C++ Gurus

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  UNSOLVED Nested Map QDialog sharing
  • ASCsk  

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  @ASCsk said in Nested Map QDialog sharing: QDate Calendario::on_calendarWidget_clicked(const QDate &date) { *selecteddate = date; qDebug() << *selecteddate; return *selecteddate; } This screams for cleanup. There's no need to allocate a QDate on the heap. In any case, from your description, you should really consider using a database. Please take the time to go through the corresponding chapter in Qt's documentation as well as read the examples.
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  UNSOLVED Static Member variable and static member function resides in ????
  • Vinoth Rajendran4  

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  In terms of memory placement static member is basically like a variable in global scope. Same with a member function - it's like a free function outside of classes. The class scope is just syntax and visibility management, sorta like a namespace.
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  SOLVED How to get std::array's size at compile time
  c++ constexpr • • kshegunov  

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  Thanks for the input guys, much appreciated!
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  SOLVED Performance difference
  • damianatorrpm  

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  @Chris-Kawa It looked a bit like "Kiwi" to me, I was way off. "("Down-under" means Australia or New Zealand.) It may be wasted on you, but the one thing I was thinking I might regret on my deathbed is that Kylie never chose to marry me. I do not believe that is because she is disappointed that I did not use compiler optimizations ;-) All the best :)
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  UNSOLVED Can someone explain this code to me please?
  • UnknownCOder  

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  @UnknownCOder But how many LEDs? If leds contains 60 elements then leds[z] will crash if z is 60 as the biggest valid index is 59!
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  UNSOLVED Singleton vs uncreatableType
  • damianatorrpm  

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  To an extend it does. The chart does not compare the advantages of using a singleton vs an uncreatable type if the singleton is not instantiated at all.
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  SOLVED Fastest way to read part of 300 Gigabyte binary file
  • Please_Help_me_D  

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  @SGaist yes thank you @JonB I was wrong. Thank you for explanation
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  SOLVED Question about "auto" keyword in combination with "new"
  • KroMignon  

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  @fcarney said in Question about "auto" keyword in combination with "new": auto i = 0, *p = &i; But this is nothing special to the auto keyword. It's C basic.
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  SOLVED Second guessing the compiler?
  • fcarney  

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  Useful reading on the topic of sharing data (including between threads): https://doc.qt.io/qt-5/implicit-sharing.html
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  UNSOLVED Pure virtual function with implementation
  • Vinoth Rajendran4  

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  I think I used it in a windowing type library to define a BaseWindow that didn't actually draw anything. It was there to provide a pointer to all classes Window related. I cannot remember if I could actually instantiate it or not. This was 20 years ago, so details for fuzzy and this was a windowing library for DOS. Edit: Maybe I misread the question, but I don't think I defined any functions for pure virtual class. Edit2: Okay, I read the SO question and realized that is just really strange thing to do. So no, I don't think I have done this.
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  SOLVED Add VTK library to Qt project
  vtk • • Please_Help_me_D  

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  @SGaist ahaha thank for gremlins)
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  UNSOLVED Send mouse click to OS's window manager
  • normvcr  

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  @normvcr Under Windows I would say SendInput is indeed the way to go. "with which I am having trouble": be aware of https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-sendinput: This function is subject to UIPI. Applications are permitted to inject input only into applications that are at an equal or lesser integrity level.
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  SOLVED Can't add HDF5 library in empty Qt-project
  • Please_Help_me_D  

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  @SGaist Thank you for the time you spent!
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  UNSOLVED Segmentation Fault on object Creation
  constructor • • Vinoth Rajendran4  

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  @Vinoth-Rajendran4 Further to @Christian-Ehrlicher, in other words that means you need to delete the statement Hello* obj = new Hello;! Your calling code will go new Hello, and that will execute your constructor and return the created instance, you don't want another new Hello anywhere inside your Hello class as it stands.
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  UNSOLVED Storage of const char* in memory layout
  • Vinoth Rajendran4  

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  Thank you @Konstantin-Tokarev . Will look into the link provided by you.
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  This topic is deleted!
  • CarterJohn11  

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  SOLVED Puning... Back where I started with unions
  • fcarney  

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  Old thread and I should know better than to chime in, but everyone is entitled to my opinion. Also, I had this very issue posed to me the other day in our embedded domain. If you are dealing with existing CAN devices that expect network transport of IEEE floating point numbers then you have to go with what is expected and my comments are moot. However, if you have control over the endpoints in your networking of devices then you should NOT transport floating point numbers as such across a network. AUTOSAR, MISRA, and JPL coding standards all recognize why this is a bad idea. Our engineer is designing a microcontroller based controller and he did the union/float thing to represent data being exchanged. I quickly got him on-board to use scaled integers instead, which are more the standard in the automotive CAN arena. There are some legacy devices that use fixed precision ASCII representations of floating point numbers, such as NMEA/GPS, so that is also an option where bandwidth is not a concern. Some points to consider regarding transport of native floating point numbers: different or adhoc endian format of devices devices that require native types to start on processor word sized boundaries devices that emulated floating point operations (no FPU) Anyway, just something to think about.
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  SOLVED Stack/Heap/smart pointers question.
  • adutzu89  

