Giuseppe D’Angelo

In the previous posts of this series (if you've missed them: parts 1, 2, 3, and 4), we have learned about relocation and trivial relocation. We have explored what relocation means, what trivial relocation means, and how it can be used to optimize the implementation of certain data structures, such as the reallocation of a […]

In the last post of this series we learned that: erasing elements from the middle of a vector can be implemented, in general, via a series of move assignments, move constructions, swaps, destructions for types with value semantics, the exact strategy does not really matter for types with write-through reference semantics, the strategy matters, because […]

In the last post of this series we started exploring how to erase an element from the middle of a vector. We discussed that in principle there are several different possible ways to implement erase().For instance, a vector could move-assign over the elements to be erased: Alternatively, a vector could use rotations or some other […]

In the last post of this series we discussed the usage of trivial relocation in order to optimize move construction followed by the destruction of the source. To quickly recap: objects of certain datatypes ("trivially relocatable" types) can be moved in memory by simply moving bytes; this can be used to optimize certain bulk operations […]

The container classes introduced in Qt 4 (Tulip, for the aficionados) had an interesting optimization: the ability to turn certain operations on the contained objects into byte-level manipulations. Example: vector reallocation Consider the reallocation of a QVector<T>: when the vector is full and we want to insert a new value (of type T), the vector […]

Today's blog post is about a small utility class in Qt with a... questionable name: QPointer. If you're new to Qt, maybe don't check out QPointer's documentation just yet, and try to guess what the class does based on its name alone. I've seen countless users being very confused by it. Some end up using […]

The familiar solution for thick value classes that want to preserve binary compatibility is to use the pimpl pattern (private implementation), also known as d-pointer (pointer to data). In future versions of our class, we can freely change the contents of the pimpl (i.e. adding, removing, and/or modifying data members) but the binary compatibility of […]

Given a strictly positive integer i, this code will calculate i+1 "equally spaced" values between 1 and 0: If you're looking for a trap, this does actually work for any i > 0. One can verify it experimentally; run the code with i from 1 to INT_MAX. For simplicity, just consider the case j = […]

A very common implementation pattern for QObject subclasses is to declare its child QObjects as data members of type "pointer to child." Raise your hand No, keep your hand on your computer input device :-) Nod if you have ever seen code like this (and maybe even written code like this yourself): A fairly common […]

If you're following our Youtube channel you might have heard me talking about QVarLengthArray. If you're not... you should follow us! But let me give you a quick recap. QVarLengthArray is a Qt container that acts like a vector; its elements are stored contiguously in memory and it has a dynamic size. At any time […]

C++23 is feature complete and on track to be released next year. While many people are complaining that it's, after all, a "minor" release (as the pandemic made the Committee work very difficult), C++23 still has a few very significant changes. In this blog post, I want to talk about what I think is my […]

Another day, another small addition to KDToolBox, KDAB's collection of miscellaneous useful C++ classes and stuff. In this post, we're going to talk about KDFunctionalSortFilterProxyModel, which is a convenience subclass of QSortFilterProxyModel. What's a proxy model to begin with, and what's QSortFilterProxyModel? Well, that explanation requires a bigger space than what I have in this […]

(Apologies for the clickbait in the post title! But I'd really like people who are searching for solutions to read this.) Between Qt 5.14 and Qt 5.15, my colleague, Marc Mutz, and I submitted a series of patches to Qt that added "range constructors" to the Qt containers. This brought Qt containers one step closer […]

In the previous blog post of this series, we discussed KDToolBox::QtHasher, a helper class that allows us to use unordered associative containers datatypes which have a qHash() overload but not a std::hash specialization. The majority of Qt value classes indeed are lacking such a specialization, even if they do provide a qHash() overload. For our […]

Sometimes it's useful to establish a connection between a signal and a slot that should be activated only once. This is not how signal/slot connections normally behave. Remember that, when a connection is established, it will stay active until: the connection is explicitly disconnected; or the sender object is destroyed; or the receiver object is […]

Another day, another blog post about KDToolBox, KDAB's collection of miscellaneous useful C++ classes and stuff. Today, we'll talk about ways to throttle your signal/slots connections -- in other words, how to activate a slot less often than the emission rate of the signal it's connected to. The usual reason why you may want something […]

In the previous blog post about qHash, we explained: how to declare an overload of qHash for your own datatypes, how to hash Qt datatypes for which Qt does not provide a qHash overload, and why you can't use a type from the Standard Library (or just another library) as a key in QHash (newsflash: […]

Another day, another blog about some of KDAB's utility classes. Now it's the turn of KDSingleApplication, a class that helps implement applications that follow the single-instance policy. What is a single-instance policy? Single-instance policy refers to those applications that want to spawn a unique instance for a given user session. Even when launched multiple times, […]