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...

In Rust, if I want to print some 32-bit unsigned value in hex, with the leading 0x, padded out with zeros, I would write that as: println!("{:#010x}", value); In C++23, if I want to do the same, that’s: std::println("{:#010x}", value); The only difference is the spelling of the name of the ...

Sometimes, we want more than to just iterate over all the elements of a range. We also want the index of each element. If we had something like a vector<T>, then a simple for loop suffices 1: for (int i = 0; i < vec.size(); ++i) { // use i or vec[i] } But for a range that can’t be...

One of the new language features for C++23 is Deducing this, which is a feature I co-authored with Gašper Ažman, Sy Brand, and Ben Deane. The interesting history there is that Sy and I were working on solving one problem (deduplication of all the const and ref-qualifier overloads) and Gašper and ...

Let’s talk about assignment for optional<T>. I realize this is a fraught topic, but I want to try to build up proper intuition about how assignment has to work, especially since the debate around this topic has been fairly underwhelming. This post will almost exclusively discuss copy assign...