Overloading unary operators (C++ only)

You overload a unary operator with either a nonstatic member function that has no parameters, or a nonmember function that has one parameter. Suppose a unary operator @ is called with the statement @t, where t is an object of type T. A nonstatic member function that overloads this operator would have the following form:

 return_type operator@()

A nonmember function that overloads the same operator would have the following form:

return_type operator@(T)

An overloaded unary operator may return any type.

The following example overloads the ! operator:

#include <iostream>
using namespace std;

struct X { };

void operator!(X) {
  cout << "void operator!(X)" << endl;
}

struct Y {
  void operator!() {
    cout << "void Y::operator!()" << endl;
  }
};

struct Z { };

int main() {
  X ox; Y oy; Z oz;
  !ox;
  !oy;
//  !oz;
}

The following is the output of the above example:

void operator!(X)
void Y::operator!()

(The compiler would not allow !oz because the ! operator has not been defined for class Z.)

Related information

• Unary expressions