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Unary operators
The following example shows the syntax to
overload all the unary
operators, in the form of both
global functions (non-member friend functions) and as member functions.
These will expand upon the Integer class shown previously and add a new
byte class. The meaning of your particular operators will depend on the
way you want to use them, but consider the client programmer before doing
something unexpected.
Here is a catalog of all the unary
functions:
//: C12:OverloadingUnaryOperators.cpp
#include <iostream>
using namespace std;
// Non-member functions:
class Integer {
long i;
Integer* This() { return this; }
public:
Integer(long ll = 0) : i(ll) {}
// No side effects takes const& argument:
friend const Integer&
operator+(const Integer& a);
friend const Integer
operator-(const Integer& a);
friend const Integer
operator~(const Integer& a);
friend Integer*
operator&(Integer& a);
friend int
operator!(const Integer& a);
// Side effects have non-const& argument:
// Prefix:
friend const Integer&
operator++(Integer& a);
// Postfix:
friend const Integer
operator++(Integer& a, int);
// Prefix:
friend const Integer&
operator--(Integer& a);
// Postfix:
friend const Integer
operator--(Integer& a, int);
};
// Global operators:
const Integer& operator+(const Integer& a) {
cout << "+Integer\n";
return a; // Unary + has no effect
}
const Integer operator-(const Integer& a) {
cout << "-Integer\n";
return Integer(-a.i);
}
const Integer operator~(const Integer& a) {
cout << "~Integer\n";
return Integer(~a.i);
}
Integer* operator&(Integer& a) {
cout << "&Integer\n";
return a.This(); // &a is recursive!
}
int operator!(const Integer& a) {
cout << "!Integer\n";
return !a.i;
}
// Prefix; return incremented value
const Integer& operator++(Integer& a) {
cout << "++Integer\n";
a.i++;
return a;
}
// Postfix; return the value before increment:
const Integer operator++(Integer& a, int) {
cout << "Integer++\n";
Integer before(a.i);
a.i++;
return before;
}
// Prefix; return decremented value
const Integer& operator--(Integer& a) {
cout << "--Integer\n";
a.i--;
return a;
}
// Postfix; return the value before decrement:
const Integer operator--(Integer& a, int) {
cout << "Integer--\n";
Integer before(a.i);
a.i--;
return before;
}
// Show that the overloaded operators work:
void f(Integer a) {
+a;
-a;
~a;
Integer* ip = &a;
!a;
++a;
a++;
--a;
a--;
}
// Member functions (implicit "this"):
class Byte {
unsigned char b;
public:
Byte(unsigned char bb = 0) : b(bb) {}
// No side effects: const member function:
const Byte& operator+() const {
cout << "+Byte\n";
return *this;
}
const Byte operator-() const {
cout << "-Byte\n";
return Byte(-b);
}
const Byte operator~() const {
cout << "~Byte\n";
return Byte(~b);
}
Byte operator!() const {
cout << "!Byte\n";
return Byte(!b);
}
Byte* operator&() {
cout << "&Byte\n";
return this;
}
// Side effects: non-const member function:
const Byte& operator++() { // Prefix
cout << "++Byte\n";
b++;
return *this;
}
const Byte operator++(int) { // Postfix
cout << "Byte++\n";
Byte before(b);
b++;
return before;
}
const Byte& operator--() { // Prefix
cout << "--Byte\n";
--b;
return *this;
}
const Byte operator--(int) { // Postfix
cout << "Byte--\n";
Byte before(b);
--b;
return before;
}
};
void g(Byte b) {
+b;
-b;
~b;
Byte* bp = &b;
!b;
++b;
b++;
--b;
b--;
}
int main() {
Integer a;
f(a);
Byte b;
g(b);
} ///:~
The functions are grouped according to
the way their arguments are passed. Guidelines for how to pass and return
arguments are given later. The forms above (and the ones that follow in the next
section) are typically what you’ll use, so start with them as a pattern
when overloading your own operators.
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