Inheriting a C++ enum type

/////////////////////////////////////////////////////////////////////////////
// 
// enum_tryout.cpp
// extend an enum in the derived class.
// Sample Created by Theo Buys, 6-1-2009
// 
/////////////////////////////////////////////////////////////////////////////

#include "stdafx.h"
#include <ostream.h>

class base {
public:
	enum Fruit { Orange, Mango, Banana };
	int eat(enum Fruit fruit);
};
 
int base::eat(enum Fruit fruit)
{
	return fruit;
}
 
/////////////////////////////////////////////////////////////////////////////

class derived : public base {
public:
	derived();
	int consume(enum Fruit fruit);
	const Fruit Apple, Pear; // extend the class with extra fruit!
};
 

derived::derived() : Apple(Fruit(Banana + 1)), Pear(Fruit(Apple + 1))
{
}
 
int derived::consume(enum Fruit fruit)
{
	return fruit;
}
 
/////////////////////////////////////////////////////////////////////////////

int main(int argc, char* argv[])
{
	derived o;
 
	cout << "Orange = " << o.eat(o.Orange) << endl;
	cout << "Mango  = " << o.eat(o.Mango)  << endl;
	cout << "Banana = " << o.eat(o.Banana) << endl;
	cout << "Apple  = " << o.eat(o.Apple)  << endl;
	cout << "Pear   = " << o.eat(o.Pear)   << endl;
	
	cout << "Orange = " << o.consume(o.Orange) << endl;
	cout << "Mango  = " << o.consume(o.Mango)  << endl;
	cout << "Banana = " << o.consume(o.Banana) << endl;
	cout << "Apple  = " << o.consume(o.Apple)  << endl;
	cout << "Pear   = " << o.consume(o.Pear)   << endl;
		
	return 0;
}
 
/*
output:
 
Orange = 0
Mango  = 1
Banana = 2
Apple  = 3
Pear   = 4
 
Orange = 0
Mango  = 1
Banana = 2
Apple  = 3
Pear   = 4
 
*/

warlock6x3 wrote:

provided that the above copyright
// notice appear in all copies and that both that copyright notice and this
// permission notice appear in supporting documentation.

Ivan Kolev wrote:

In that case consume(orange) won't compile.

class Base {};
class Derived: public Base {};
 
void foo(Base);
void bar()
{
  foo(Base());
  foo(Derived());
}

void test() {
  MyFruit orange (Orange), apple (Apple);
  if (orange == apple) {
    cout << "oranges are apples" << endl; 
  }
} 

oranges are apples

It is important to ensure that the values on the two enum types don't overlap. There is no way I know of finding out the last enum value in an enum list. If anyone knows a way, please let me know. In the meantime, a workaround in the updated version looks like this:

enum Fruit { Orange, Mango, Banana, FruitCount };

enum MyFruit { Apple = FruitCount, Pear, MyFruitCount };

Roland Pibinger wrote:

This works only for one 'derived' enum. The users of your template need to define ranges for their enum values upfront.

Roland Pibinger wrote:

Another constraint - from Stroustrup p.77: "The range of an enumeration holds all the enumerator's enumerator values rounded up to the nearest larger binary power minus 1. The range goes down to 0 if the smallest enumerator is non-negative and to the nearest lesser negative binary power if the smallest enumerator is negative."

Roland Pibinger wrote:

Have you considered operator int() instead of operator EnumT()? I'm not sure which is better though.

Roland Pibinger wrote:

Your InheritEnum template already 'decays' to an int (via operator EnumT()).

Lidzhade Fhulu wrote:

It is important to ensure that the values on the two enum types don't overlap. There is no way I know of finding out the last enum value in an enum list. If anyone knows a way, please let me know. In the meantime, a workaround in the updated version looks like this:
 

enum Fruit { Orange, Mango, Banana, FruitCount };

 
and MyFruit becomes
 

enum MyFruit { Apple = FruitCount, Pear, MyFruitCount };

enum MyFruit { Apple = (Orange|Mango|Banana)+1, Pear };