// This code illustrates implementing the preincrement and postincrement
// operators in a class.
//
// Notes:
// • The letters[] array should really be static.  As it is, there's
//   a copy of it in each Greek object.
// • preincrement returns by reference, because that’s faster than
//   making a copy of the Greek object.
// • preincrement uses the C++17 free function size(), which returns
//   the number of elements in letters (24).  DRY!
// • postdecrement returns by value, which is slower.  However, it’s
//   returning a copy, so it can’t return by reference, because that
//   copy vanishes when the method returns.
// • Postincrement is implemented ENTIRELY in terms of preincrement.
//   This technique can be used in almost any class that implements
//   postincrement.
// • operator<< does NOT have to be a friend.  It just uses the
//   public accessor str().

#include <iostream>
#include <string>

using namespace std;

class Greek {
    unsigned short state{0};	    // index to current greek letter
    const string letters[24] {
	"α","β","γ","δ","ε","ζ","η","θ","ι","κ","λ","μ",
	"ν","ξ","ο","π","ρ","σ","τ","υ","φ","χ","ψ","ω"};
  public:
    const string &str() const {
	return letters[state];		// return reference to const string
    }

    Greek& operator++() {		// No argument means pre-increment.
	if (++state >= size(letters))	// Went past ω, the last letter?
	    state = 0;			// Wrap around to α, the first letter.
	return *this;			// Return reference to current object.
    }

    Greek operator++(int) {		// The (int) means post-increment.
	const auto save = *this;	// Save the state before incrementing.
	++*this;			// Let pre-increment do the work.
	return save;			// Return local variable BY VALUE.
    }
};

ostream& operator<<(ostream &os, const Greek &rhs) {
    return os << rhs.str();		// Just use the public getter.
}

int main() {
    Greek g, h;				// αβγδεζηθικλμνξοπρστυφχψω

    for (int i=0; i<50; i++)
	cout << ++g;			// preincrement, so start with β
    cout << '\n';

    for (int i=0; i<50; i++)
	cout << h++;			// postincrement, so start with α
    cout << '\n';
}