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  @JonB said in Stack/Heap/smart pointers question.: the size of an object which is to be allocated on the stack must be known at compile time, there is no workaround for this, it's an architectural "issue". Now, you know I always read what you write with interest, my friend! But I don't understand why you pick this out as a "stricture" of stack allocation? For the purpose of pedagogy. There's a gnu extension which allows you a stack based array with a runtime constant as size, as well, but it's not standard. What about alloca(), which has been there since year dot in Unix, and I see is still supported by gcc and in msvc (_alloca())? Did not mention it, and specifically avoided it, because the memory isn't freed when you leave the current scope, it's freed when the stack frame's popped. This is rather dangerous and can be messy. And in the C++ world *alloc() functions are discouraged in most cases. If you want an array of objects and you take the memory block through alloca you need to do a placement new on top of it to follow C++ semantics, so your constructors are called, and finally you need to manually invoke the destructors before exiting. This isn't ideal in the world of RAII. I'm not suggesting people go out and start using it, but it does not require a compile-time constant argument, so how does this relate to your "architectural" claim above? Not too well. I was specifically thinking of push and pop. Granted, alloca will move the top of the stack frame, and as with any integer that can be done at runtime, but it's a bad idea generally. In fact this is what the compiler does when you do a regular stack allocation - it moves the stack pointer the size of the object off the stack frame pointer (i.e. move the esp register in relation to the ebp register), with the exception that it validates the consistency at compile time (and static code analysis can be easily supported). Also the meaning is clearer, as you know the memory is local. With alloca you can pass the pointer to somewhere else, and without realizing that pointer's data block can get invalidated the moment you leave the current stack frame. It's not for the faint of heart.
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  SOLVED compiler optimization
  • damianatorrpm  

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  @sierdzio said in compiler optimization: I have no idea if it matters in this case, though. A great deal. QString has an additional indirection through the d_ptr and this is the great wall of china standing in front of the optimizations the compiler could possibly make.
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  • Please_Help_me_D  

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  SOLVED Can't create variable of weigth more than 2 Gb
  • Please_Help_me_D  

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  @JKSH thank you!
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  UNSOLVED How does programs like Fraps or Dxtory get FPS?
  • Engelard  

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  @Engelard this involves Windows API functions which find the window from the cursor point, possibly with WindowFromPoint which returns an HWND. From that HWND you can resolve the parent process with GetProcessHandleFromHwnd. At the core of Windows applications is the message loop within which it calls a callback function, or window procedure, to handle event messages. There's a function called SetWinEventHook which allows intercepting certain window messages in a given range. Now, if fraps can get the HWND of your app it can either intercept the WM_PAINT message to get a device context to draw over the top of the content, or it could create a child window displaying the FPS. But those are guesses. Other than what I've speculated on, if you want to find out what's actually happening, you could use OllyDbg to reverse engineer the program.
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  UNSOLVED Statistics c++
  • fatimaabdawa  

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  @SGaist said in Statistics c++: histogram from the frequencies of cats running through a street by moonless night I have been scouring the internet for this! 😉
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  SOLVED Simple big-looped program break down
  big loop break down • • Please_Help_me_D  

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  @JonB said in Simple big-looped program break down: Doesn't "doubling" mean "the next power of 2"? Yeah. I'm not at the top of my game, so you can probably safely ignore me.
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  SOLVED How to use Qt documentation?
  documentation qtendian • • Please_Help_me_D  

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  @KroMignon said in How to use Qt documentation?: @Please_Help_me_D said in How to use Qt documentation?: In reality I also need to use Little/Big conversation with float data-type. Do you know how to solve that? As far as I know, a float is stored as a 32 bits word and a double as 64 bits word. So my suggestion would be to cast your float value to quint32 and your double to quint64 float floatBE = static_cast<float>(qToBigEndian(static_cast<quint32>(val)); double doubleBE = static_cast<double>(qToBigEndian(static_cast<quint64>(valDouble)); nope, that does NOT work! Casting float to int truncates it. What you can do instead, is to reinterpret_cast the memory occupied by the float - or use a union. We currently have a similar topic in the forum on this. Regards
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  SOLVED General question about arrays and I/o
  • Please_Help_me_D  

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  @Please_Help_me_D said in General question about arrays and I/o: I need to extract {4, 2, 3} You will need to do it by yourself. There is nothing for that neither in Qt nor in C++ stdlib. Even if there would be it would do nothing else as iterating over {4, 2, 3} in a loop. So, I doubt you can optimise here much.
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  SOLVED QList and smart pointers
  • Exotic_Devel  

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  @KroMignon said in QList and smart pointers: auto obj=QSharedPointer<MyCustomObj>(new MyCustomObj, &QObject::deleteLater); Your post interested me. I read some articles on this and I get the idea that you should not mix ownership methods. QObject based system uses parent/child to express ownership. So if you use smart pointers you should not use the parent/child ownership. I explored your custom deleter strategy with std smart pointers: { // smart pointers combined with custom deleter for use with QObject based objects using namespace std; shared_ptr<QObject> ptr(new QObject, [](QObject* p){ p->deleteLater(); }); unique_ptr<QObject,function<void(QObject*)>> ptr2(new QObject, [](QObject* p){ p->deleteLater(); }); } I am filing this away for future use. Thanks for sharing.
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  UNSOLVED QObject: Cannot create children for a parent that is in a different thread. When Deleting Object.
  • jfinn88  

