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Forum Update on Monday, May 27th 2025

Increasing usage for C++ new operators based on data model indexes?

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data modelscreateindexallocationnew operatorssoftware design
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  • V VRonin
    15 Oct 2018, 11:53

    @elfring said in Increasing usage for C++ new operators based on data model indexes?:

    The safe handling of object lifetimes is a general software development challenge.
    But this class is the only way to get data from a model so far, isn't it?

    Yes, what I'm saying is that if you want to store a pointer in a QVariant you probably want a non-owning pointer. Keep the owning pointer somewhere else.

    Can it be more convenient to let a C++ new operator (which can work also with extra allocation parameters directly) perform the desired data type conversion?

    No, new allocates memory. static_cast just tell the compiler to treat a piece of memory as it was a different type.

    But this class is the only way to get data from a model so far, isn't it?

    And here we come back to the fact that having your objects implicitly shared stored in a QVariant directly is much more practical and has a negligible impact on performance

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    elfring
    wrote on 15 Oct 2018, 13:01 last edited by
    #24

    new allocates memory.

    Only if you do not use the construct “placement new”.

    And here we come back to the fact that having your objects implicitly shared stored in a QVariant directly is much more practical

    I guess that the run time consequences are more interesting then for the distinction if only pointers are transferred or complete “value objects” are copied.

    and has a negligible impact on performance

    There are software applications where more data transfers might not have a directly noticeable effect. But I guess that some C++ programmers have got a strong focus on software efficiency.

    K V 2 Replies Last reply 15 Oct 2018, 13:09
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    • E elfring
      15 Oct 2018, 13:01

      new allocates memory.

      Only if you do not use the construct “placement new”.

      And here we come back to the fact that having your objects implicitly shared stored in a QVariant directly is much more practical

      I guess that the run time consequences are more interesting then for the distinction if only pointers are transferred or complete “value objects” are copied.

      and has a negligible impact on performance

      There are software applications where more data transfers might not have a directly noticeable effect. But I guess that some C++ programmers have got a strong focus on software efficiency.

      K Offline
      K Offline
      kshegunov
      Moderators
      wrote on 15 Oct 2018, 13:09 last edited by
      #25

      This is getting pretty ridiculous ...

      @elfring said in Increasing usage for C++ new operators based on data model indexes?:

      Only if you do not use the construct “placement new”.

      Yeah, which I have a creeping suspicion you haven't. The so called placement new is nothing more than going around calling constructors on a preallocated memory block. So where is this preallocated memory going to come from? It's going to materialize from the ether?

      If you're intent on pushing this make-you-own-heap-for-models, I advise to create a proof-of-concept first, then we can have something to base a discussion on, otherwise - thanks but no thanks, I'm out of this conversation.

      Read and abide by the Qt Code of Conduct

      E 1 Reply Last reply 15 Oct 2018, 13:22
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      • K kshegunov
        15 Oct 2018, 13:09

        This is getting pretty ridiculous ...

        @elfring said in Increasing usage for C++ new operators based on data model indexes?:

        Only if you do not use the construct “placement new”.

        Yeah, which I have a creeping suspicion you haven't. The so called placement new is nothing more than going around calling constructors on a preallocated memory block. So where is this preallocated memory going to come from? It's going to materialize from the ether?

        If you're intent on pushing this make-you-own-heap-for-models, I advise to create a proof-of-concept first, then we can have something to base a discussion on, otherwise - thanks but no thanks, I'm out of this conversation.

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        elfring
        wrote on 15 Oct 2018, 13:22 last edited by
        #26

        The so called placement new is nothing more than going around calling constructors on a preallocated memory block.

        I agree on this aspect.

        But can you pass a “model index” as another allocation parameter (for existing data) to this C++ operator?

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        • E elfring
          15 Oct 2018, 13:01

          new allocates memory.

          Only if you do not use the construct “placement new”.

          And here we come back to the fact that having your objects implicitly shared stored in a QVariant directly is much more practical

          I guess that the run time consequences are more interesting then for the distinction if only pointers are transferred or complete “value objects” are copied.

          and has a negligible impact on performance

          There are software applications where more data transfers might not have a directly noticeable effect. But I guess that some C++ programmers have got a strong focus on software efficiency.

          V Offline
          V Offline
          VRonin
          wrote on 15 Oct 2018, 13:24 last edited by VRonin
          #27

          @elfring said in Increasing usage for C++ new operators based on data model indexes?:

          Only if you do not use the construct “placement new”.

