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: C++
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';
}
}
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.
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;
}
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;
}
};
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;
};
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;
}
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
C++ tasks from Yandex
Task 1
At an interview I was asked how to by a given vector of integers build resulting vector containing the products of all the elements except current. Below I provided my solution in C++:
#include <vector>
#include <iostream>
using V = std::vector<int>;
V func(const V& v)
{
V result;
result.resize(v.size());
int product = 1;
for (size_t pos = 0; pos != v.size(); ++pos)
{
result[pos] = product;
int a = v[pos];
product *= a;
}
Range-based for loop with universal reference.
It is possible to iterate over std::vector with &&:
#include <vector>
class A {};
int main()
{
std::vector<A> vec;
for (auto&& v : vec)
{
static_cast<void>(v);
}
return 0;
}
