C++ Gurus

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  SOLVED What's the meaning of marking a whole class volatile?
  • PikaCat  

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  I'm not aware of a volatile class. You can have volatile functions and volatile objects, but that specifier makes no sense for classes ...
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  UNSOLVED dynamicly creating an 2D array, but how come array is bigger than ...
  • Natural_Bugger  

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  bool element_exist(const std::vector<std::array<int,2> >& input, int key) { auto it = std::find_if(input.begin(), input.end(), [&](auto& elem) { return elem[0] == key || elem[1] == key; }); return it != input.end(); }
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  SOLVED static min max values from object
  • Natural_Bugger  

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  @JonB thnx, i got it working now, i had to do some minor improvements and pass the tests.
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  SOLVED std::regex issue
  • Natural_Bugger  

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  @SGaist hi, thnx for your reply, afterwards, i found this one. https://stackoverflow.com/questions/5844657/splitting-a-c-string-into-two-parts size_t found = str.find(" "); if(found != string::npos) { string temp1 = (str.substr(0,found)); string temp2 = (str.substr(found+1, string::npos)); } it did the job. i can't use the QT parts for these assignments.
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  UNSOLVED loop through a vector using iterator and test for end of loop-
  • Natural_Bugger  

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  @JonB thank you for the reply. well, i saw that some people or most did that for the same problem, but in Java and apparently, they also sorted the "array/map" like Chris Kawa mentioned it worked for them. so i tried that as well in c++. bit i also think the "solutions" they published do not work, because they only compare 2 (if) or else (nothing). no taking in consideration if the list of values is either even or uneven in length or the numbers of equal value is either even or uneven. now i'm using nested "Do While" loops instead of For loop in combination with Iteration. what to do next? .
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  • Natural_Bugger  

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  SOLVED funtion pointer
  • Natural_Bugger  

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  @jsulm ay, i didn't notice.. thanks alot.
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  UNSOLVED how to use std::getline(std::cin, s);
  • Natural_Bugger  

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  std::getline() knows about std::string but it doesn't know about QString. So, store your input in a std::string first and then call QString::fromStdString(). See https://doc.qt.io/qt-5/qstring.html#fromStdString
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  SOLVED Linux linking and dependencies
  • JonB  

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  @sierdzio Blimey, yes, that's a bit to read that I am unfamiliar with! I will do so if I want to go down that route, thank you. As the second reply there states, the fact that Ubuntu is now at libreadline.so.8 implies that it will not be binary compatible with expectations for libreadline.so.7. The change in version number tends to indicate this. That is fair enough. But then I am equally "surprised" that when I look Ubuntu 20.04 only offers an apt-get install ... choice of libreadline8 or libreadline5. No libreadline7. Anyway, I have just spoken to a Linux Admin friend, and he says he's not keen on version-independent linking, as the app may indeed fail to work with a newer version. He says he would expect either Go find the correct old version in an older Ubuntu, and get that. Or, recompile & relink against what's available now. Which is what I will do, when I have the time. So @sierdzio thanks for your answer, it's useful to know about the version-independent, but I am going to mark this as my correct solution.
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  SOLVED Error while compiling Qt mySQL driver
  • denko  

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  @chrabaszcz-patryk Registered here to thank you for your useful message :) This problem is still actual in 5.15.0. Solved it as you mentioned.
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  UNSOLVED Directory Structure
  • Shruthi  

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  @Shruthi said in Directory Structure: So what is the directory structure of the project will be? It's up to you. And no, you should never ever put your projects inside Qt installation folder! Why would you want this?! Keep your projects inside your home folder as anything else user related...
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  SOLVED new type in template function
  • qwe3  

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  @kshegunov Perfect!
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  UNSOLVED intel intrinsic 8bit int to float
  • Q139  

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  @Christian-Ehrlicher Thanks for the flags , will check the results.
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  SOLVED Need advice with QtConcurrency & lambda expression
  • r3d9u11  