          Is your plan to do exactly what dynamic_cast does but using a new operator? I mean it's possible but I don't see why you'd want to do it. How would you implement it?

          guess that the run time consequences are more interesting then for the distinction if only pointers are transferred or complete “value objects” are copied.
          There are software applications where more data transfers might not have a directly noticeable effect. But I guess that some C++ programmers have got a strong focus on software efficiency.

          @VRonin said in Increasing usage for C++ new operators based on data model indexes?:

          having your objects implicitly shared

          With implicitly shared objects, the distinction from a pointer to a complete value object when copying is just an operator++ on an int. 1 atomic instruction. 100000 times more efficient than a dynamic_cast.
          If you want to go into memory difference it's 32bits more. If 32 bits break your design then it's the design itself that comes into question

          "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
          ~Napoleon Bonaparte

          On a crusade to banish setIndexWidget() from the holy land of Qt

          E 1 Reply Last reply 15 Oct 2018, 13:32
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          • V VRonin
            15 Oct 2018, 13:24

            @elfring said in Increasing usage for C++ new operators based on data model indexes?:

            Only if you do not use the construct “placement new”.

            Is your plan to do exactly what dynamic_cast does but using a new operator? I mean it's possible but I don't see why you'd want to do it. How would you implement it?

            guess that the run time consequences are more interesting then for the distinction if only pointers are transferred or complete “value objects” are copied.
            There are software applications where more data transfers might not have a directly noticeable effect. But I guess that some C++ programmers have got a strong focus on software efficiency.

            @VRonin said in Increasing usage for C++ new operators based on data model indexes?:

            having your objects implicitly shared

            With implicitly shared objects, the distinction from a pointer to a complete value object when copying is just an operator++ on an int. 1 atomic instruction. 100000 times more efficient than a dynamic_cast.
            If you want to go into memory difference it's 32bits more. If 32 bits break your design then it's the design itself that comes into question

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            elfring
            wrote on 15 Oct 2018, 13:32 last edited by
            #28

            Is your plan to do exactly what dynamic_cast does but using a new operator?

            Maybe.

            But these C++ operations provide different functionality, don't they?

            With implicitly shared objects, the distinction from a pointer to a complete value object when copying is just an operator++ on an int.

            Will it become nicer to avoid (or reduce) even the influence of object reference counting?

            V 1 Reply Last reply 15 Oct 2018, 14:39
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            • E elfring
              15 Oct 2018, 13:32

              Is your plan to do exactly what dynamic_cast does but using a new operator?

              Maybe.

              But these C++ operations provide different functionality, don't they?

              With implicitly shared objects, the distinction from a pointer to a complete value object when copying is just an operator++ on an int.

              Will it become nicer to avoid (or reduce) even the influence of object reference counting?

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              VRonin
              wrote on 15 Oct 2018, 14:39 last edited by
              #29

              @elfring said in Increasing usage for C++ new operators based on data model indexes?:

              Will it become nicer to avoid (or reduce) even the influence of object reference counting?

              C++ is moving in the opposite direction actually. If you try to suggest replacing std::shared_ptr (which is a pointer + a reference counter) with raw pointers on Stack Overflow you'd better bring a helmet and kevlar vest because you are going to get shot.
              The ISO C++ style guide goes one step further and even discourages the use of owning raw pointer and suggest ownership encapsulation (an implicitly shared object behaves as a smart pointer). If you don't like it, good luck convincing Herb Sutter and Bjarne Stroustrup himself.

              But these C++ operations provide different functionality, don't they?

              Yes but so far you did not highlight any need for custom allocation, your problems all seem to come from casting.

              "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
              ~Napoleon Bonaparte

              On a crusade to banish setIndexWidget() from the holy land of Qt

              E 1 Reply Last reply 15 Oct 2018, 16:08
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              • V VRonin
                15 Oct 2018, 14:39

                @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                Will it become nicer to avoid (or reduce) even the influence of object reference counting?

                C++ is moving in the opposite direction actually. If you try to suggest replacing std::shared_ptr (which is a pointer + a reference counter) with raw pointers on Stack Overflow you'd better bring a helmet and kevlar vest because you are going to get shot.
                The ISO C++ style guide goes one step further and even discourages the use of owning raw pointer and suggest ownership encapsulation (an implicitly shared object behaves as a smart pointer). If you don't like it, good luck convincing Herb Sutter and Bjarne Stroustrup himself.

