C++ Cookbook

15.5. Writing an Operator That Isn’t a Member Function

Problem

You have to write a binary operator, and you can’t or don’t want to make it a class member function.

Solution

Use the operator keyword, a temporary variable, and a copy constructor to do most of the work, and return the temporary object. Example 15-5 presents a simple string concatenation operator for a custom String class.

Example 15-5. Concatenation with a nonmember operator

#include <iostream>
#include <cstring>

class String {  // Assume the String class declaration
                // has at least everything shown here
public:
   String();
   String(const char* p);
   String(const String& orig);
  ~String() {delete buf_;}

   String& append(const String& s);
   size_t length() const;
   const char* data() const;
   String& operator=(const String& orig);

   // ...
};

String operator+(const String& lhs, const String& rhs) {

   String tmp(lhs); // Copy construct a temp object
   tmp.append(rhs); // Use a member function to do the real work

   return(tmp);     // Return the temporary
}

int main() {

   String s1("banana ");
   String s2("rancher");
   String s3, s4, s5, s6;

   s3 = s1 + s2;           // Works fine, no surprises
   s4 = s1 + "rama";       // Constructs "rama" automatically using
                           // the constructor String(const char*)
   s5 = "ham " + s2;       // Hey cool, it even does it backward
   s6 = s1 + "rama " + s2;

   std::cout << "s3 = " << s3.data() << '\n';
   std::cout << "s4 = " << s4.data() << '\n';
   std::cout << "s5 = " << s5.data() << '\n';
   std::cout << "s6 = " << s6.data() << '\n';
}

Discussion

A standalone operator ...