C++ Gurus

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  Unsolved How to increase speed of large for loops
  • kane9x

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  @JonB said in How to increase speed of large for loops:

  Maybe a touch harsh :)

  Oh, you know I wrote that with loving condescension, as I usually do! ;)

  I thought when I saw templates they were to do with providing type-"independent" generic functions, aka "generics", e.g. in C#.

  Well, yes, templates are for that - providing type independent code, or rather as you put it generic, because there may be limitations put on the types involved (i.e. the type may be required to be floating point, or integral). This is fine and very useful for many purposes. Consider writing an algorithm that operates on a matrix, the matrix may contain rational numbers, or floating point, or complex, or quaternions. The point is the algorithm is the same regardless of the type it operates on (with some sane limitations).

  C# isn't a good example as it doesn't compile to native assembly. It has, much like its mother Java, an interpreter for opcode. That is, source is compiled to an intermediary opcode (which is similar to assembly), which is then interpreted by a virtual machine.

  Nothing to do with in-lining....

  Well, not nothing. Templates' instantiations are known fully, including all dependent types and the whole source. While you can have them hidden in a source file and prevent inlining, that's an extremely rare case. Usually the point of them being stored in the headers without exposing only the instantiated types is to allow the compiler to freely inline everything it wants to. So they're also used to hint that to the compiler. In this case that's the idea, otherwise you'd just write the simple fast exponentiation which takes 2 arguments (i.e. basically a rewrite of std::pow) instead of giving the compiler enough rope to hang itself. As I said, every instantiation is different, so the compiler is going to generate one function for each template argument, so calling power<8>, leads to your compiler generating code for power<8>, power<4>, power<2>, power<1> and so on. These are separate functions, mind you. Then it may (or may not) decide to inline some (or all) of them into the others.

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  Unsolved Loops, Constructors and Performance
  • beecksche

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  @beecksche said in Loops, Constructors and Performance:

  For me it's a general problem: it's a juggling act between readability, compactness and speed.

  It's a problem we're all facing :-) But as @aha_1980 said, there is no need to worry about performance too early. Our intuitive take on what is faster is often wrong, too - his hint at profiling the code is a very good advice.

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  Solved PIMPL for Multiple Inheritance
  • ModelTech

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  @kshegunov Thanks for your clarification! Very much appreciated. Indeed, I avoid the diamond situation in any case and therefore also for QObject. I have just used doxygen to generate a class diagram (inheritance graph) for my architecture at the moment.

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  Solved std::string error
  • mzimmers

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  they were referring to/pointing to 2 different object types. I'd expect C++ to have been stricter about that...

  It'd depend on the compiler really, but from the machine's point of view it's all the same. Everything is/is converted to an address to some region in memory. Type-safety is something people invented to have at least some idea about what we are working with, and I agree, the compiler should've warned you at least. But, well, as I said almost everything decays to void * and from there the step into the abyss is just tiny ... ;)

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  This topic is deleted!
  • mrdebug

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  Unsolved Friendship with method of other class in other File
  • Josz

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  @karlheinzreichel solution requires mySubClass to know the full definition of myClass. You can't just pre-declare class myClass; //forward declaration is needed

  An alternative is

  class FriendHelper{ public: static void actualPrint(myClass& sender); };

  now move the body of void print(); inside the body of actualPrint and change it to void print() {FriendHelper::actualPrint(*this);}

  No you can make FriendHelper a friend of both myClass and mySubClass

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  Solved Link errors; Connect a Signal of an object in main.cpp to a Slot in an object in MainWindow.cpp
  • Josz

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  @SGaist said in Link errors; Connect a Signal of an object in main.cpp to a Slot in an object in MainWindow.cpp:

  AFAIK

  I don't know. I have a .pro and a .pri. For me was needed.

