This should have been part 2 of my comparison series, and I have almost finished it, but due to university stuff I just haven’t found the time to polish it.

But the optional discussion started again, so I just wanted to really quickly share my raw thoughts on the topic. In case you are lucky and don’t know what I mean: std::optional<T&> doesn’t compile right now, because the behavior of assignment wasn’t clear (even though it actually is). There are basically four questions in the discussion I want to answer:

1. Is std::optional<T&> the same as a pointer?
2. Do we need std::optional<T&>?
3. Should the assignment operator rebind or assign through?
4. Should it even have an assignment operator?

tl;dr: no, I don’t, rebind, no.

In order to sort a collection of elements you need to provide a sorting predicate that determines when one element is less than the other. This predicate must “induce a strict total ordering on the equivalence classes” according to cppreference. Wait, what?

The upcoming C++ spaceship operator implements a three way comparison, i.e. it is a single function that can return the results of <, == and > combined. But related to it are terms like “strong equality” and “weak ordering” which are somewhat confusing if you don’t have the mathematical background.

So let’s untangle it: This series will explain both the mathematics behind equality and ordering, as well as give concrete guidelines for implementing the comparison operators and the spaceship operator.

This part covers equality and equivalence relations. What does it mean for two objects to be equal? What are the mathematical properties and C++ semantics it needs to fulfill? How do I implement proper equality comparison in C++?

In the following parts we’ll look at ordering relations, the new three way comparison and algorithms like sorting and searching on various orderings.

At C++Now 2018 I gave a talk about rethinking pointers: jonathanmueller.dev/talk/cppnow2018.

I highly recommend you check it out, even if you watched the similar talk I gave at ACCU, as that version is a lot better. It rediscovers and discusses the common guidelines about when to use references over pointers, when smart pointers, etc.

If you’re an expert, you might get a deeper meaning from the structured analysis. And if you’re a beginner, you get the condensed guidelines.

However, I think the most valuable thing is the taxonomy of pointer types. It gives new vocabulary when talking about T* vs std::optional<T&> which gives the whole discussion an obvious answer.

And here is also the big problem: Naming is hard.

In particular, my naming of the taxonomy in the talk is bad, so let’s introduce new names.

Okay, so in the previous post I talked about putting optional<T> in container. I came to conclusions which I though were reasonable at the time, however, people — rightfully — pointed out some flaws in my argumentation.

As I was at ACCU last week, I wasn’t able to respond to them earlier (note to self: don’t publish and then fly away to a conference), so I’m doing that now. Let’s revisit my argumentations and see where I was wrong.

Title says it all: should you put std::optional<T> in a container?

To answer that we have to take a slight detour first.