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  @Christian-Ehrlicher said in Need advice with QtConcurrency & lambda expression: var is local inside calllambda() and gets destroyed when the function exits Aha, sure, local variable will be destroyed before lambda function will try to get access from parallel thread. To test I can add QThread::msleep(10); at the end of calllambda function (after QtConcurrency::run). There is no problems in Qt or pure C++, the problem is in logic of example. It's clear now, thank you!
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  • PikaCat  

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  UNSOLVED Qt Creator, SQLite3, and JSON1 example?
  • BSL787  

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  Don't know what you really want to achieve by including the sqlite3 sources but Qt will not use it - it will use the QSqlite3 plugin provided by your Qt installation. And there json support is enabled since Qt 5.13
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  UNSOLVED C++, Functional Programming, and Immutability
  • fcarney  

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  For a briefer discussion than a book, see https://softwareengineering.stackexchange.com/a/149576
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  UNSOLVED Access statisBar from member function
  • MatFox  

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  Well, why are you creating a local pointer to a QtDemo object within a method that is part of QtDemo ?
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  UNSOLVED What is best way to use 4bits for storing numbers.
  • Q139  

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  https://en.cppreference.com/w/cpp/language/bit_field Get familiar with this site. It contains just about everything with really good explanations of C and C++. It also has an offline copy you can download.
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  UNSOLVED Simplest way to for loop
  • Q139  

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  The other day I learned that gcc will convert a tail call optimized recursive routine into a while loop and thus no limit to recursion iterations (there are probably caveats to this, but it won't likely cause an SO). That is when I realized I should not spend my time on second guessing the compiler unless I am presented with a reason. Optimizing for the sake of optimizing is wasted effort. Find bottlenecks first. They are probably not where you think they are.
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  SOLVED Pointers and object initialization
  • Aymeric_Qt  

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  Hey, Sorry for the delay, I didn't had/take enough time for Qt these days. I think this is going a little bit far/too complex for me at the moment. I need to learn and pratice more on pointers. Also my question was too 'general' so it could not be really answered (except with a all disseration and that was not the goal). But I've learn some really usefull informations and that's already something! So thank you all for your replies. Have a nice day.
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  SOLVED QString/C++ convert to number
  • JonB  

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  it's a bit heavy weight but I always like a good RE engine for parsing most things. [-+]?(\d+)%
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  SOLVED How rigorous are you about using const?
  • JonB  

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  I am glad to have started such a big discussion :) For my own part, I shall: Obviously, for any global/member variables which are genuinely meant to be const I will put that in. That was never the question. When I choose to create a temporary pointer, I will (try to) make the effort to put in const (e.g. const QWidget *widg = something; widg->constMethod();) where I only need to use the variable to call something const. Just as I would it it were a formal parameter to a method. But for non-pointer simple value types, whether as local variables or formal parameters, for right or for wrong I 'm not going to cover my code with const. It's just too much to type, and I'd rather save const for where it actually matters. Thanks to all. I'll close this in a couple of days, give y'all some more time to disagree with each other ;-)
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  • Q139  

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  SOLVED Unable to cast
  • mrdebug  

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  I too very much dislike c++ style casts... too fracking wordy to use them in places like uint8_t c{ 0x43 }; cout << static_cast<int>(c); // would be shorter and just as obvious when done as cout << (int)c;
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  UNSOLVED Convert QStringList to QVector<const char*>
  • magrif  

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  Since qtSt.toStdString() is returning a temporary why not just turn QStringList into a std::vector<string> and maintain for the lifetime or your pointers. That way the pointer returned by c_str remains valid. Probably better than managing the memory with new/delete.
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  UNSOLVED Not sure about using vectors to store object of different Types
  • jkwok678  

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  @fcarney For this to work you have to use pointers: QVector<drink*> drinks;
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  UNSOLVED Return type of lambda function
  • Vinoth Rajendran4  

