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;
}
Category Archives: Programming languages
Range-based for loop with universal reference.
An annoying question about auto in C++
I was recently asked during a C++ job interview what are the types of riN variables in the code below:
int val = 25;
int foo() { return val; }
int& foo1() { return val; }
//warning: type qualifiers ignored on function return type
/*const*/ int foo2() { return val; }
const int& foo3() { return val; }
int main()
{
auto ri = foo();
auto ri1 = foo1();
auto ri2 = foo2();
auto ri3 = foo3();
//cannot bind non-const lvalue reference of type 'int&' to an rvalue of type 'int'
//auto& ri4 = foo();
auto& ri5 = foo1();
//cannot bind non-const lvalue reference of type 'int&' to an rvalue of type 'int'
//auto& ri6 = foo2();
auto& ri7 = foo3();
auto&& ri8 = foo();
auto&& ri9 = foo1();
auto&& ri10 = foo2();
auto&& ri11 = foo3();
return 0;
}
auto ignores the type qualifiers and references, so looks like the types are simply int, int& and int&&.
Negating the minimal integer results in an overflow in C++
The code below compiles with a warning:
#include <iostream>
#include <limits>
#include <cstdint>
int main()
{
int64_t m1 = -std::numeric_limits<int64_t>::min();
int64_t m2 = -m1;
std::cout << m1 << std::endl << m2 << std::endl << std::numeric_limits<int64_t>::max() << std::endl;
return 0;
}
Dragging a link from a browser to a QML app
When a link is dragged from a browser on Window 10 platform it contains the page title, but they did not make it accessible in QML:
import QtQuick 2.15
import QtQuick.Controls 2.15
ApplicationWindow {
visible: true
width: 640
height: 480
title: qsTr("Scroll")
id: root
DropArea {
id: dropArea;
anchors.fill: parent
onEntered: {
root.color = "gray";
drag.accept (Qt.LinkAction);
}
The sum of signed and unsigned is unsigned in C++
#include <iostream>
template <typename T>
void PrintValue(T val)
{
if constexpr (std::is_signed_v<T>)
{
std::cout << "signed";
}
else
{
std::cout << "unsigned";
}
std::cout << std::endl << val << std::endl;
}
None of C++ compilers have heterogeneous lookup for unordered containers except MSVC
At 10/20/2020:
And std::vector is not constexpr yet:
And there is no text formatting also.
Coroutines:
std::chrono::days:
Assignment a property of type function in QML
It is possible to do this in QML:
Item
{
property var refreshChart: function () {}
Component.onCompleted:
{
refreshChart = function ()
{
console.log("Hello!")
}
}
}
Brain collapsing rules in C++
Consider the declaration of a class that contains a lambda function or a reference to it:
#include <utility>
template <class Func>
struct Holder
{
Holder(Func && func) : m_func(std::forward<Func>(func))
{
}
Func m_func;
};
template <class Func>
auto MakeHolder(Func && func)
{
return Holder<Func>(std::forward<Func>(func));
}
Why my QT app crashes at the destructor of std::thread on Android 10 devices?
I did some research on why my QT app crashes at the destructor of std::thread on Android 10 devices at the user side with the following call stack:
#00 /apex/com.android.runtime/lib64/bionic/libc.so (abort+160)
#01 /system/lib64/libc++.so (abort_message+232)
#02 /system/lib64/libc++.so (demangling_terminate_handler()+44)
#03 /system/lib64/libc++.so (std::__terminate(void (*)())+12)
#04 /system/lib64/libc++.so (std::terminate()+52)
#05 /system/lib64/libc++.so (std::__1::thread::~thread()+20)
#06 /apex/com.android.runtime/lib64/bionic/libc.so (__cxa_finalize+212)
#07 /apex/com.android.runtime/lib64/bionic/libc.so (exit+24)
#08 /data/app/com.domain.myapp-Rs_sm5VrLR1Jj8QW6oYByA==/lib/arm64/libplugins_platforms_qtforandroid_arm64-v8a.so
and figured out that its likely because std::thread destructor is being invoked while the thread is still joinable at some point of the application execution (thanks to G. M. on stackoverflow.com).
(more…)C++ binary serialization speed is comparable with std::memmove
Measuring std::memmove speed
I wrote a simple test that outputs std::memmove speed to the console:
AWT_ATTRIBUTE(size_t, element_count, 1000000);
std::unique_ptr<uint8_t> p_src(new uint8_t[element_count]);
std::memset(p_src.get(), 25u, element_count);
std::unique_ptr<uint8_t> p_dst(new uint8_t[element_count]);
context.out << _T("std::memmove: ");
awl::StopWatch w;
std::memmove(p_dst.get(), p_src.get(), element_count);
ReportSpeed(context, w, element_count);
context.out << std::endl;
And the similar tests for std::memset and std::vector::insert.
(more…)