People frequently ask how to do things like this in C++20: Make a concept Renderer that is satisfied if and only if t.render(u) is valid for all Printable types U. (From /r/cpp.) Make a concept ValidSize that is satisfied if and only if t is convertible to some integral type U. (From the standard.)…

Someone on StackOverflow just asked about the silly-looking requires requires syntax that’s used a few times in the C++2a Working Draft and in the Ranges library implementation. Since I had just written slides on this for my presentation at the NYC C++ meetup last week, and my explanation seemed…

Here’s a slogan that needs more currency: const is a contract. Let’s start off with the one place this is not true: variable declarations. Here y is an object of const type. This means that it’s constant, immutable; nobody can change its value, and its value cannot change. This is what newcomers to…

In my C++ training classes, I often explain the meaning of =delete as The library author is saying, “I know what you’re trying to do, and what you’re trying to do is wrong.” In some situations there’s not much difference between =delete’ing a function and not-=delete’ing it. In fact, the most common…

C++20 introduces new library functions std::erase and std::erase_if. Notice that I said “functions,” not “algorithms”: these are not implemented as generic function templates, but rather as a closed set of function overloads, scattered across the STL’s many container-related headers. There’s a…

Here’s another slogan that needs more currency: Value category is not lifetime. What I mean by this mantra is probably best shown by example. Two examples of a common C++ problem Suppose we design a String class that looks like this: And suppose we design a TokenIterator class that looks like this:…

I was all set to write a blog post about hidden friends, and then I realized that it was turning into a blog post largely ranting about ADL. So I figure I should rant about ADL first, and then talk about hidden friends in Part 2. First things first. ADL stands for “argument-dependent [name] lookup.”…

This blog post has now been more than a year in the making; it’s the one for which I was laying the groundwork when I wrote “What is ADL?” (2019-04-26). Since I used the term again the other day, let’s finally define it: What is the std::swap two-step? The name “std::swap two-step” comes (as far as…

Recall that in C++ there’s a difference between a using-declaration and a using-directive. Using-declarations A using-declaration is semantically similar to a declaration; it introduces into the current scope a new meaning for a name. If that name already had one or more meanings from outer scopes,…

I just watched Odin Holmes’s half-session from C++Now 2017 titled “Type-Based Template Metaprogramming is Not Dead.” Honestly, I found it mostly pretty hard to follow. (Note to presenters: The video recording will pick up your mouse cursor, but not your laser pointer!) But there was one “oooh”…

Thanks to Jeff Trull for this example of metaprogramming. Our goal is to make a filter metafunction that takes a typelist and returns a new typelist with all non-matching elements filtered out. Here’s our unit test: And here’s our original metaprogram (courtesy of Jeff). Most C++11 metaprograms, in…

C++2a concepts (formerly known as “Concepts Lite” and/or the Concepts TS) famously do not support “definition checking.” The idea of definition checking is that the programmer might write and then an algorithm such as Now, in C++2a–concepts-land, where templates are still effectively duck-typed, we…

This post describes how to compile a single C++ source file to an object file with the Clang API. Here is the code. It behaves like a simplified clang executable that handles -c and -S. 1 cat > main.cc <<eof 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34…

March 26, 2025 Volume 23, issue 1 PDF String Matching at Scale A call for interdisciplinary collaboration and better-directed resources Dennis Roellke Freshly brewed coffee steaming from my favorite mug and an open newspaper in my hands, I am ready to enjoy this beautiful Sunday. I take a bite from…

Opinion Computing Applications Get out of town with style. By Posted Mar 28 2025 Consider a set of n items I. You have purchased k copies of each item as well as k bags. Each bag can hold all or any subset of the n items of I. The goal is to preload the k bags so that if you need a certain subset S…

Functional programming languages see types not just as buckets of things that can contain values, but as assertions, e.g., this is not just a string that I vaguely hope is a Username, but this is a validated, certain-to-exist Username for a user, and anything that receives an input parameter to some…

There are two main types of fixed-precision integers in modern software: unsigned and signed. In C++20 and above, the signed integers must use the two’s complement convention. Other programming languages typically specify two’s complement as well. Two’s complement is a method for representing signed…

In my last post, I presented a hazard pointer implementation: A Lock-Free Stack: A Hazard Pointer Implementation. Today, I will explain the implementation. MyNode is a class template, parametrized by the type it holds: data. MyNode models the concept Node. template concept Node = requires(T a) {…