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  Hi and welcome to devnet, Where is your threading code ? What do you do in it ? Are you creating widgets in that thread ? If that's the case then it's wrong. Widgets creation and manipulation should be done in the QApplication thread i.e the main thread
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  SOLVED Returning unique_ptr and preventing move
  • Exotic_Devel  

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  @kshegunov said in Returning unique_ptr and preventing move: The typical problem being that returning the unique_ptr transfers ownership. I am confused then. I thought he had trouble doing this. I didnt understand that he didnt want it to move.
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  SOLVED Setters with and without this pointer.
  • Exotic_Devel  

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  @Exotic_Devel said in Setters with and without this pointer.: @J-Hilk said in Setters with and without this pointer.: But why would you insist on always writing this-> this isn't Python ! Habit I acquired when I started using Java. :) Ah, my condolences! 😉
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  UNSOLVED typeid in a tempate derived class
  template typeid • • addebito  

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  The only way I see to write this "genericly" would be to have some virtual QVariant getValue() const and virtual QVariant setValue(QVariant v) in your base class and implement it in Derived. This way you wouldn't have to do any casting, simply b2->setValue(b1->getValue()). The implementation in Derived would do the cast to T via QVariant's value().
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  • JorkGomes  

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  SOLVED Class properties in stack or heap?
  memory stack • • Exotic_Devel  

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  @Exotic_Devel In the link I posted is a full list of implicitly shared classes: list of classes. What I said applies to all of them and yes, QMap is one of them.
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  • JubimGomes  

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  • NuzakFarnas  

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  UNSOLVED How to resolve this warning
  • mrdebug  

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  @mrdebug said in How to resolve this warning: QtAndroid: Wait, this is a warning and not an error? Maybe a declaration to satisfy the template?: void QAndroidJniObject::callMethod(const char *methodName, const char *sig, ...) const; I don't know if that is the right sig though: https://doc.qt.io/qt-5/qandroidjniobject.html#callMethod-1
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  SOLVED Performance of signals/slots for plotting/oscilloscope (performance questions)
  • fcarney  

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  Yeah, sorry for not writing, I was rather busy. This indeed was my point.
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  UNSOLVED Memory Layout for Static Variable in Class
  • Vinoth Rajendran4  

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  @Vinoth-Rajendran4 Depends what you mean by "class memory", but as you say a static variable is never part of a class instance, so it will be stored (once) outside of any memory area allocated on creating an instance. Yes, it can be accessed across objects/instances precisely because it's in some shared area.
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  UNSOLVED Adding MySQL driver to Qt
  • uuecm  

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  These are the instruction that I made after installed it on my Ubuntu 18.04 linux install. Maybe it will help you Download QT 5.12.4 from QT website using the downloads buttons and then select open source and download After Download is complete go to Downloads folder and run the installer (for new ubuntu users – right click on file in downloads and select permissions and select option at bottom) Download latest QT that is known to have LTS (long term support) -Run installer -deselect android stuff and make sure that you select the dev tools and sources at the bottom install QT download and install mysql and dev packages sudo apt-get install libglu1-mesa-dev sudo apt-get install build-essential sudo apt-get install libmysqlclient-dev sudo apt-get install libssl-dev navigate to your Qt folder/5.12.4/Src -run config file -./configure -plugin-sql-mysql Navigate to qt source ie...Qt/5.12.4/Src/qtbase/src/plugins/sqldrivers/mysql -run the QT qmake /home/bj/Qt/5.12.4/gcc_64/bin/qmake MYSQL_PREFIX=/usr/include/mysql/ mysql.pro locations may vary depending on where you installed Qt during the installation process and what version of Ubuntu you may be using after running qmake -run make -make install you should be set from here...if you have any more issues then refer to these articles to seek clarification https://forum.qt.io/topic/59449/qsqldatabase-qmysql-driver-not-loaded-build-mysql-plugin https://stackoverflow.com/questions/37369480/qsqldatabase-qmysql-driver-not-loaded-on-ubuntu-15-04-64bits https://doc.qt.io/qt-5/sql-driver.html#qmysql-for-mysql-4-and-higher https://forum.qt.io/topic/36072/qmysql-driver-not-loaded-on-ubuntu/15
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  SOLVED An experiment to understand ways of copying C arrays to vector
  • fcarney  

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  @JKSH said in An experiment to understand ways of copying C arrays to vector: No resizing was done. // using copy vector<int> vec3; qInfo() << vec3.capacity(); copy(arr, arr+arrsize, back_inserter(vec3)); loop("vec3", vec3); Output: 0 vec3 10 16 std::vector(0, 1, 2, 3, 4, 5, 6, 7, 8, 9) Maybe I said the wrong term. A reserve was done.