Skip to content
  • Categories
  • Recent
  • Tags
  • Popular
  • Users
  • Groups
  • Search
  • Get Qt Extensions
  • Unsolved
Collapse
Brand Logo
  1. Home
  2. General talk
  3. The Lounge
  4. Recurring C++ and Qt anti-patterns
Forum Updated to NodeBB v4.3 + New Features

Recurring C++ and Qt anti-patterns

Scheduled Pinned Locked Moved The Lounge
126 Posts 17 Posters 71.3k Views 10 Watching
  • Oldest to Newest
  • Newest to Oldest
  • Most Votes
Reply
  • Reply as topic
Log in to reply
This topic has been deleted. Only users with topic management privileges can see it.
  • sierdzioS sierdzio

    @Asperamanca said in Recurring C++ and Qt anti-patterns:

    This brings me to a philosophical question: Do I want to be able to emit a signal from a const method, although the slot(s) attached to the signal may well modify data the originating const method could not itself modify?

    Yes, it's very debatable :D I did find a few occasions where it was useful (latest example: modifying behaviour of QTreeView without patching Qt - I have emitted a signal from const overloaded method and did my modifications there), but I agree it does not feel "right".

    Kent-DorfmanK Offline
    Kent-DorfmanK Offline
    Kent-Dorfman
    wrote on last edited by
    #38

    @sierdzio said in Recurring C++ and Qt anti-patterns:

    Yes, it's very debatable :D I did find a few occasions where it was useful (latest example: modifying behaviour of QTreeView without patching Qt - I have emitted a signal from const overloaded method and did my modifications there), but I agree it does not feel "right".

    Actually, I have zero problem with this. The way my mind works it makes perfect sense, as the signal is a message to a receiving class (any class). It's not the sender method that modifies the object state. It is the message. My mind differentiates between the two.

    1 Reply Last reply
    0
    • fcarneyF fcarney

      @aha_1980
      Apparently the standard allows for it:
      https://stackoverflow.com/questions/704466/why-doesnt-delete-set-the-pointer-to-null

      The creator himself wonders why it isn't so. Its like C++ is this beautiful, amazing, and now, WILD animal roaming free in cyberspace... Yeah, maybe the analogy isn't all that great, but it does conjure up a cool picture.

      Kent-DorfmanK Offline
      Kent-DorfmanK Offline
      Kent-Dorfman
      wrote on last edited by
      #39

      @fcarney said in Recurring C++ and Qt anti-patterns:

      Apparently the standard allows for it:
      https://stackoverflow.com/questions/704466/why-doesnt-delete-set-the-pointer-to-null
      The creator himself wonders why it isn't so. Its like C++ is this beautiful, amazing, and now, WILD animal roaming free in cyberspace... Yeah, maybe the analogy isn't all that great, but it does conjure up a cool picture.

      Jumping back a few months on this one, but I think the decision to leave alone the pointer value upon an object delete is solid. If I understand the standard properly, the target of a delete can be an lvalue or and rvalue. So delete 0x34fc3d2200 should be a valid operation, right? How ya gonna change the value of an rvalue (in a traditional sense)?

      1 Reply Last reply
      0
      • Chris KawaC Offline
        Chris KawaC Offline
        Chris Kawa
        Lifetime Qt Champion
        wrote on last edited by Chris Kawa
        #40

        Imagine clearing some sort of array:

        for(type* ptr = some_array; something ; ++ptr)
        {
             delete ptr;
        }
        

        Now imagine delete would zero that pointer. Do you see the problem? You would have to make another, temporary, pointer just so you zero the copy and your original doesn't get changed. In other words you're paying for what you don't use or even want. There's also problem of const pointers or pointers that you got from external APIs that do their own bookkeeping and might actually need that pointer value even after delete. It would create more problems than it solves.

        1 Reply Last reply
        5
        • fcarneyF Offline
          fcarneyF Offline
          fcarney
          wrote on last edited by
          #41
                  int 🥩=1;
                  int 🧀=1;
                  int 🥬=1;
                  int 🍞=1;
                  int 🍅=1;
                  int 🥪=🥩+🥬+🍅+🧀+🥩;
                  cout << 🥪 << endl;
          

          Fails to compile in C++17...

          C++ is a perfectly valid school of magic.

          1 Reply Last reply
          1
          • Kent-DorfmanK Offline
            Kent-DorfmanK Offline
            Kent-Dorfman
            wrote on last edited by
            #42

            @fcarney said in Recurring C++ and Qt anti-patterns:

                int 🥩=1;
                int 🧀=1;
                int 🥬=1;
                int 🍞=1;
                int 🍅=1;
                int 🥪=🥩+🥬+🍅+🧀+🥩;
                cout << 🥪 << endl;
            

            Fails to compile in C++17...