                But these C++ operations provide different functionality, don't they?

                Yes but so far you did not highlight any need for custom allocation, your problems all seem to come from casting.

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                elfring
                wrote on 15 Oct 2018, 16:08 last edited by
                #30

                but so far you did not highlight any need for custom allocation,

                I suggest to reconsider the software situation once more.

                A pair of row and column values (data model index, address or coordinate) can be passed as a parameter to a C++ new operator.
                You can choose then if you need to work with fresh objects (using dynamic memory allocation) or would like to reuse existing data.

                Is the usage of the construct “placement new” a kind of customised operation?

                your problems all seem to come from casting.

                I hope that specific software development challenges can be adjusted when the new operator call takes care of casting to the desired target (pointer) data type already.

                V 1 Reply Last reply 15 Oct 2018, 17:09
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                • E elfring
                  15 Oct 2018, 16:08

                  but so far you did not highlight any need for custom allocation,

                  I suggest to reconsider the software situation once more.

                  A pair of row and column values (data model index, address or coordinate) can be passed as a parameter to a C++ new operator.
                  You can choose then if you need to work with fresh objects (using dynamic memory allocation) or would like to reuse existing data.

                  Is the usage of the construct “placement new” a kind of customised operation?

                  your problems all seem to come from casting.

                  I hope that specific software development challenges can be adjusted when the new operator call takes care of casting to the desired target (pointer) data type already.

                  V Offline
                  V Offline
                  VRonin
                  wrote on 15 Oct 2018, 17:09 last edited by
                  #31

                  @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                  A pair of row and column values (data model index, address or coordinate) can be passed as a parameter to a C++ new operator.
                  You can choose then if you need to work with fresh objects (using dynamic memory allocation) or would like to reuse existing data.
                  Is the usage of the construct “placement new” a kind of customised operation?

                  Once again, the model implementation might or might not allocate data. This should not be a problem of the API user.

                  Could you please provide a concrete example of what you are suggesting?
                  Otherwise we are just discussing of theory and nobody is gaining any value out of it.
                  I will not continue the discussion until you provide at least a code snippet

                  "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
                  ~Napoleon Bonaparte

                  On a crusade to banish setIndexWidget() from the holy land of Qt

                  E 2 Replies Last reply 15 Oct 2018, 17:27
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                  • V VRonin
                    15 Oct 2018, 17:09

                    @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                    A pair of row and column values (data model index, address or coordinate) can be passed as a parameter to a C++ new operator.
                    You can choose then if you need to work with fresh objects (using dynamic memory allocation) or would like to reuse existing data.
                    Is the usage of the construct “placement new” a kind of customised operation?

                    Once again, the model implementation might or might not allocate data. This should not be a problem of the API user.

                    Could you please provide a concrete example of what you are suggesting?
                    Otherwise we are just discussing of theory and nobody is gaining any value out of it.
                    I will not continue the discussion until you provide at least a code snippet

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                    elfring
                    wrote on 15 Oct 2018, 17:27 last edited by
                    #32

                    Could you please provide a concrete example of what you are suggesting?

                    Does the software situation become really more challenging for passing a few extra parameters to a (member) function when it is “accidentally” (or intentionally) called “new”?

                    1 Reply Last reply
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                    • V VRonin
                      15 Oct 2018, 17:09

                      @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                      A pair of row and column values (data model index, address or coordinate) can be passed as a parameter to a C++ new operator.
                      You can choose then if you need to work with fresh objects (using dynamic memory allocation) or would like to reuse existing data.
                      Is the usage of the construct “placement new” a kind of customised operation?

                      Once again, the model implementation might or might not allocate data. This should not be a problem of the API user.

                      Could you please provide a concrete example of what you are suggesting?
                      Otherwise we are just discussing of theory and nobody is gaining any value out of it.
                      I will not continue the discussion until you provide at least a code snippet

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                      elfring
                      wrote on 15 Oct 2018, 19:55 last edited by
                      #33

                      … concrete example of what you are suggesting?

