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Thinking in C++
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Operator overloading & inheritance

Except for the assignment operator, operators are automatically inherited into a derived class. This can be demonstrated by inheriting from C12:Byte.h:

//: C14:OperatorInheritance.cpp
// Inheriting overloaded operators
#include "../C12/Byte.h"
#include <fstream>
using namespace std;
ofstream out("ByteTest.out");

class Byte2 : public Byte {
  // Constructors don't inherit:
  Byte2(unsigned char bb = 0) : Byte(bb) {}  
  // operator= does not inherit, but 
  // is synthesized for memberwise assignment.
  // However, only the SameType = SameType
  // operator= is synthesized, so you have to
  // make the others explicitly:
  Byte2& operator=(const Byte& right) {
    return *this;
  Byte2& operator=(int i) { 
    return *this;

// Similar test function as in C12:ByteTest.cpp:
void k(Byte2& b1, Byte2& b2) {
  b1 = b1 * b2 + b2 % b1;

  #define TRY2(OP) \
    out << "b1 = "; b1.print(out); \
    out << ", b2 = "; b2.print(out); \
    out << ";  b1 " #OP " b2 produces "; \
    (b1 OP b2).print(out); \
    out << endl;

  b1 = 9; b2 = 47;
  TRY2(+) TRY2(-) TRY2(*) TRY2(/)
  TRY2(%) TRY2(^) TRY2(&) TRY2(|)
  TRY2(<<) TRY2(>>) TRY2(+=) TRY2(-=)
  TRY2(*=) TRY2(/=) TRY2(%=) TRY2(^=)
  TRY2(&=) TRY2(|=) TRY2(>>=) TRY2(<<=)
  TRY2(=) // Assignment operator

  // Conditionals:
  #define TRYC2(OP) \
    out << "b1 = "; b1.print(out); \
    out << ", b2 = "; b2.print(out); \
    out << ";  b1 " #OP " b2 produces "; \
    out << (b1 OP b2); \
    out << endl;

  b1 = 9; b2 = 47;
  TRYC2(<) TRYC2(>) TRYC2(==) TRYC2(!=) TRYC2(<=)
  TRYC2(>=) TRYC2(&&) TRYC2(||)

  // Chained assignment:
  Byte2 b3 = 92;
  b1 = b2 = b3;

int main() {
  out << "member functions:" << endl;
  Byte2 b1(47), b2(9);
  k(b1, b2);
} ///:~

The test code is identical to that in C12:ByteTest.cpp except that Byte2 is used instead of Byte. This way all the operators are verified to work with Byte2 via inheritance.

When you examine the class Byte2, you’ll see that the constructor must be explicitly defined, and that only the operator= that assigns a Byte2 to a Byte2 is synthesized; any other assignment operators that you need you’ll have to synthesize on your own.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire