Solving Undefined Behavior in Factories with constinit from C++20

by Bartlomiej Filipek

Let’s bring back the topic:

Here’s a typical factory function. It creates unique_ptr with ZipCompression or BZCompression based on the passed name/filename:

Here are some issues with this approach:

• Each time you write a new class, and you want to include it in the factory, you have to add another if in the Create() method. Easy to forget in a complex system.
• All the types must be known to the factory.
• In Create(), we arbitrarily used strings to represent types. Such representation is only visible in that single method. What if you’d like to use it somewhere else? Strings might be easily misspelled, especially if you have several places where they are compared.

All in all, we get a strong dependency between the factory and the classes.

But what if classes could register themselves? Would that help?

C++20’s Coroutines for Beginners

by Andreas Fertig

You've heard about this new feature in C++20, coroutines, but it's the first time you have encountered this term? Then this talk is what you're looking for. We start from the beginning with just "normal" functions. Next, we introduce coroutines.

Using them, we explore the various customization points C++ offers. We look at what the new keywords co_await, co_yield, and co_return are for.

Sadly, we also have to talk about how to write a generator for a coroutine since there is no STL part for that in C++20.

Another distinction we make is between cooperative and preemptive multitasking, opening the door for another beauty of coroutines, why we don't need locks.

By the end of this talk, you've learned what coroutines are and where you can use them.

The Move Constructor That You Have to Declare, Even Though You Don’t Want Anyone to Actually Call It

by Raymond Chen

Blah blah blah C++ return value optimization (RVO), named return value optimization (NRVO), and copy elision.

RVO support was optional in C++11 but became mandatory in C++17. NRVO support remains optional (but recommended).

To allow NRVO in C++17 (or RVO and NRVO in C++11), a move constructor must be available, even though the compiler will not call it if the optimization is employed.

You may have a class that you want to participate in RVO or NRVO, but you also don’t want it to be moved. For example, it may contain a std::mutex, which is not movable. But you nevertheless have to declare a move constructor. What can you do?

Declare the move constructor, but...

Need Something Sorted? Sleep On It!

by Kevlin Henney

A decade ago, I first presented a lightning talk entitled ‘Cool Code’. This short talk evolved into a full talk whose iterations I presented over the next half decade. The focus? Code that, for some reason or other, can be considered cool. For example, code that has played a significant role in historical events, such as the source for the Apollo Guidance Computer [Apollo]. Or code that is audacious – if not seemingly impossible – given its constraints, such as David Horne’s 1K chess [Frogley01]. There is code that is both simple and profound, such as Peter Norvig’s fits-on-a-slide spelling corrector [Norvig16]. And code that demonstrates ingenuity and humour, such as Yusuke Endoh’s Qlobe [Endoh10].

Leaving aside its content for a moment, one of the most interesting things about the talk was its...

Modern C++ In-Depth — Transparent Comparisons

by Michael Kristofik

Searching for an element in a container normally requires constructing an object of the container’s key_type. If the key is small, such as one you’d normally pass by value, it isn’t a performance concern. But what if the key is large?

Using Modern C++ to Eliminate Virtual Functions

by Jonathan Gopel

Why You Should Only Rarely Use std::move

by Andreas Fertig

The example in Listing 1 is the code I used to make my point: don’t use std::move on temporaries! Plus, in general, trust the compiler and only use std::move rarely. For this article, let’s focus on the example code.

class S {
public:
  S() { printf("default constructor\n"); }
  ~S() { printf("deconstructor\n"); }
  // Copy constructor ①
  S(const S&) { printf("copy constructor\n"); }
  // Move constructor ②
  S(S&&) { printf("move constructor\n"); }
};
void Use()
{
  S obj{
    S{} // Creating obj with a temporary of S ③
  };
}

Here we see a, well, perfectly movable class. I left the assignment operations out. They are not relevant. Aside from the constructor and destructor, we see in ① the copy constructor and in ② the move constructor. All special members print a message to identify them when they are called.

Type Safe C++ enum Extensions

by Alf Steinbach

Consider if an enum like the following,

  enum class Suit{
  spades, hearts, diamonds, clubs };

could be extended like

  enum class Suit_with_joker extends Suit {
    joker };

where

• Suit_with_joker has all the enumerators of Suit plus the joker enumerator; and
• enumerators introduced in Suit_with_joker get integer values following those of Suit; and
• any Suit value is also a Suit_with_joker value.

This would be an example of what I’ll call a value type extension.

The apparently backwards is-a relationship in the last point, where any value of the original type is-a value of the derived type, is characteristic of value type extensions.

C++20 totally lacks support for value type extensions, of enum types or other types.

Lightning Talk: Finding Whether a Number is a Power of 2

by Ankur Satle

Lightning Talk: Who is Looking for a C++ Job?

by Jens Weller

A few insights on who has been applying for C++ jobs and visited the Meeting C++ online job fairs.