += a.im;
return *this;
}
};
inline complex operator+(const complex& a, const complex& b)
{
return complex(a.re + b.re, a.im + b.im);
}
You can use them as follow:
complex a(1,-1), b(2,2);
complex c = a + b; // c will be (3,1). Same as c = operator+(a,b);
c += a; // c will be (4,0). Same as c.operator+=(a);
Note: operator+() could also have been defined as a member function.
Some special points of operators:
— You can only overload operators if at least one operand is a class,
structure or reference to class or structure. If every operand were built-in types, that would mean redefining an existing or forbidden
operation.
— You cannot overload "::" (scope operator) "?:" (ternary operator) "."
and ".*" (member resolution operators).
— "->" and "->*" are special, can be only defined as unary member functions
returning a type supporting the "->" or "->*" operator (generally a pointer
to a class or structure)
— operator "=" (assignment operator) have special behavior like copy
constructors (and it's one of the most important operator...) and can only be
defined as a member function.
— postcript "++" and "--" operator differs from their non-postfix
counterpart using a dummy "int" argument: "operator++(A)" defines "++a" while
"operator++(A,int)" defines "a++".
— special operators called cast operators (always member functions) defined
as "operator ()" without return types are used to define a conversion
to another type.
— the call operator (member function) emulates a function call. Example:
struct functor
{
int operator()(int, int);
};
functor f;
int result = f(2,3);
7/2/98 10:32:32 AM
Last Modified: 01-SEP-99