Meeting 9 August 2018

Article, Reddit

Follow-up: Toby Allsopp - An Introduction to the Proposed Coroutine Support for C++

How to use coroutines today

  • Visual Studio 2017 (/await)
  • Clang 5.0 with libc++ 5.0 (-fcoroutines-ts -stdlib=libc++)

Abstraction libraries

  • cppcoro
    • task, generator, async_generator, async_mutex, …
  • range-v3
    • generator

Clang Concepts is feature-complete

  • Announcement
  • Compiler Explorer
  • Code
    • Andrew Sutton’s reply: “If I had not gone on vacation, I might have beaten you to the punch in GCC ;) I’m in the process of working through older TS tests.”

The optimal way to return from a function, by Jason Turner

Single return (20%):

1string val(const bool b) {
2    string ret;
3    if (b) ret = "Hello"; else ret = "World";
4    return ret;
5}

versus multiple return (61%):

1string val(const bool b) {
2    if (b) return "Hello"; else return "World";
3}

versus:

1string val(const bool b) {
2    return b ? "Hello" : "World";
3}

Practical C++17, by Jason Turner

Slides

C++17: std::apply and std::invoke

C++17 in libsigc++ : invoke, apply, and constexpr if

libsigc++ implements a typesafe callback system for standard C++. It allows you to define signals and to connect those signals to any callback function, either global or a member function, regardless of whether it is static or virtual.

Modules TS example

pet.cpp:

1module pets.pet;
2import std.core;
3
4export class Pet
5{
6public:
7  virtual char const* pet() = 0;
8};

dog.cpp:

 1module pets.dog;
 2import std.core;
 3import pets.pet;
 4
 5export class Dog : public Pet
 6{
 7public:
 8  char const* pet() override;
 9};
10
11char const* Dog::pet()
12{
13  return "Woof!";
14}

interface.cpp (or maybe pets.cpp?):

1module pets;
2
3export module pets.pet;
4export module pets.dog;

main.cpp:

1import pets;
2import std.core;
3import std.memory;
4
5int main()
6{
7  std::unique_ptr<Pet> pet = std::make_unique<Dog>();
8  std::cout << "Pet says: "<< pet->pet() << std::endl;
9}

More on Modules

A C++ Puzzle by Leor Zolman

Question

Write a portable C++ (or C) program that displays:

Hello World

on the standard output when executed, WITHOUT USING ANY SEMICOLONS (;)

  • Don’t worry what’s in standard header files (and in the C version you don’t need any headers)
  • No preprocessor directives are required (aside from #include for C++)
  • No assembly language required

Solutions

C++:

1#include <iostream>
2int main() {
3    if (std::cout << "Hello World") {}
4}

C:

1int main() {
2    if (printf("Hello World")) {}
3}

CppChat: Volatile Is the Embedded Keyword

Episode

  • For many embedded or kernel developers using C++ for anything is anathema: “Here is a thing I made in C++ which solves this problem in the kernel/embedded system” – “Why are you even using C++? You should use C!”
  • C and C++ compiler defaults differ, so compiling C code with a C++ compiler will make it slower. When you disable certain C++ defaults (RTTI, exceptions) it becomes faster than C.
  • Freestanding proposals by Ben Craig:
  • Static exceptions by Herb Sutter

Volatile

  • volatile is needed:
    • John Regehr’s tweet: “I think it’s 100% clear the C++ committee should remove volatile”
      • JF Bastien’s reply: “No! I used volatile recently, and advocated for its use too!!! It’s great for signals, and TOCTOU, at a minimum. I’m wondering if we should deprecate volatile-qualified functions though. I don’t think they’re useful anymore.”
      • JF Bastien’s tweet: “<…> it defines the member function to call if the this pointer is volatile. That’s been standard C++ forever. Same for const member function overloads. Don’t forget about ref and rvalue member functions! <…>”
1class Foo {
2    void bar() volatile;
3};

Spdlog V1.0

A very fast header-only C++ logging library

Interview