Thread-Safe Queue - Two Serious Errors

by Rainer Grimm

First, I want to show you again the erroneous implementation from my last post to understand the context.

// monitorObject.cpp

#include <condition_variable>
#include <functional>
#include <queue>
#include <iostream>
#include <mutex>
#include <random>
#include <thread>

class Monitor {
public:
    void lock() const {
        monitMutex.lock();
    }

    void unlock() const {
        monitMutex.unlock();
    }

    void notify_one() const noexcept {
        monitCond.notify_one();
    }

    template <typename Predicate>
    void wait(Predicate pred) const {                 // (10)
        std::unique_lock<std::mutex> monitLock(monitMutex);
        monitCond.wait(monitLock, pred);
    }
  
private:
    mutable std::mutex monitMutex;
    mutable std::condition_variable monitCond;
};

template <typename T>                                  // (1)
class ThreadSafeQueue: public Monitor {
public:
    void add(T val){
        lock();
        myQueue.push(val);                             // (6)
        unlock();
        notify_one();
    }
  
    T get(){
        wait( [this] { return ! myQueue.empty(); } );  // (2)
        lock();
        auto val = myQueue.front();                    // (4)
        myQueue.pop();                                 // (5)
        unlock();
        return val;
    }

private:
    std::queue<T> myQueue;                            // (3)
};


class Dice {
public:
    int operator()(){ return rand(); }
private:
    std::function<int()> rand = std::bind(std::uniform_int_distribution<>(1, 6),
                                          std::default_random_engine());
};


int main(){
  
    std::cout << '\n';
  
    constexpr auto NumberThreads = 100;
  
    ThreadSafeQueue<int> safeQueue;                      // (7)

    auto addLambda = [&safeQueue](int val){ safeQueue.add(val);          // (8)
                                            std::cout << val << " "
                                            << std::this_thread::get_id() << "; ";
                                          };
    auto getLambda = [&safeQueue]{ safeQueue.get(); };  // (9)

    std::vector<std::thread> addThreads(NumberThreads);
    Dice dice;
    for (auto& thr: addThreads) thr = std::thread(addLambda, dice());

    std::vector<std::thread> getThreads(NumberThreads);
    for (auto& thr: getThreads) thr = std::thread(getLambda);

    for (auto& thr: addThreads) thr.join();
    for (auto& thr: getThreads) thr.join();
  
    std::cout << "\n\n";
   
}

C++ Lambda Idioms

by Timur Doumler

Ever since they were introduced in C++11, lambdas have been an essential language feature. Each subsequent standard added more functionality: we got generic lambdas and init captures in C++14, constexpr lambdas in C++17, default-construction and assignment, explicit template arguments and more in C++20, and even more possibilities are coming in the upcoming C++23 standard.

In this talk, we are looking at various interesting things you can do with lambdas. Some of these are well-established, useful idioms; others are lesser known, surprising tricks. Have you ever inherited from a lambda? Can you think of three different ways to call a lambda recursively? Do you know what happens if we assign an immediately-invoked lambda expression to a static variable? If not, then this talk is for you.

Object Ownership

By Ilya Doroshenko

Since we know the rules of new and delete, namely new allocates and delete destroys, we never really cared about who is responsible for the object. This caused a lot of confusion in the past. For instance, some API codes from Win32 return strings that should be LocalFree()d, like FormatMessage or GetEnvironmentStrings. POSIX, on the other hand, has strdup as a common example of you should free it yourself. This model is confusing because you may have a lot of return statements, before which you should always call free or delete, depending on the operation. However, we have RAII since the very beginning of C++, which adds constructors and destructors. So, in 1998 resourceful people decided to add auto_ptr to the standard.

The premise was simple:

• a simple explicit constructor, that took raw pointer from new
• destroyed by either an explicit release/reset or a destructor on the end of the block

This was the first attempt at a structured cleanup. As time passed, the initial point began to crumble. More issues arose and the question came up: Who owns the data?

