I recently ran into an interesting bug that I thought was worth sharing. The Setup We have an abstract base class, with polymorphic equality: struct Base { virtual ~Base() = default; virtual auto operator==(Base const&) const -> bool = 0; }; And then we have a bunch of derived...

One of the reasons that I’m excited for Reflection in C++ is that it can permit you to implement, as a library, many things that previously required language features. In this post, I’m going to walk through implementing P2786R8 (“Trivial Relocatability For C++26”). Or, at least, just the tri...

One of the things I like to do is compare how different languages solve the same problem — especially when they end up having very different approaches. It’s always educational. In this case, a bunch of us have been working hard on trying to get reflection — a really transformative language featu...

Setting the Stage Previously, I tried to answer the question: what’s so hard about constexpr allocation?. Today, we continue what will become a series of posts about attempting to explain the issues behind a bunch of hard problems we’re trying to solve. The next problem: class types as non-type ...

Setting the Stage Before C++20, we couldn’t have any allocation during constant evaluation at all. Any attempt to do so would fail the evaluation — it would no longer be constant. In C++20, as a result of P0784R7, that changed. Finally we could do allocation during constant evaluation. However,...

Let’s say we’re writing one container that we’re implementing in terms of another container. If we want to make my container a range based entirely on that other container, that’s easy: template <typename T> class StableVector { std::vector<T*> impl; public: auto begin() con...

The Associative Container API When it comes to a hash table, there are basically only two interesting operations from an API perspective: lookup and insertion 1. The standard library containers are all very consistent in how these operations are provided, across all the associative containers (m...

Introduction Nico Josuttis gave a talk recently that included an example like this: std::vector<int> coll{1, 4, 7, 10}; auto isEven = [](int i) { return i % 2 == 0; }; // increment even elements: for (int& i : coll | std::views::filter(isEven)) { ++i; // UB: but works } I want...

With the adoption of Ranges in C++20 (and with a lot of new additions in C++23), we have a lot of new, composable, range-based algorithms at our disposal. There are, however, several ways to spell the usage of such algorithms: std::vector<int> v = {1, 2, 3}; auto square = [](int i){ return...

Several years ago, I wrote a post about the complexities of implementing comparison operators for optional<T>: Getting in trouble with mixed comparisons. That post was all about how, even just for ==, making a few seemingly straightforward decisions leads to an ambiguity that different libr...