  here is my .pro

  QT += core gui charts greaterThan(QT_MAJOR_VERSION, 4): QT += widgets TARGET = drawRange TEMPLATE = app include(drawRangeGUI/drawRangeGUI.pri) include(hmisg/hmisg.pri) INCLUDEPATH += tmp/moc/release_shared DEFINES += QT_DEPRECATED_WARNINGS CONFIG += c++11 SOURCES += \ main.cpp HEADERS += FORMS += \ # Default rules for deployment. qnx: target.path = /tmp/$${TARGET}/bin else: unix:!android: target.path = /opt/$${TARGET}/bin !isEmpty(target.path): INSTALLS += target DISTFILES += \ drawRangeGUI/drawrange.pri

  and my .pri

  INCLUDEPATH += $$PWD \ $$PWD/erpc-1.7.1/erpc_c/port \ $$PWD/erpc-1.7.1/erpc_c/infra \ $$PWD/erpc-1.7.1/erpc_c/setup \ $$PWD/erpc-1.7.1/erpc_c/config \ $$PWD/erpc-1.7.1/erpc_c/transports DEPENDPATH += $$PWD \ $$PWD/erpc-1.7.1/erpc_c/port \ $$PWD/erpc-1.7.1/erpc_c/infra \ $$PWD/erpc-1.7.1/erpc_c/setup \ $$PWD/erpc-1.7.1/erpc_c/config \ $$PWD/erpc-1.7.1/erpc_c/transports HEADERS += \ $$PWD/hmisg.h \ $$PWD/erpc-1.7.1/erpc_c/setup/erpc_server_setup.h \ $$PWD/erpc-1.7.1/erpc_c/setup/erpc_client_setup.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_basic_codec.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_client_manager.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_codec.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_common.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_crc16.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_framed_transport.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_manually_constructed.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_message_buffer.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_server.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_simple_server.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_transport.h \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_version.h \ $$PWD/erpc-1.7.1/erpc_c/setup/erpc_mbf_setup.h \ $$PWD/erpc-1.7.1/erpc_c/setup/erpc_transport_setup.h \ $$PWD/erpc-1.7.1/erpc_c/config/erpc_config.h \ $$PWD/erpc-1.7.1/erpc_c/port/erpc_config_internal.h \ $$PWD/erpc-1.7.1/erpc_c/port/erpc_port.h \ $$PWD/erpc-1.7.1/erpc_c/port/erpc_threading.h \ $$PWD/erpc-1.7.1/erpc_c/transports/erpc_tcp_transport_win.h SOURCES += \ $$PWD/hmisg.cpp \ $$PWD/erpc-1.7.1/erpc_c/setup/erpc_server_setup.cpp \ $$PWD/erpc-1.7.1/erpc_c/setup/erpc_client_setup.cpp \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_basic_codec.cpp \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_client_manager.cpp \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_crc16.cpp \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_framed_transport.cpp \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_message_buffer.cpp \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_server.cpp \ $$PWD/erpc-1.7.1/erpc_c/infra/erpc_simple_server.cpp \ $$PWD/erpc-1.7.1/erpc_c/setup/erpc_setup_mbf_dynamic.cpp \ $$PWD/erpc-1.7.1/erpc_c/setup/erpc_setup_tcp.cpp \ $$PWD/erpc-1.7.1/erpc_c/port/erpc_threading_windows.cpp \ $$PWD/erpc-1.7.1/erpc_c/transports/erpc_tcp_transport_win.cpp

  regards

• 

  Unsolved Pointer to SubObject Member
  • Josz

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  @Josz

  //(mO.mS.*pSOprint)(); //don't work

  Where is your MySubObject::pSOprint defined and initialized to MySubObject::print(), if that's what you're trying to do?

  m0.(*pSOprint)(); //dont work, I would like to make anything so

  Similarly for MyObject::pSOprint? [And btw that m0 should be mO.]

  In your original you have

  void (MyObject::*pointerToPrint)() = &MyObject::print; //<- I make here a function pointer to own methrod

  Where are you doing the equivalent of this for the function you want to access in your MySubObject? I'm not C++, but don't you need something like one of (hopefully the first):

  void (MyObject::*pSOPrint)() = &MySubObject::print; void (MyObject::*pSOPrint)() = &MyObject::mS::print;

  Also, when you say "don't work" for things, it would save a lot of guessing if you said it doesn't compile/crashes/doesn't do what you want/... :)

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  Unsolved Is it possible to use static arrays in std Algorithms?
  • Engelard

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  To summarise: std::begin() and std::end() are the way to go

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  Solved How to write a common callback function?
  • Mr Pang

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  @JKSH
  Great! I never thought of it before.

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  Unsolved unable to open or create a project with QT creator
  • tesnime

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  Hi and welcome to devnet,

  What version of Qt are you using ?
  What version of Qt Creator are you using ?
  How did you install them ?
  What OS are you running on ?
  What compiler are you using ?

