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  4. [SOLVED] Get all Objects with same class ?
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[SOLVED] Get all Objects with same class ?

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  • D Offline
    D Offline
    deleted28
    wrote on last edited by
    #25

    If i understnad you correctly, you recomment to use this QSet<T*>
    ( set of gathered objects created by gather method (?) )
    as public and not to use JKSH's suggestion to make it private
    and not avoid globally-accessible variables for now, because
    to much efforts to implement it (overkill).
    Is this correct ?

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    • SGaistS Offline
      SGaistS Offline
      SGaist
      Lifetime Qt Champion
      wrote on last edited by
      #26

      No, you are just missing the static qualifier in from of m_instances, JKSH's technique is fine

      Interested in AI ? www.idiap.ch
      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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      • D Offline
        D Offline
        deleted28
        wrote on last edited by
        #27

        I set the static already and it works now.
        My above question is meant to this :

        bq. still, i have to say it again, using getters (not to mention static getters) is in my opinion an overkill when it’s not a “general public” object. If i were you i’d just use the variable ‘KNhead::instances’ directly where i needed it.

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        • JKSHJ Offline
          JKSHJ Offline
          JKSH
          Moderators
          wrote on last edited by
          #28

          [quote author="wally123" date="1422790846"]If i understnad you correctly, you recomment to use this QSet<T*>
          ( set of gathered objects created by gather method (?) )
          as public and not to use JKSH's suggestion to make it private
          and not avoid globally-accessible variables for now, because
          to much efforts to implement it (overkill).
          Is this correct ?
          [/quote]Yes, that is what gyll recommended.

          Both approaches work.

          • The global approach is the "fast" way, and saves you a few lines of code at the start
          • The the private approach is the "safe" way, and reduces the risk of you making mistakes in the future.

          As the program designer, you need to compare the costs and benefits of the different approaches and decide which one you want.

          I leave you with this piece of wisdom from xkcd:
          !http://imgs.xkcd.com/comics/good_code.png(Good Code)!
          (source: http://xkcd.com/844/ )

          Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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          • D Offline
            D Offline
            deleted28
            wrote on last edited by
            #29

            nice !
            now the Iterator ... :)

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            • D Offline
              D Offline
              deleted28
              wrote on last edited by
              #30

              this works to find by variable name:
              @
              QSet<KNhead*>::iterator it = qFind(KNhead::instances.begin(), KNhead::instances.end(), cell_2);@

              I introduced a public property to KNhead class as
              @int testProp;@

              @KNhead *cell_2 = new KNhead;
              cell_2->testPop = 222;
              @

              Can i find/search the object in the entire QSet by this property ?
              Means, find an object in KNhead::instances with testProp == 222 ?

              Of course i can go through each object an check each on the contents of the testProp,
              i want to ask if there is a direct possibility by STL or Qt .

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              • JKSHJ Offline
                JKSHJ Offline
                JKSH
                Moderators
                wrote on last edited by
                #31

                If you want to search by one particular property, use a QMap instead of a QSet, and use the property as the map key.

                @
                // Declaring the map
                QMap<int, KNhead*> instances;
                @

                @
                // Inserting into the map
                KNhead::KNhead(int prop)
                {
                this->testProp = prop;
                instances[prop] = this;

                // ASSUMPTION: The value of this property never changes
                

                }
                @

                @
                // Retrieving from the map
                KNhead *cell = instances[222];
                @

                If you want to search by multiple properties, use an in-memory SQLite database.

                Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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                • G Offline
                  G Offline
                  gyll
                  wrote on last edited by
                  #32

                  just a quick correction to my last reply, dunno what was in my mind when i wrote it: i said that if declaring a static getter it should be declared as 'const', and this is not possible for static methods. however, i was wrong: in order to prevent using the getter method for changing the data value, it is the return value that should be declared as 'const', not the method itself.