            What's this "int" stuff? Doesn't the 17 standard deduce the type based on the rvalue? Not that I think that is necessarily a good thing though.

            fcarneyF 1 Reply Last reply
            0
            • Kent-DorfmanK Kent-Dorfman

              @fcarney said in Recurring C++ and Qt anti-patterns:

                  int 🥩=1;
                  int 🧀=1;
                  int 🥬=1;
                  int 🍞=1;
                  int 🍅=1;
                  int 🥪=🥩+🥬+🍅+🧀+🥩;
                  cout << 🥪 << endl;
              

              Fails to compile in C++17...

              What's this "int" stuff? Doesn't the 17 standard deduce the type based on the rvalue? Not that I think that is necessarily a good thing though.

              fcarneyF Offline
              fcarneyF Offline
              fcarney
              wrote on last edited by
              #43

              @kent-dorfman said in Recurring C++ and Qt anti-patterns:

              🍞

              The real problem is this variable is unused.

              C++ is a perfectly valid school of magic.

              1 Reply Last reply
              1
              • Kent-DorfmanK Offline
                Kent-DorfmanK Offline
                Kent-Dorfman
                wrote on last edited by
                #44

                @fcarney said in Recurring C++ and Qt anti-patterns:

                The real problem is this variable is unused.

                So in 17 unused variables are errors instead of warnings?

                1 Reply Last reply
                0
                • fcarneyF Offline
                  fcarneyF Offline
                  fcarney
                  wrote on last edited by
                  #45

                  @kent-dorfman said in Recurring C++ and Qt anti-patterns:

                  So in 17 unused variables are errors instead of warnings?

                  No, its just a bug in the code for a samich.

                  C++ is a perfectly valid school of magic.

                  1 Reply Last reply
                  0
                  • Kent-DorfmanK Offline
                    Kent-DorfmanK Offline
                    Kent-Dorfman
                    wrote on last edited by
                    #46

                    @fcarney said in Recurring C++ and Qt anti-patterns:

                    No, its just a bug in the code for a samich.

                    Samich... Are you a yinzer?

                    1 Reply Last reply
                    0
                    • fcarneyF Offline
                      fcarneyF Offline
                      fcarney
                      wrote on last edited by
                      #47

                      @kent-dorfman said in Recurring C++ and Qt anti-patterns:

                      Are you a yinzer?

                      Had to look it up. Based on what I read, no. Not sure where I heard sandwich being called samich though. I am in western USA.

                      C++ is a perfectly valid school of magic.

                      1 Reply Last reply
                      0
                      • Chris KawaC Offline
                        Chris KawaC Offline
                        Chris Kawa
                        Lifetime Qt Champion
                        wrote on last edited by
                        #48

                        Being an a-hole as a recruiter:

                        What does o() mean?
                        What does o.o mean?
                        What does o->o mean?
                        What does o-->o mean?
                        What does o()--<=>--o() mean? Fun fact - crashes MSVC (yes, the compiler, not the compiled program)
                        What does [](){;o()++<=>++o();}() mean?

                        J.HilkJ 1 Reply Last reply
                        3
                        • Chris KawaC Chris Kawa

                          I had to dig through this thing once, only the real code was like a hundred times longer and more convoluted.

                          // Library.h statically linked to and included in DLL and EXE
                          struct SomeType
                          {
                             int typeId();
                          };
                          Q_DECLARE_METATYPE(SomeType);
                          
                          // Library.cpp
                          int SomeType::typeId()
                          {
                              return qMetaTypeId<SomeType>();
                          }
                          
                          // main app
                          SomeType& var1 = getItFromDLL();
                          SomeType& var2 = getItFromEXE();
                          
                          bool same = var1.typeId() == var2.typeId(); // nope
                          

                          Pretty ugly thing to debug, especially since once in blue moon it actually works :/

                          kshegunovK Offline
                          kshegunovK Offline
                          kshegunov
                          Moderators
                          wrote on last edited by
                          #49

                          @chris-kawa said in Recurring C++ and Qt anti-patterns:

                          I had to dig through this thing once, only the real code was like a hundred times longer and more convoluted.
                          [Snip]
                          Pretty ugly thing to debug, especially since once in blue moon it actually works :/

                          Indeed. Although, this is windows specific. It works correctly on Linux as the symbol resolution happens at run time.

                          Read and abide by the Qt Code of Conduct

                          1 Reply Last reply
                          1
                          • Chris KawaC Chris Kawa

                            Being an a-hole as a recruiter:

                            What does o() mean?
                            What does o.o mean?
                            What does o->o mean?
                            What does o-->o mean?
                            What does o()--<=>--o() mean? Fun fact - crashes MSVC (yes, the compiler, not the compiled program)
                            What does [](){;o()++<=>++o();}() mean?

                            J.HilkJ Offline
                            J.HilkJ Offline
                            J.Hilk
                            Moderators
                            wrote on last edited by
                            #50

                            @chris-kawa wow, some of those you don't encounter every day...


                            Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                            Q: What's that?
                            A: It's blue light.
                            Q: What does it do?
                            A: It turns blue.