                      • https://isocpp.org/wiki/faq/dtors#placement-new
                      • https://en.wikipedia.org/wiki/Placement_syntax
                      • https://stackoverflow.com/questions/222557/what-uses-are-there-for-placement-new
                      • https://www.geeksforgeeks.org/placement-new-operator-cpp/
                      • http://blog.aaronballman.com/2011/08/the-placement-new-operator/
                      • https://thispointer.com/whats-placement-new-operator-and-why-do-we-need-it/
                      • https://eli.thegreenplace.net/2011/02/17/the-many-faces-of-operator-new-in-c
                      • https://archive.org/details/TICPP2ndEdVolOne
                      1 Reply Last reply
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                      • S Offline
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                        sierdzio
                        Moderators
                        wrote on 16 Oct 2018, 05:45 last edited by
                        #34

                        The first link clearly states:

                        ADVICE: Don’t use this “placement new” syntax unless you have to. Use it only when you really care that an object is placed at a particular location in memory. For example, when your hardware has a memory-mapped I/O timer device, and you want to place a Clock object at that memory location.

                        In case of model indexes, there seems to be no need to use placement new.

                        (Z(:^

                        E 1 Reply Last reply 16 Oct 2018, 08:43
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                        • S sierdzio
                          16 Oct 2018, 05:45

                          The first link clearly states:

                          ADVICE: Don’t use this “placement new” syntax unless you have to. Use it only when you really care that an object is placed at a particular location in memory. For example, when your hardware has a memory-mapped I/O timer device, and you want to place a Clock object at that memory location.

                          In case of model indexes, there seems to be no need to use placement new.

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                          elfring
                          wrote on 16 Oct 2018, 08:43 last edited by
                          #35

                          In case of model indexes, there seems to be no need to use placement new.

                          • Do you care if customised model data were “placed at a particular location in memory”?
                          • How do you think about to get direct access to object data which are managed by a class like QVariant?
                          S 1 Reply Last reply 16 Oct 2018, 08:56
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                          • E elfring
                            16 Oct 2018, 08:43

                            In case of model indexes, there seems to be no need to use placement new.

                            • Do you care if customised model data were “placed at a particular location in memory”?
                            • How do you think about to get direct access to object data which are managed by a class like QVariant?
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                            sierdzio
                            Moderators
                            wrote on 16 Oct 2018, 08:56 last edited by
                            #36

                            @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                            In case of model indexes, there seems to be no need to use placement new.

                            • Do you care if customised model data were “placed at a particular location in memory”?

                            No I don't.

                            • How do you think about to get direct access to object data which are managed by a class like QVariant?

                            I don't understand the question. Accessing data of a QVariant is possible and trivial, if necessary.

                            (Z(:^

                            E 1 Reply Last reply 16 Oct 2018, 09:03
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                            • S sierdzio
                              16 Oct 2018, 08:56

                              @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                              In case of model indexes, there seems to be no need to use placement new.

                              • Do you care if customised model data were “placed at a particular location in memory”?

                              No I don't.

                              • How do you think about to get direct access to object data which are managed by a class like QVariant?

                              I don't understand the question. Accessing data of a QVariant is possible and trivial, if necessary.

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                              elfring
                              wrote on 16 Oct 2018, 09:03 last edited by
                              #37

                              Accessing data of a QVariant is possible and trivial, if necessary.

                              Can the data access become more convenient (and safe) for customised data types?

                              V 1 Reply Last reply 16 Oct 2018, 09:53
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                              • E elfring
                                16 Oct 2018, 09:03

                                Accessing data of a QVariant is possible and trivial, if necessary.

                                Can the data access become more convenient (and safe) for customised data types?

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                                VRonin
                                wrote on 16 Oct 2018, 09:53 last edited by
                                #38

                                @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                                Can the data access become more convenient (and safe) for customised data types?

                                No.
                                get->change->set is mildly less convenient but infinitely safer.

                                Can you think of an example code, and paste it below, where it would?

                                "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
                                ~Napoleon Bonaparte

                                On a crusade to banish setIndexWidget() from the holy land of Qt

                                E 1 Reply Last reply 16 Oct 2018, 10:44
                                1
                                • V VRonin
                                  16 Oct 2018, 09:53

                                  @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                                  Can the data access become more convenient (and safe) for customised data types?

                                  No.
                                  get->change->set is mildly less convenient but infinitely safer.

                                  Can you think of an example code, and paste it below, where it would?

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                                  elfring
                                  wrote on 16 Oct 2018, 10:44 last edited by
                                  #39

                                  Can you think of an example code, and paste it below, where it would?

                                  I suggest to take another look at a specific implementation detail: How many function calls do you need finally to get access to a member variable within a customised data model so far?

                                  V 1 Reply Last reply 16 Oct 2018, 12:03
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                                  • E elfring
                                    16 Oct 2018, 10:44

                                    Can you think of an example code, and paste it below, where it would?