• 

  Solved scope problems
  • CharlesHuff

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  Qt is just C++. Creating a SysManager object in each function might not be the best idea. Having one instance in your class will make you code cleaner. Note that this doesn't apply to every situation.

• 

  Unsolved Recursiveness in "catch enter key" example?
  • Josz

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  You're welcome!

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  Solved Algorithm to find all possible combinations
  • DavidM29

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  https://en.wikipedia.org/wiki/Constraint_programming and https://en.wikipedia.org/wiki/Gecode?

• 

  Solved How to implement Oscilloscope for realtime-plotting?
  • _bhum

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  @_bhum Ok, so Qwt library should help. You may want to use one of the examples I mentioned (i.e. the one from MagPi magazine) as a guideline to what/how they draw on screen (I know, it's Python code but it will help...)

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  This topic is deleted!
  • chmod

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  No one has replied

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  Unsolved How does programs like Fraps or Dxtory get FPS?
  • Engelard

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  No one has replied

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  Unsolved Default special member binary compatibility
  • VRonin

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  @Konstantin-Tokarev said in Default special member binary compatibility:

  That's wrong

  Ok, I was told the opposite and I have the maximum respect for your and David's opinion so I don't know who to believe anymore :(

• 

  Unsolved QObject::connect: No such slot Widget::on_lineEdit_textEdited()
  • Rahul.k

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  Hi,

  You wrote:
  @Rahul.k said in QObject::connect: No such slot Widget::on_lineEdit_textEdited():

  QObject::connect(arduino,SIGNAL(readyRead()),this,SLOT(on_lineEdit_textEdited()));

  Yet your slot implementation signature is:
  @Rahul.k said in QObject::connect: No such slot Widget::on_lineEdit_textEdited():

  void Widget::on_lineEdit_textEdited(const QString)

  So which one is the correct ?

  In any case, that name is completely misleading with respect to what that function does and to what it's connected to. Thus you should rather give it a proper name.

• 

  Unsolved Seismic Model code runs in CodeBlocks but not in Qt.
  c++ seismic modell academic stuff research • • Flavio Mesquita

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  Are you implementing technique number 4 described here ?

• 

  Rants about auto
  • jsulm

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  @JonB said in Rants about auto:

  And the relevance of it being specifically zsh is... what?

  Environment of the interpreter.

  You just looking for extra points?

  Yep! Gimme, gimme!

• 

  Solved QRegExp. Extract sentence containing keyword to the end
  • mandruk1331

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  Hello,

  I have tried your variant it returns string which I do not need. I am parsing out an HTML code of the page. When I run you variant it returns all kind of string, and I only need the sentence where the "[error]" key word is present.

  Update:
  This variant of RegExp works like a charm:

  \\[error\\](.*)

  Thank you for your time. The issue has been resolved

• 

  Unsolved Letting the User set the respective directory within a qt wigdet application
  • Pippapo

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  @JonB Thank you so much for your reply. I am sorry I did not get back to you sooner, life kept me busy. I'll get right back to my project and I'm really thankful for your help and the information you provided.

• 

  This topic is deleted!
  • nasir

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  Unsolved C++ - Can't access function in another class
  • mad-hatter

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  Hi @mad-hatter

  Is there something I should add in the header file?

  That would help to resolve compile errors, when the compiler cannot find the definition of a function.

  Linker errors mean, the linker cannot find the implementation of the function.

  Usually this means either the implementation is missing in your source file or you have it in a source file that is not compiled/linked.

  Regards

• 

  Solved C++ % " ?
  • JonB

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  @JKSH said in C++ % " ?:

  Wait till you try + or += on QJsonArray ;)

  QJsonArray a1{1, 2, 3}; QJsonArray a2{4, 5, 6}; a1 += a2; // a1 now has 4 elements: 3 numbers and 1 array

  😑😔
  of course it does.

  Not a fan!

• 

  Solved QSettings ini format special chars in key
  • 6thC

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  @6thC said in QSettings ini format special chars in key:

  @JonB you mean me specifically or it's been discussed before, I don't remember but me forgetting something wouldn't be the first time that's happened. I did search quite a bit so got the feeling this was the case.

  Gosh, no! I meant the "Royal We" :) I recall answering about this in another thread a while ago (e.g. https://forum.qt.io/topic/92858/logging-rules-in-ini-file-with-qsetting-and-asterisk/2 and https://forum.qt.io/topic/91362/how-to-edit-a-ini-file/11).

