When to Overload the Comma Operator?

Question

I see questions on SO every so often about overloading the comma operator in C++ (mainly unrelated to the overloading itself, but things like the notion of sequence points), and it makes me wonder:

When should you overload the comma? What are some examples of its practical uses?

I just can't think of any examples off the top of my head where I've seen or needed to something like

foo, bar; 

in real-world code, so I'm curious as to when (if ever) this is actually used.

Answer 1

I have used the comma operator in order to index maps with multiple indices.

enum Place {new_york, washington, ...};  pair<Place, Place> operator , (Place p1, Place p2) {     return make_pair(p1, p2); }   map< pair<Place, Place>, double> distance;  distance[new_york, washington] = 100; 

Answer 2

Let's change the emphasis a bit to:

When should you overload the comma?

The answer: Never.

The exception: If you're doing template metaprogramming, operator, has a special place at the very bottom of the operator precedence list, which can come in handy for constructing SFINAE-guards, etc.

The only two practical uses I've seen of overloading operator, are both in Boost:

• Boost.Assign
• Boost.Phoenix – it's fundamental here in that it allows Phoenix lambdas to support multiple statements