C++ Gurus

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  This topic is deleted!
  • PikaCat  

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  SOLVED Efficiency of std::vector copy?
  • fcarney  

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  @fcarney said in Efficiency of std::vector copy?: Interesting. This will affect my decision for references. I will have to play with this a bit. It really depends upon what my life cycle ends up being for each module involved. Do I pass a football around or do I copy and keep them more independent? That will be something I need to figure out. If we are talking performance, the best would always be having a common buffer that's not locked at all (i.e. every thread working on its piece). Especially when you manage to align the pieces on the cache line size to have better cache coherency. It'd really depend on what exactly you're doing though, as for most cases the signal-slot mechanism is just fine. @Chris-Kawa said in Efficiency of std::vector copy?: Using QVector in this scenario is pretty much like using std::shared_ptr<std::vector<double>>. Sharing is protected but access to the data is not and is up to you. Although to QVector's advantage in that case it's internally ref-counted, thus you get one indirection less accessing the underlying object (independent of syncing that actual data that is).
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  SOLVED Unable to find the platform plugin while deploying application
  • Please_Help_me_D  

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  If your program starts, then you can put in a delay somewhere, for example: for (;;); so that you will have time to run ListDLLs
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  UNSOLVED Static Member variable and static member function resides in ????
  • Vinoth Rajendran4  

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  Referring to a 25 year old framework in a discussion of modern C++ best practices really doesn't help your case though Why? Things haven't changed that much really. Some new stuff came along but the old is very much still there. There's still a lot of software using MFC today. I'm not saying you should use it in new projects or anything like that, but it's still there, still works and the rules of how it works didn't change, so I think it's a perfectly valid point in this discussion. I'm not talking about some obscure tech that fell out of use. It's (sadly) very much alive. and even humble MFC had a main(), they just didn't call it that. I think it was _tmain() or wmain(). Right, well, it's, again, a little bit more complicated. Yes, that framework provides a _tWinMain that called AfxWinMain and that's one way to make an MFC app, but you can also skip that and start the event pump another way. Anyways, I don't want to get into MFC discussion, that wasn't the point :P The point was you don't need a main() or any other function to initialize global statics and run some code in their constructors. Another example is global statics in dynamically loaded libraries. I hope this one is less controversial :) The best practice in the regulated field I work in is to use the "construct on first use" paradigm. I think we have a misunderstanding coming from that name, but the thing it names is a perfectly valid strategy, I agree. Global (static) objects are bad when they have interdependence upon each other. I absolutely agree. If I seemed to advocate for using static globals let me be very clear - I'm not. They have their uses, sure, but also the order of initialization problem you mentioned, so that's something to take into account in your app design. If it matters - don't do it, if it doesn't - go ahead. I was merely trying to convey how they work on a technical level. that's why it is often better to use global static pointers to those object and construct them in a well-defined order in main(). See, I think this is the point of confusion we had. If you use global pointers then those pointers get initialized (zeroed) in the way I was taking about. The pointers, not the objects that they later point to and you can obviously assign to them in main() or wherever else in a controlled order that you like. The pointers themselves though are originally initialized for you in an unspecified order. It doesn't really matter in that scenario of course as you overwrite them later in a way you see fit. The gist of it is that the pointers are the global static objects we're talking about here, not the things they later point to. I suspect we'll continue to agree to disagree. LOL I think we actually agree. We just misunderstood each other on some technicalities ;)
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  SOLVED Trouble assigning value to boolean in the struct
  • artwaw  

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  @kshegunov Thank you, will remember that.
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  SOLVED Error using qEndian
  • Please_Help_me_D  

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  @Please_Help_me_D said in Error using qEndian: Is that beacause my computer native is little endian? Yes.
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  SOLVED QRunnable unavailable dynamically allocated member variables
  • RBLL  

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  Wow. How did I miss that... Well that should solve it. Thanks!
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  SOLVED Read some bytes from binary file with qfile
  • Please_Help_me_D  

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  @Please_Help_me_D OK, but in case someone observes that this is not the way to do it (which they are entitled to do!), be aware of that. Given: There is official description of the file format that tells us that number in 3225-3226 bytes should be in range from 1 to 5. somewhere in such an official description there should also be a statement as to how they store 2-byte/16-bit numbers!
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  SOLVED QHash<int,QString> vs QHash<int,QByteArray>
  • fcarney  