  As @sierdzio said, the .ini file support from QSettings is only for its own puprposes. All it really guarantees is that you can read back a file that it has written. You may be able to read an external .ini file, but if you write it back it can change to non-.ini format. Remember Qt is platform-independent, and the files you are talking about are really Windows-only.

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  Unsolved TableView background color
  • Mogli123

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  @Mogli123
  just to make sure: indexes are 0-based. So first row/column has an index of 0 not 1.
  In case you only showing 1 row of data in your table and wondering why the background doesn't get applied.

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  Unsolved QSharedDataPointer detach too much ( don't call const )
  • miskol

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  @miskol It outputs 2 times "bbb". Why do you think there is a detach in your example?

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  Solved how to write PID control algorithm to control temperature
  • _bhum

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  @_bhum said in how to write PID control algorithm to control temperature:

  how i execute my program on raspi terminal????

  do you know how to run a program in a Linux terminal?

• 

  Unsolved How to use QTimer connect without Object and without slot
  • mrudula

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  Hi,
  You can use QTimer with lamda functions like this.

  connect(pTimerValidateScheduleTimeoutr, &QTimer::timeout, []() { ValidateScheduleEvent(); });
• 

  Unsolved Is this intentional?
  • digital_light

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  @digital_light said in Is this intentional?:

  Why are we defining Ui::LoadScreen and ::LoadScreen? Doesn't that defeat the purpose of the namespace?

  They are 2 separate classes.

  ::LoadScreen is your "actual" class where you write your code Ui::LoadScreen is a fully auto-generated class that's embedded inside ::LoadScreen.
• 

  Unsolved print function char FirstDriveFromMask( ULONG unitmask ) with qDebug()
  • filipdns

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  Hi,

  Not a direct answer but did you consider KDE’s Solid framework for that ?

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  Unsolved Cgal Computational Geometry algorithms integration with QT
  • ujur007

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  @ujur007

  Why should you?

  The example names are just for explanation of how to integrate a third party library. The 3rd party library is integrated through relative path within your actual project. However, you may place the 3rd party stuff anywhere and your PC.

  Basically you have to download and install the 3rd party library independent of Qt. You need to know the path to the includes and add this as relative or absolute path to INCLUDEPATH

  You have to ensure that the libraries are compiled correctly for Qt installation (32 vs 64 bit, compiler andsome stuff). You need to locate the required libraries and setup absolute or relative path and libraries in LIBS

• 

  Solved What parentheses at the beginning mean?
  • Engelard

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  @Engelard

  It is a C-style cast to an unspecified long pointer. When some more is around in the line you are casting to your needs.

  As you show it above you get rid of compile warnings.

• 

  Solved Logical public variable in object programming
  • elicat

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  @elicat said in Logical public variable in object programming:

  it is clear that * pA is a pointer to a new class A instance
  what type of object is * pA ??
  is an instance of the class but how do I declare it?

  You already answered for yourself what type of object pA is --- it's a pointer to a class instance.

  How to declare it? I gave you:

  # Outside world goes: classA *pA = new classA("Hello world");

  but if I have set this variable in a program phase and then I will have to read it later in other classes and at other times of the process .. . how can I do? Do I have to carry the variable with me at every step?

  Yes, like any other variable, if you need to access the object all over the place you have to pass it around. If it turns out(?) you often access the classA instance from your classB instance, you could create a classA * member variable in your classB and then if you're passing a classB instance around it will carry with it a classA * to save you having to pass that around separately.

• 

  Unsolved Why does const auto& lead to empty lists?
  • Asperamanca

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  @Asperamanca said in Why does const auto& lead to empty lists?:

  So you never do stuff like this?

  for(const auto& item : container) { // Do stuff }

  I do, I do.

  I just I think of this like I do with function parameters, but I think of function parameters differently from local variables inside a function block (like const auto& list = ...;). Once I remember they're basically the same, it now makes sense.

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  Solved Best design for code used in many classes
  • Weichao Wang

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  @Gojir4
  Many thanks!

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  Solved Constructor of a derived class
  • Weichao Wang

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  @SGaist
  Many thanks!

• 

  Solved abstract class
  • Weichao Wang

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  @jsulm
  Thank you! I'm going to use this mothode. The code would be a little extensive and I want to avoid the situation that it turns out to be not workable.