Effortless Performance Improvements in C++: std::vector

by Julien Jorge

This is the third post in the series about effortless performance improvements in C++. Check the first post to get the full story!

Last time we switched from std::map to std::unordered_map and got 30% performance improvements. After these changes we observed that the tokenize function was a top hotspot, mostly due to std::vector::push_back. How can we improve the performance of this function? ...

Merging intervals in next-gen C++

by Marco Arena

A few weeks ago, I set this problem at Coding Gym: given an array of intervals where intervals[i] = [starti, endi], merge all overlapping intervals, and return an array of the non-overlapping intervals that cover all the intervals in the input...

Effortless Performance Improvements in C++

Part 1: Analyzing a program

Part 2: std::unordered_map

by Julien Jorge

In this series of blog posts we are going to explore the implications on processing time for typical C++ way-of-doing by implementing a non-trivial task using idiomatic C++. Then we will benchmark this program and improve its performance.

Optimization is a never-ending quest that can lead to huge sacrifices in terms of code readability. ... In these blog posts we will restrict ourselves into transformations that do not diminish the expressiveness of the existing interfaces, and we will stay withing the scope of the standard library. We will see that even within these limits one can easily reduce processing times by a factor of two, and maybe three with some compromises. ...

Decreasing the Number of Memory Accesses, 1.2

by Johnny's Software Lab

When we are trying to speed up a memory-bound loop, there are several different paths. We could try decreasing the dataset size. We could try increasing available instruction level parallelism. We could try modifying the way we access data. Some of these techniques are very advanced. But sometimes we should start with the basics.

One of the ways to improve on memory boundness of a certain piece of code is the old-fashioned way: decrease the total number of memory accesses (loads or stores). Once a piece of data is in the register, using it is very cheap, to the point of being free (due to CPU’s ability to execute up to 4 instructions in a single cycle and their out-of-order nature). So all techniques that try to lower the total number of loads and stores should result in speedups. ...

Fun with printing tables with std::format and C++20

by Bartlomiej Filipek

Today’s experiment went from a simple table printing code using C++17 into a fancier version from C++20. The improved code was “nicer” and easier to write and maintain. Additionally, we got working date-time handling capabilities in just a couple of lines of code! Which wasn’t possible until C++20. ...

The Iterator Protocol

by Rainer Grimm

Here is the question I want to answer: What interface must a user-defined type support to be usable in a range-based for-loop. ...

Insights from the second year of running online C++ job fairs

by Jens Weller

An overview on the second year of organizing online job fairs for the C++ community.

In 2022 Meeting C++ organized 5 online job fairs for C++. Originally 4 were planned, but in May only one company could attend and two other companies approached Meeting C++ after the event wondering if another fair in June was possible. With better planning this could have been one fair. The next job fair is on March 14th & 15th.

Value Objects

by Rainer Grimm

A value object is a small object whose equality is based on state, but not identity. Typical value objects are money, numbers, or strings. ...

...  For a user-defined type, you have to choose the appropriate equality semantics. ... In C++20, the compiler can auto-generate the equality operator and use it as a fallback for the inequality operator. The auto-generated equality operator applies value equality. To be more precise, the compiler-generated equality operator performs a lexicographical comparison. Lexicographical comparison means that all base classes are compared left to right and all nonstatic members of the class in their declaration order.

I have to add two important points:

• For strings or vectors, there is a shortcut: the compiler-generated == and != operators compare first their lengths and then their content if necessary.
• The compiler-generated three-way comparison operator (<=>) also applies value equality. You can read more about the three-way comparison operator in my previous posts:
  • The Three-Way Comparision Operator
  • More Details to the Spaceship Operator
  • Optimized Comparision with the Spaceship Operator

Honestly, the example works as expected but does not seem right. ...

std::initializer_list in C++, Caveats and Improvements

by Bartłomiej Filipek

In this article, you’ll learn why std::initializer_list has a bad reputation in C++...

... We can make the following conclusion:

std::initializer_list is a “view” type; it references some implementation-dependent and a local array of const values. Use it mainly for passing into functions when you need a variable number of arguments of the same type. If you try to return such lists and pass them around, then you risk lifetime issues. Use with care.

Let’s take on another limitation: ...

Abseil Strings Library

by Yury Fedorov

Abseil is an open source solution from Google that implements many basic and very helpful construction blocks for C++. Thanks to Abseil you may create a great isolation layer from your version of C++ compiler. In this article I will focus on few common functions from one of the most used Abseil libraries: absl::strings.