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Recurring C++ and Qt anti-patterns

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  • sierdzioS sierdzio

    Small things from me:

    • lack of const in signal declarations. A signal will never modify an object so it can always be const. Qt will const_cast it away anyway, but it enables you to emit signals from const methods and (possibly) compiler to optimize a bit more
    • overuse of lambdas in slot connections even when a normal slot just makes more sense
    A Offline
    A Offline
    Asperamanca
    wrote on last edited by
    #33

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

    Small things from me:

    • lack of const in signal declarations. A signal will never modify an object so it can always be const. Qt will const_cast it away anyway, but it enables you to emit signals from const methods and (possibly) compiler to optimize a bit more
    • overuse of lambdas in slot connections even when a normal slot just makes more sense

    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?

    sierdzioS 1 Reply Last reply
    0
    • A Asperamanca

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

      Small things from me:

      • lack of const in signal declarations. A signal will never modify an object so it can always be const. Qt will const_cast it away anyway, but it enables you to emit signals from const methods and (possibly) compiler to optimize a bit more
      • overuse of lambdas in slot connections even when a normal slot just makes more sense

      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?

      sierdzioS Offline
      sierdzioS Offline
      sierdzio
      Moderators
      wrote on last edited by
      #34

      @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".

      (Z(:^

      Kent-DorfmanK 1 Reply Last reply
      0
      • fcarneyF Offline
        fcarneyF Offline
        fcarney
        wrote on last edited by fcarney
        #35

        Given the code:
        modules.h

        #ifndef MODULES_H
        #define MODULES_H
        
        #include <string>
        
        void reg_module(int type, std::string name, int initedValue);
        
        #endif // MODULES_H
        

        modules.cpp

        #include "modules.h"
        
        using namespace std;
        
        struct Modules
        {
            Modules(): m_initedValue(0){}
            int m_type;
            string m_name;
            int m_initedValue;
        } global_modules_struct[128];
        
        void reg_module(int type, std::string name, int initedValue){
            global_modules_struct[type].m_type = type;
            global_modules_struct[type].m_name = name;
            global_modules_struct[type].m_initedValue = initedValue;
        }
        

        moduletype.h

        #ifndef MODULETYPE_H
        #define MODULETYPE_H
        
        // nothing here
        
        #endif // MODULETYPE_H
        

        moduletype.cpp

        #include "moduletype.h"
        #include "modules.h"
        
        struct SomeModule{
            SomeModule(){
                reg_module(10, "some type", 5);
            }
        } SomeModuleInstance;
        

        Ignore obvious indexing bounds checking issues for the array itself. Also ignore external array indexing possibly being out of bounds.

        I just ran into a form of this problem in our code and it did not exhibit issues in Linux (that we know of) and did show issues in Windows. Linux used gcc and Windows used mingw. Same version of Qt 5.12.2 etc. Once identified it was really easy to see why this is a big issue.

        Edit:
        Technically global_modules_struct is not really global either. So ignore the misleading name.

        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 Chris Kawa
          #36

          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 1 Reply Last reply
          2
          • B Offline
            B Offline
            Brunner2
            Banned
            wrote on last edited by
            #37
            This post is deleted!
            1 Reply Last reply
            0
            • 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 Online
                                      J.HilkJ Online
                                      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

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