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  { auto x1 = [](int i) -> int { qInfo() << "Invoked"; return i; }; typedef typename std::invoke_result<decltype(x1), int>::type x1ret_type; typedef decltype (x1(1)) rtype; Q_ASSERT(typeid(rtype) == typeid(x1ret_type)); qInfo() << typeid(rtype).name() << typeid(x1ret_type).name(); qInfo() << "before typeid"; typeid(x1(1)); qInfo() << "after typeid"; } Pretty sure, I saw this behavior before (tested again): i i before typeid after typeid
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  • yu167bewds3d  

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  SOLVED Adding salt to hashing in C++ - looking for library
  c++ qt hash • • bwylegly  

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  A quick look on bcrypt's webpage tells me this is not a library, but a program. If you want to have it as a library, there is not support for this out of the box. You need to figure it out by yourself. The general instructions for building are to use make. (Usually, nmake is the equivalent on Windows.) I don't see any instructions for Windows on the webpage. You'll have to look through the README's, etc. to figure it out. cmake and qmake will not help as these will just generate makefiles and not compile anything. However, there is already a makefile, so you don't have to run cmake or qmake to create them.
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  UNSOLVED Deserializing QSqlQuery using submethod in efficient way
  mysql c++ qt • • bwylegly  

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  No it's not, you can see that with its constructors that take a QSqlDatabase parameter. No clear is not called, the destructor is. There's no need to create QSqlQuery objects on the heap.
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  JSON/XML Conversion in C++ ?
  • IamSumit  

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  @IamSumit said in JSON/XML Conversion in C++ ?: i want to convert JSON to an XML I think Qt probably has enough in it to do this. Read up on how to read and parse through a JSON document using the Qt library. Do a recursive printout of the structure to text. Then once you are confident you are reading the entire document then use the Qt library to create your XML document. Also print this out to make sure it matches what you want. You will have to determine a convention of how to translate and this might just be a simple lookup table or a set of rules.
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  SOLVED Squarefunction
  • felix6262  

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  yes thats true. But if you want to get a chart with a fourier transformation, i found out, that you have to type in: fx=(4/M_PI)*((sin(x))+((1/3.0)sin(3x))+((1/5.0)*sin(5x))+((1/7.0)sin(7x))+((1/9.0)sin(9x))); Because you are trying to divide a integer by integer. That gives you a integer back. In this case its zero. When you typ for example 3.0 insted of 3 you divide integer by double and then you get a double back. Now simply find a good solution for x to get a nice chart. Should be working. But, how VRonin said, its not a real square Chart (watch youtube). In my program i programmed some if else cases that gives me for y value 1 or 0 back.
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  UNSOLVED class Widget const * const &
  • Vinoth Rajendran4  

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  I think it might be a valid template parameter, like template <class Widget const * const &> void doSomething(); In any case, const refers to the item on its left unless it has nothing on its left so it refers to the item on it's right. Then you start reading right to left. In your case it's a reference to a const pointer to a const widget. You can imagine it as using cpWidget = const Widget* and then const cpWidget&. Having said that it's not very useful as a construct given pointers and references occupy the same amount of memory so const Widget* const would have been identical
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  SOLVED 16 bit float precision
  • Q139  

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  @Q139 said in 16 bit float precision: It worked out as good idea Nice, are you able to mark your post as solved then? Thanks.
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  • Q139  

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  SOLVED Compiling a lambda using a signal thinks signal is undefined?
  • fcarney  

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  @fcarney You're missing Q_OBJECT macro so moc did not generate signal's body. Unrelated - You should use const QModelIndex& parameters and make your signals const.
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  SOLVED A question about getting the address of a class static const.
  • PikaCat  

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  @Konstantin-Tokarev You are right, but I am surprised, I thought constexpr gave you more of a compile-time constant. (I am not a C++ expert.)
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  UNSOLVED QSqlError("", "Parameter count mismatch", "")
  • Hasibul Hasan Chowdhury  

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  show the database table definition. You probably have a keyed field (not null) that you are not supplying a default value for, as was mentioned by @JonB .