                                    I suggest to take another look at a specific implementation detail: How many function calls do you need finally to get access to a member variable within a customised data model so far?

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                                    V Offline
                                    VRonin
                                    wrote on 16 Oct 2018, 12:03 last edited by VRonin
                                    #40

                                    @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                                    How many function calls do you need finally to get access to a member variable within a customised data model so far?

                                    2 chained. QModelIndex::data and QVariant::value<T>. e.g.: index.data().value<QString>();. Don't think you can do better.
                                    Can you show us how would your new implementation look? Call this a test. You haven't wrote a single line of code in all your posts. I want to check that at least you have a rough idea of what you are talking about

                                    "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
                                    ~Napoleon Bonaparte

                                    On a crusade to banish setIndexWidget() from the holy land of Qt

                                    E 1 Reply Last reply 16 Oct 2018, 13:09
                                    4
                                    • V VRonin
                                      16 Oct 2018, 12:03

                                      @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                                      How many function calls do you need finally to get access to a member variable within a customised data model so far?

                                      2 chained. QModelIndex::data and QVariant::value<T>. e.g.: index.data().value<QString>();. Don't think you can do better.
                                      Can you show us how would your new implementation look? Call this a test. You haven't wrote a single line of code in all your posts. I want to check that at least you have a rough idea of what you are talking about

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                                      elfring
                                      wrote on 16 Oct 2018, 13:09 last edited by
                                      #41

                                      Don't think you can do better.

                                      I imagine that a single function call will be a bit nicer. It can be that it will still need to combine the other mentioned functions.
                                      (Reminder: The usage of the function “QVariant::value” is “unsafe” for non-core data types so far, isn't it?)

                                      You haven't wrote a single line of code in all your posts.

                                      • I find this information partly inappropriate because I contributed a (questionable) test case in this forum.
                                      • Source code might distract from the really relevant software development ideas, doesn't it?

                                      Can you show us how would your new implementation look?

                                      Did other information sources show in sufficient ways already how such a function can be written?

                                      V 1 Reply Last reply 16 Oct 2018, 13:42
                                      0
                                      • E elfring
                                        16 Oct 2018, 13:09

                                        Don't think you can do better.

                                        I imagine that a single function call will be a bit nicer. It can be that it will still need to combine the other mentioned functions.
                                        (Reminder: The usage of the function “QVariant::value” is “unsafe” for non-core data types so far, isn't it?)

                                        You haven't wrote a single line of code in all your posts.

                                        • I find this information partly inappropriate because I contributed a (questionable) test case in this forum.
                                        • Source code might distract from the really relevant software development ideas, doesn't it?

                                        Can you show us how would your new implementation look?

                                        Did other information sources show in sufficient ways already how such a function can be written?

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                                        VRonin
                                        wrote on 16 Oct 2018, 13:42 last edited by
                                        #42

                                        @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                                        I find this information partly inappropriate because I contributed a (questionable) test case in this forum.

                                        This is a great starting point. Let's assume the overloaded new existed. How would you use it in your example?

                                        Source code might distract from the really relevant software development ideas, doesn't it?

                                        No, it really helps focusing on the problem, what really matters and what is overhead.

                                        "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
                                        ~Napoleon Bonaparte

                                        On a crusade to banish setIndexWidget() from the holy land of Qt

                                        E 1 Reply Last reply 16 Oct 2018, 14:15
                                        1
                                        • V VRonin
                                          16 Oct 2018, 13:42

                                          @elfring said in Increasing usage for C++ new operators based on data model indexes?:

                                          I find this information partly inappropriate because I contributed a (questionable) test case in this forum.

                                          This is a great starting point. Let's assume the overloaded new existed. How would you use it in your example?

                                          Source code might distract from the really relevant software development ideas, doesn't it?

                                          No, it really helps focusing on the problem, what really matters and what is overhead.

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                                          elfring
                                          wrote on 16 Oct 2018, 14:15 last edited by
                                          #43

                                          How would you use it in your example?

                                          I would not use extra statements in the implementation of the constructor from the class “my_views” because this test case tried to check other details.

                                          No, it really helps focusing on the problem, what really matters and what is overhead.

                                          I prefer an other clarification approach.

                                          Which names can you imagine for functions which determine a pointer data type for a model index?

                                          V 1 Reply Last reply 16 Oct 2018, 14:23
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