                  So it is entirely possible to declare a clean getter method which returns a const reference:

                  @
                  // knhead.h
                  #include <unordered_set>

                  class KNhead
                  {
                  static std::unordered_set<KNhead*> m_instances;
                  public:
                  KNhead();
                  ~KNhead();
                  static const std::unordered_set<KNhead*>& instances();
                  };

                  // knhead.cpp
                  #include "knhead.h"
                  #include <assert.h>

                  std::unordered_set<KNhead*> KNhead::m_instances;

                  KNhead::KNhead() {
                  m_instances.insert(this);
                  }

                  KNhead::~KNhead() {
                  assert(m_instances.count(this));
                  m_instances.erase(this);
                  }

                  const std::unordered_set<KNhead*>& KNhead::instances() {
                  return m_instances;
                  }
                  @

                  With the KNhead::instances() as defined above you're safe (the compiler won't let you do something like e.g. KNhead::instances().insert(x) , it will complain about you trying to change a const reference)

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                  • D Offline
                    D Offline
                    deleted28
                    wrote on last edited by
                    #33

                    great ! thanks a lot

                    and it works !

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                    • D Offline
                      D Offline
                      deleted28
                      wrote on last edited by
                      #34

                      Hello,

                      i have problem with the javastyle QMapiterator and need your help.
                      Seems i do not understand the concept at all.

                      the STL style iterator works nice;
                      @ QMap<quint32, Indiv*>::iterator it4 = m_instances.find(this->devID);
                      qDebug() << it4.value();

                      // Indiv(0x8b1c378, name = "cell_0")
                      // Indiv(0x8b23f08, name = "cell_1")
                      // Indiv(0x8ac1138, name = "cell_2")
                      // Indiv(0x8b2dbf8, name = "cell_3")
                      // Indiv(0x8af8870, name = "cell_4")
                      // Indiv(0x8b249f0, name = "cell_5")
                      // OK@

                      I played the entire afternoon with the below code,
                      mostly the application crashes, sometimes i end up in a
                      neverending loop. No further ideas any more now ...

                      // java iterator
                      @
                      QMapIterator<quint32, Indiv*> it5(m_instances);

                      qDebug() << it5.hasNext();  // true ok
                      
                      while (it5.hasNext()) {
                          qDebug() << it5.value();
                          it5.next();
                      }@
                      

                      @#include <QDebug>
                      #include <QObject>
                      #include <QMap>
                      #include <QMapIterator>@

                      thx wally

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                      • JKSHJ Offline
                        JKSHJ Offline
                        JKSH
                        Moderators
                        wrote on last edited by
                        #35

                        [quote author="wally123" date="1422983820"]
                        @
                        while (it5.hasNext()) {
                        qDebug() << it5.value();
                        it5.next();
                        }
                        @
                        [/quote]You need to call next() before value().

                        Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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                        • D Offline
                          D Offline
                          deleted28
                          wrote on last edited by
                          #36

                          Yes, a fundamental issue :D

                          thank you

                          @
                          | item 0 | item 1 | item 2 | ... | item n |

                          A B C

                          @

                          Does the it5.next() moves the iterator from initial
                          position A to B and then read item 0 ?

                          I'm much too stupid for java stuff

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                          • JKSHJ Offline
                            JKSHJ Offline
                            JKSH
                            Moderators
                            wrote on last edited by
                            #37

                            [quote author="wally123" date="1422987332"]Yes, a fundamental issue :D

                            thank you [/quote]You're welcome :)

                            [quote author="wally123" date="1422987332"]
                            @
                            | item 0 | item 1 | item 2 | ... | item n |

                            A B C

                            @

                            Does the it5.next() moves the iterator from initial
                            position A to B and then read item 0 ?

                            I'm much too stupid for java stuff[/quote]I'm not sure (I don't use Java-style iterators myself), but yes that's what the "documentation":http://doc.qt.io/qt-5/qmapiterator.html says.

                            Qt's documentation is very comprehensive -- it's good to search it. If you use Google Chrome, "this tool":http://qt-project.org/forums/viewthread/36199 makes it easy to search.

                            Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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