Semantic requirements in concepts

by Andrzej Krzemieński

The word ‘concept’ in the context of C++ generic programming has two meanings. The first is more abstract: it is the notion from the domain of Generic Programming (GP) in general. GP is not tied to any specific language: it is an approach to writing programs, and concepts are part of this approach. In this sense concepts have been with us since the inception of the STL. The second meaning is the keyword concept in C++20 with its associated semantics: its goal is to approximate the more abstract notion of a concept from GP, and this works only to some extent. One notable difference is that concepts in GP specify semantic requirements on types they constrain, and C++ concepts cannot express them directly.

In this post we will see how semantic requirements in concepts can break your program if you don’t pay attention to them, and what can be done in C++20 concepts to account for semantic requirements...

Tricks with Default Template Arguments

by Jonathan Müller

Just like regular function parameters, template parameters can also have default parameters. For class templates, this behaves mostly just like default function arguments: if you pass fewer template arguments than required, default template arguments are used to fill the remaining places. However, for function templates, it gets more complicated as template parameters for functions can be deduced by the normal function arguments. This leads to some interesting side-effects. In particular, default arguments of template parameters don’t need to be put at the end!

Let’s take a look at a couple of things we can do with default template arguments...

Calendar and Time-Zones in C++20: Time of Day

By Rainer Grimm

With C++20, the chrono library from C++11 becomes important additions. The most prominent ones are a calendar and time-zones support. But this is by far not all. C++20 gets new clocks, powerful formatting functionality for time durations, and a time of day type

17 Smaller but Handy C++17 Features

by Bartlomiej Filipek

When you see an article about new C++ features, most of the time you’ll have a description of major elements. Looking at C++17, there are a lot of posts (including articles from this blog) about structured bindings, filesystem, parallel algorithms, if constexpr, std::optional, std::variant… and other prominent C++17 additions.

But how about some smaller parts? Library or language improvements that didn’t require decades to standardise or violent “battles” at the ISO meetings.

In this article, I’ll show you 17 (plus a few extra!) smaller C++17 things that will improve your code...

More and More Utilities in C++20

by Rainer Grimm

Today, I present a few utilities for calculating the midpoint of two values, check if a std::string starts or ends with a substring, and create callables with std::bind_front. These little utilities may not seem so little when you need them...

constexpr is a Platform

by Jonathan Müller

Let me share a useful insight with you: constexpr is a platform...

A brief introduction to C++ structured binding

by Raymond Chen

C++17 introduced a feature known as structured binding. It allows a single source object to be taken apart...

C++20: Extend std::format for User-Defined Types

by Rainer Grimm

Peter Gottschling presented in his last post "std::format in C++20" the basics to the new formatting library in C++20. In today's post Peter writes about the formatting of user-defined types...

Increased Complexity of C++20 Range Algorithms Declarations - Is It Worth?

by Bartlomiej Filipek

With the addition of Ranges and Concepts in C++20, our good old algorithm interfaces got super long “rangified” versions. For example, copy is now 4 lines long… and it’s just the declaration!

C++ Compile time conditional struct member variables

by Saleem Ahmad

C++ has rich set of features to do compile time calculations and optimizations to generate a better code. In one of code segment I have very large data structure in which few member variables are not used based on compile time if constexpr condition, but these variables are logged in the log line...