Signals and slots are my favourite Qt feature, as they make loose coupling between components or between layers super easy. I miss them most when I must write pure C++ code without the Qt goodies. I invite you to follow along while I translate signals and slots into function-object member variables and lambda functions, respectively.
Read More »Implementing Qt Signals and Slots in Pure C++I evaluated the dependency analyser CppDepend on a real-life embedded application. My goal was to find all dependency cycles between classes in the application. I… Read More »CppDepend: A C++ Dependency Analyser
Two years ago, I spent three days to figure out why the driver terminal of a sugar beet harvester crashed (see my original post). The crash happened after going through the same six-step interaction at least four times. The reason was that the C++ code accessed an object that the QML engine had already deleted. For the last two years, I have heard a lot of good things about address sanitizers. When I threw address sanitizers at the old problem, they identified the problem right away. Recently, address sanitizers helped me to locate and fix some strange crashes on a legacy application. Sanitizers will be part of my debugging toolbox from now on.
Read More »QML Engine Deletes C++ Objects Still In Use – Revisited with Address SanitizersCorrection: In the original post, I stated that ownership is transferred from C++ to QML by READ functions of Q_PROPERTYs. This is wrong. Ownership is only transferred by Q_INVOKABLE functions and slots, which return a QObject pointer. I corrected my post and my code example. Now, the simple code example crashes as desired. Many thanks to Richard and Simon for pointing out my mistake.
I recently spent three days on a customer project to figure out why my QML application crashed with a segmentation fault. The crash happened after a long sequence of interactions, which was hard to reproduce. I finally managed to reproduce the crash reliably after going through the same six-step interaction four times.
After many hours of debugging and scrutinising my code, I had an epiphany about the stack trace leading to the crash. The stack trace originated from deep inside the QML engine and ended in calling the destructor of a C++ object, which was still in use on the C++ side. The trace never touched any of the application’s C++ code in between.
So far, I had only asked myself the question: Where in my C++ code do I corrupt the memory? After my little epiphany, I changed my question: Why does the QML engine delete a C++ object still in use? What do I overlook in the interaction between QML and C++?
Read More »QML Engine Deletes C++ Objects Still In Use
Move semantics is faster than copy semantics, when the compiler can replace expensive copy operations by cheaper move operations, that is, when it can replace a deep copy of a big object by a shallow copy of the pointer to the big object. Hence, classes using the pimpl idiom in combination with move semantics should see a considerable speed-up. As Qt applies the pimpl idiom consistently to every non-trivial Qt class, we should see a speed-up by simply using Qt classes instead of their STL counterparts. I’ll compare the performance of classes that use move semantics with Qt and STL classes with and without applying the pimpl idiom.
Read More »Best Friends: C++11 Move Semantics and Pimpl
We explore when and how our code benefits most from equipping classes with move constructors and assignment operators. We analyse in detail, which constructors and assignment operators (move or copy) are called for C++98 and C++11 when we insert or emplace objects into containers, when we initialise containers and when we shuffle and sort containers. Finally, we run benchmarks for these operations and discuss the actual speed-up, which ranges from 1% to 70%.
Read More »Performance Gains Through C++11 Move Semantics
Recently, I looked through the code base of a medium-sized project to see how I could simplify handwritten for-loops by using C++11’s new range-based for and STL algorithms with lambda expressions. The results in short: Range-based for makes loops simpler, easier to understand and often faster. STL algorithms are often a bit harder to read and write than range-based for loops, because lambda expressions are pretty clumsy, algorithm naming is inconsistent, and algorithm interfaces are inconvenient. But, they are still better than handwritten for-loops.
Read More »Simplifying Loops with C++11 in Qt Ways
In this post, I want to show some examples how C++11’s initializer lists can improve some typical Qt code. I’ll show how C++11 makes the initialisation of container-type variables very simple and how it makes static const data members possible at all.
Read More »Using C++11 Initializer Lists in Qt Ways