Category Archives: Programming languages

Waitable QTimer with C++20 coroutines

Found an example of overriding co_await operator and was able to implement my own version:

#include <chrono>
#include <coroutine>

//QTimer::singleShot accepts only milliseconds and a template version of the operator does not compile.
auto operator co_await(std::chrono::milliseconds d)
{
    class awaiter
    {
        std::chrono::milliseconds m_d;

    public:

        explicit awaiter(std::chrono::milliseconds d) : m_d(d) {}

        bool await_ready() const noexcept
        {
            return m_d <= std::chrono::milliseconds(0);
        }

        void await_suspend(std::coroutine_handle<> h) const noexcept
        {
            QTimer::singleShot(m_d, [h]() -> void
            {
                h.resume();
            });
        }

        void await_resume() const noexcept {}
    };

    return awaiter { d };
}

For those who did not work with QT, QTimer::singleShot simply calls the lambda on the current thread after a given interval.

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An example of real-time TableView in QML

Looks like CheckBox-es respond to clicks and work correctly even if they are in a TableView that is updated once a second:

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Replace a substring in source file names

For example, the command below

find -name "*android*" -exec rename 's/android/windows/' {} \;

renames the following source files:

androidinappproduct.cpp
androidinappproduct.h
androidinapppurchasebackend.cpp
androidinapppurchasebackend.h
androidinapptransaction.cpp
androidinapptransaction.h
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Using QML_ELEMENT with CMake

There is an example of using QML_ELEMENT in qt5\qtdeclarative\examples\quick\scenegraph\openglunderqml and qt_add_qml_module in CMake:

qt_add_qml_module(openglunderqml
    URI OpenGLUnderQML
    VERSION 1.0
    QML_FILES main.qml
    RESOURCE_PREFIX /scenegraph/openglunderqml
    NO_RESOURCE_TARGET_PATH
)

In QML it imports the module:

import OpenGLUnderQML 1.0
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Adding a header to source files

Put the header content into a text file and load it into a variable:

cd ~/temp/
nano h.txt
h=$(<h.txt)
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Did you already learn what are C++ concepts?

In C++20 we can do this:

#include <ranges>
#include <vector>

template <std::ranges::range Range>
    requires std::same_as<std::ranges::range_value_t<Range>, int>
auto TransformIt(Range r)
{
    return r | std::views::transform([](int n) { return n * n; });
}

int main()
{
    std::vector<int> v;
    auto r = TransformIt(v);
    return 0;
}
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std::unique_ptr with a deleter that throws an exception

The program below is terminated when the deleter throws the exception:

#include <iostream>
#include <memory>

struct X
{
    int value;
};

struct Deleter
{
    void operator()(X* p) noexcept(false)
    {
        if (p->value == 0)
        {
            throw std::logic_error("Can't delete a zero.");
        }

        delete p;
    }
};
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An example of heterogeneous lookup with std::unordered_set in C++20

std::unordered_set has a template find function in C++20. To make it work I define a custom hash and a custom equality compares as follows:

struct BotSettings
{
    std::string type;
    std::string name;
    bool started;
};
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Array subscript -N is outside array bounds warning in GCC11

Consider the code below with UB:

template <class Derived>
class A
{
public:

    Derived * get() { return static_cast<Derived *>(this);}

private:

    int m_a;
};

class B
{
public:

    double m_b;
};

class X : public B, public A<X>
{
public:

    int m_x;
};

int main()
{
    A<X> a;
    std::cout << a.get();
    return 0;
}
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Compiling Awl on Ubuntu 18 and 20 with GCC 11

Below I provided the instruction on how to build Awl on Ubuntu 18 and 20.

On Ubuntu 18 you do an extra step to install CMake version >= 3.12:

wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | sudo apt-key add -
sudo apt-add-repository 'deb https://apt.kitware.com/ubuntu/ bionic main'
sudo apt update

Then do the following steps on both Ubuntu 18 and 20:

sudo apt install cmake
cmake --version
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