                            1 Reply Last reply
                            0
                            • Chris KawaC Offline
                              Chris KawaC Offline
                              Chris Kawa
                              Lifetime Qt Champion
                              wrote on last edited by Chris Kawa
                              #51

                              Yeah, it was a bit out of topic. Here's one anti-pattern I encounter something like 9/10 code reviews:

                              auto widget = new SomeWidget(some_widget);
                              auto layout = new SomeLayout(some_other_widget);
                              layout->addWidget(widget);
                              

                              It's not a correctness bug. It's a subtle performance one. Compare this with:

                              auto widget = new SomeWidget();
                              auto layout = new SomeLayout();
                              layout->addWidget(widget);
                              some_other_widget->setLayout(layout);
                              

                              If you don't see it - count how many times parents need to be changed and imagine there's not one but, say, 50 widgets and layouts.
                              For extra sweetness do the same when the parent widget is visible - how many times layouts need to be recalculated?

                              jsulmJ 1 Reply Last reply
                              6
                              • Chris KawaC Chris Kawa

                                Yeah, it was a bit out of topic. Here's one anti-pattern I encounter something like 9/10 code reviews:

                                auto widget = new SomeWidget(some_widget);
                                auto layout = new SomeLayout(some_other_widget);
                                layout->addWidget(widget);
                                

                                It's not a correctness bug. It's a subtle performance one. Compare this with:

                                auto widget = new SomeWidget();
                                auto layout = new SomeLayout();
                                layout->addWidget(widget);
                                some_other_widget->setLayout(layout);
                                

                                If you don't see it - count how many times parents need to be changed and imagine there's not one but, say, 50 widgets and layouts.
                                For extra sweetness do the same when the parent widget is visible - how many times layouts need to be recalculated?

                                jsulmJ Offline
                                jsulmJ Offline
                                jsulm
                                Lifetime Qt Champion
                                wrote on last edited by
                                #52

                                @chris-kawa said in Recurring C++ and Qt anti-patterns:

                                auto widget = new SomeWidget(widget);

                                I hope this is a typo :-)

                                https://forum.qt.io/topic/113070/qt-code-of-conduct

                                Chris KawaC 1 Reply Last reply
                                1
                                • jsulmJ jsulm

                                  @chris-kawa said in Recurring C++ and Qt anti-patterns:

                                  auto widget = new SomeWidget(widget);

                                  I hope this is a typo :-)

                                  Chris KawaC Offline
                                  Chris KawaC Offline
                                  Chris Kawa
                                  Lifetime Qt Champion
                                  wrote on last edited by
                                  #53

                                  @jsulm Sure, sorry, fixed :)

                                  1 Reply Last reply
                                  0
                                  • Kent-DorfmanK Offline
                                    Kent-DorfmanK Offline
                                    Kent-Dorfman
                                    wrote on last edited by
                                    #54

                                    @chris-kawa said in Recurring C++ and Qt anti-patterns:

                                    It's not a correctness bug. It's a subtle performance one. Compare this with:

                                    It took me a few minutes but ok, I'm convinced... LOL

                                    1 Reply Last reply
                                    0
                                    • fcarneyF Offline
                                      fcarneyF Offline
                                      fcarney
                                      wrote on last edited by
                                      #55
                                      mtime.toString(tr("M/d/yyyy hh:mm AP"));
                                      

                                      C++ is a perfectly valid school of magic.

                                      aha_1980A 1 Reply Last reply
                                      2
                                      • fcarneyF fcarney
                                        mtime.toString(tr("M/d/yyyy hh:mm AP"));
                                        
                                        aha_1980A Offline
                                        aha_1980A Offline
                                        aha_1980
                                        Lifetime Qt Champion
                                        wrote on last edited by
                                        #56

                                        Hi @fcarney,

                                        mtime.toString(tr("M/d/yyyy hh:mm AP"));

                                        OMG. Yeah, that's a good (bad) one :)

                                        Qt has to stay free or it will die.

                                        1 Reply Last reply
                                        0
                                        • fcarneyF Offline
                                          fcarneyF Offline
                                          fcarney
                                          wrote on last edited by
                                          #57

                                          Just created this pattern today:

                                          if(condition == somevalue)
                                          somestatement.append(whatever);
                                          

                                          I forgot the indentation so it didn't look like and if statement.
                                          I I know this is really simple and not an error/bad practice. It is more a readability issue.

                                          Going to be more rigorous in the future:

                                          if(condition == somevalue){
                                              somestatement.append(whatever);
                                          }
                                          

                                          C++ is a perfectly valid school of magic.

                                          1 Reply Last reply
                                          2

                                          • Login

                                          • Login or register to search.
                                          • First post
                                            Last post
                                          0
                                          • Categories
                                          • Recent
                                          • Tags
                                          • Popular
                                          • Users
                                          • Groups
                                          • Search
                                          • Get Qt Extensions
                                          • Unsolved