An example of extending the lifetime of a temporary:
#include <iostream>
int main()
{
int a = 1;
int&& b = std::move(a);
int&& c = b + 1;
b = b + 1;
c = c + 1;
std::cout << a << b << c;
}
The output is:
223
Category Archives: Programming languages
An example of how r-value reference works in C++
Using std::future as a coroutine in C++20
In C++20 it is possible to do this:
std::future<int> compute(as_coroutine) {
int a = co_await std::async([] { return 6; });
int b = co_await std::async([] { return 7; });
co_return a * b;
}
std::future<void> fail(as_coroutine) {
throw std::runtime_error("bleah");
co_return;
}
int main() {
std::cout << compute({}).get() << '\n';
try {
fail({}).get();
} catch (const std::runtime_error &e) {
std::cout << "error: " << e.what() << '\n';
}
}
Switching to a different thread with async/await in C#
The code below switches to a different thread when the execution of Main() is resumed at line 11:
internal class Program
{
static async Task Main(string[] args)
{
try
{
HttpClient client = new HttpClient();
Console.WriteLine($"Thread: {Thread.CurrentThread.ManagedThreadId}");
HttpResponseMessage response = await client.GetAsync("https://developernote.com/");
Console.WriteLine($"Thread: {Thread.CurrentThread.ManagedThreadId}");
response.EnsureSuccessStatusCode();
string responseBody = await response.Content.ReadAsStringAsync();
Console.WriteLine($"Thread: {Thread.CurrentThread.ManagedThreadId}");
Console.WriteLine(responseBody);
}
catch (HttpRequestException e)
{
Console.WriteLine("\nException Caught!");
Console.WriteLine($"Thread: {Thread.CurrentThread.ManagedThreadId}");
Console.WriteLine($"Message: {e.Message}");
}
}
}
Waitable QTimer with C++20 coroutines
Found an example of overriding co_await operator and was able to implement my own version:
#include <QTimer>
#include <QMetaObject>
#include <chrono>
#include <coroutine>
//QTimer accepts only milliseconds.
class TimeAwaitable
{
public:
explicit TimeAwaitable(std::chrono::milliseconds d) : m_d(d) {}
~TimeAwaitable()
{
if (m_connection)
{
QObject::disconnect(m_connection);
}
}
bool await_ready() const noexcept
{
return m_d <= std::chrono::milliseconds(0);
}
void await_suspend(std::coroutine_handle<> h) noexcept
{
m_connection = QObject::connect(&m_timer, &QTimer::timeout,
[this, h]()
{
QObject::disconnect(m_connection);
h.resume();
});
m_timer.setSingleShot(true);
m_timer.start(m_d);
}
void await_resume() const noexcept {}
private:
std::chrono::milliseconds m_d;
QTimer m_timer;
QMetaObject::Connection m_connection;
};
inline TimeAwaitable operator co_await(std::chrono::milliseconds d) noexcept
{
return TimeAwaitable{ d };
}
For those who did not work with QT, QTimer::singleShot simply calls the lambda on the current thread after a given interval.
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:
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
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
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)
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;
}