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  @Kent-Dorfman said in QHash<int,QString> vs QHash<int,QByteArray>: Where would those roleNames commonly be used? Since they are themselves a type of code metadata (possibly used internally by the Qt framework), I don't think you'd want any QString locale conversion done on them. Also, for ASCII-only strings QString takes twice as much memory than QByteArray because of UTF16 encoding.
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  SOLVED object address
  • LeLev  

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  @Christian-Ehrlicher, @Konstantin-Tokarev thank you very much for your inputs.
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  SOLVED Iterator algorithm: assignment
  • Weichao Wang  

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  @Weichao-Wang said in Iterator algorithm: assignment: double instead of doppelt (which is German). You can edit your posts by clicking on the three vertical dots right down of the post. And when the topic is solved you can mark it as such with the 'Topic Tools' button.
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  SOLVED How to use Qt documentation?
  documentation qtendian • • Please_Help_me_D  

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  I have written the following code: if (fileEndian == "Little"){ for(quint32 i = 0; i < nTrc; i++){ FFID(i) = *util::bit_cast<qint32*>(qFromLittleEndian(qFile->map(3608+i*bytesPerTrc, 1))); } } else if (fileEndian == "Big"){ for(quint32 i = 0; i < nTrc; i++){ FFID(i) = *util::bit_cast<qint32*>(qFromBigEndian(qFile->map(3608+i*bytesPerTrc, 1))); } } It gives me errors: readsegy.obj:-1: ошибка: LNK2019: unresolved external symbol "unsigned char * __cdecl qbswap<unsigned char *>(unsigned char *)" (??$qbswap@PEAE@@YAPEAEPEAE@Z) referenced in function "unsigned char * __cdecl qFromBigEndian<unsigned char *>(unsigned char *)" (??$qFromBigEndian@PEAE@@YAPEAEPEAE@Z) debug\ReadSegy.exe:-1: ошибка: LNK1120: 1 unresolved externals The compilator output: jom: C: \ Users \ tasik \ Documents \ Qt_Projects \ build-ReadSegy-Desktop_x86_windows_msvc2017_pe_64bit-Debug \ Makefile.Debug [debug \ ReadSegy.exe] Error 1120 jom: C: \ Users \ tasik \ Documents \ Qt_Projects \ build-ReadSegy-Desktop_x86_windows_msvc2017_pe_64bit-Debug \ Makefile [debug] Error 2 19:05:31: The process "C: \ Qt \ Tools \ QtCreator \ bin \ jom.exe" ended with code 2. Error during assembly / deployment of ReadSegy project (bundle: Desktop (x86-windows-msvc2017-pe-64bit)) During the execution of the "Assembly" Actually the problem was that I didn't know the line which throws these errors but by intuation I just commented the BigEndian part of the code and it works: if (fileEndian == "Little"){ for(quint32 i = 0; i < nTrc; i++){ FFID(i) = *util::bit_cast<qint32*>(qFromLittleEndian(qFile->map(3608+i*bytesPerTrc, 1))); } } else if (fileEndian == "Big"){/* for(quint32 i = 0; i < nTrc; i++){ FFID(i) = *util::bit_cast<qint32*>(qFromBigEndian(qFile->map(3608+i*bytesPerTrc, 1))); }*/ } I use little endian Windows 10 x64, Qt 5.14.0, MSVC 2017 x64. Why do I can use qFromLittleEndian but I can't qFromBigEndian?? By the way the endian of my file is LITTLE now I think I just found a solution. If I change the order of performing bit_cast and qFromBigEndian it works: if (fileEndian == "Little"){ for(quint32 i = 0; i < nTrc; i++){ FFID(i) = qFromLittleEndian(*util::bit_cast<qint32*>(qFile->map(3608+i*bytesPerTrc, 1))); } } else if (fileEndian == "Big"){ for(quint32 i = 0; i < nTrc; i++){ FFID(i) = qFromBigEndian(*util::bit_cast<qint32*>(qFile->map(3608+i*bytesPerTrc, 1))); } } I don't understand why but that works fine
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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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  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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  This topic is deleted!
  • 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 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! 😉