C++ Coding with Neovim

by Prateek Raman

If the command line has ever piqued your interest, specially using Vim/Neovim for C++ coding, but you have some reservations about losing access to IDE like features, wonder no more. This talk will present a productive C++ development environment on the Command Line using Neovim and LSP. LSP (Language Server Protocol) is the protocol which powers semantic auto-completion and intellisense in modern IDEs and we'll learn how Neovim's built-in LSP client integrates seamlessly with language servers to provide a rich editing experience.

Neovim is a modern editor built on top off the giant shoulders of Vim, and lets us have all the speed and efficiency of Vim as we edit code, while also providing rich support for C++ coding with intellisense and auto-completion via LSP. Along the way we'll also demonstrate Neovim's rich LSP aware plugin ecosystem for project navigation and IDE like workflows, all on the command line.

Even if you're comfortable with Vim/Neovim, but would like to learn how to level up the experience with modern IDE features, you will find this talk interesting.

Parsing Time Stamps Faster with SIMD Instructions

by Daniel Lemire

You can generate time stamps using any programming language. In C, the following program will print the current time (universal, not local time):

We are interested in the problem of parsing these strings. In practice, this means that we want to ...

AddressSanitizer continue_on_error

by Jim Radigan

C++23 - What's In It For You?

by Marc Gregoire

C++23, the next release of the C++ standard, is scheduled to be released in 2023. The update introduces new features to the core language and to the Standard Library. If you want to stay up to date with the latest features that are upcoming with this new release, then this session is for you.

The session includes core language topics such as consteval if statements, multidimensional subscript operators, decay copy, unreachable code, and more. New Standard Library features that will be shown include monadic operations for std::optional, std::flat_map, std::flat_set, a stacktrace library, changes to the ranges library, improvements to std::format, std::expected, and many more.

The session will include references to other sessions at CppCon, if applicable, for more deep-dive information on any particular topic.

Monitor Object

by Rainer Grimm

Problem

If many threads access a shared object concurrently, the following challenges exist.

• Due to the concurrent access, the shared object must be protected from non-synchronized read and write operations to avoid data races.
• The necessary synchronization should be part of the implementation, not the interface.
• When a thread is done with the shared object, a notification should be triggered so the next thread can use the shared object. This mechanism helps avoid and improves the system's overall performance.
• After the execution of a member function, the invariants of the shared object must hold.

Solution

A client (thread) can access the Monitor Object's synchronized member functions, and due to the monitor lock, only one synchronized member function can run at any given time. Each Monitor Object has a monitor condition that notifies the waiting clients.

The Most Important Optimizations to Apply in Your C++ Programs

by Jan Bielak

Writing efficient programs is hard. This is because it requires a lot of knowledge, experience and strategic thinking. There have been many talks on optimization and often each addresses a single concept. Being able to achieve a bird’s eye view of factors affecting performance often requires many hours of researching the topic. To lessen the mental burden of optimizing programs, I have picked out the techniques, I believe are most important. During the talk, I will present them in an organized manner and provide practical examples of how they can be applied.

I will first discuss what I believe are the main goals efficient programs strive to achieve. Then, I will present the general methods of achieving those goals. Then, for the majority of the talk, we will discuss a few dozen performance opportunities. For each of them, I will explain the underlying mechanism of how the optimisation works. I will avoid bluntly giving guidelines to follow without explanation. Each of the techniques naturally comes with its costs, and those will be discussed as well.

I will additionally discuss various performance pitfalls. These are sometimes called “premature pessimisations” in contrast to the often used term of “premature optimizations”. I will show examples of optimizations which do not incur any cost on program readability or maintainability and as such should be considered performance best practices. Avoiding their use doesn’t improve code in any manner, while making it slower.

This talk is intended for a diverse audience, as after all, probably most of the C++ community is interested in performance. It is appropriate for hobbyists and professionals alike, with varying experience with the language, due to the gradual increase in difficulty of examples. It will be a time productively spent.

Finite State Machine with std::variant

by Bartlomiej Filipek

Let’s start with a basic example:

• we want to track a game player’s health status
• we’d like to respond to events like “Hit by a monster” or “Healing bonus.”
• when the health point goes to 0, then we have to restart the game only if there are some remaining lives available

Here’s a basic diagram with states and transitions: