saving "this" in base class

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  • Martin Drautzburg
    #1

    saving "this" in base class

    For reasons related to python/swig and garbage collecting I want to
    remember every object in a linked list. I have a common base class
    RCObj which in turn has a static std::list<RCObj *memory.

    My problem is that I cannot get RCObj store the true this pointer. Even
    if I pass it explicitly from a derived class (called "Part") as in:

    Part::Part() {
    this->RCObj::init(th is);

    the pointer that arrives at RCObj::init() is not the same as "this"
    inside Part::Part()

    (gdb) p this
    $4 = (pyramid::Part * const) 0x81b62e0
    (gdb) s
    RCObj::init (this=0x81b6378 , that=0x81b6378) at common.cc:65

    I assume C++ treats an object as composed of several sections one of
    which is the RCObj section and is silently add an offset (0x81b6378 -
    0x81b62e0 = 152) to "this" if this shall be treated as an RCObj.

    (1) what happens when I "delete this" from within the RCObj scope? Will
    it only delete part of the object?
    (2) how can I possibly keep track of all my created objects?
    Tags: None
    • Victor Bazarov
      #2
      Re: saving &quot;this&quot ; in base class

      Martin Drautzburg wrote:
      For reasons related to python/swig and garbage collecting I want to
      remember every object in a linked list. I have a common base class
      RCObj which in turn has a static std::list<RCObj *memory.
      >
      My problem is that I cannot get RCObj store the true this pointer.
      Why would you need to?
      Even if I pass it explicitly from a derived class (called "Part") as
      in:
      >
      Part::Part() {
      this->RCObj::init(th is);
      >
      the pointer that arrives at RCObj::init() is not the same as "this"
      inside Part::Part()
      Of course it isn't. So?
      (gdb) p this
      $4 = (pyramid::Part * const) 0x81b62e0
      (gdb) s
      RCObj::init (this=0x81b6378 , that=0x81b6378) at common.cc:65
      >
      I assume C++ treats an object as composed of several sections one of
      which is the RCObj section and is silently add an offset (0x81b6378 -
      0x81b62e0 = 152) to "this" if this shall be treated as an RCObj.
      >
      (1) what happens when I "delete this" from within the RCObj scope?
      If your destructor is virtual (as it should be), the object is
      destroyed correctly and the memory is freed. You shouldn't attempt
      to access any non-static members of that object after that point.
      Will it only delete part of the object?
      Not if the destructor is virtual.
      (2) how can I possibly keep track of all my created objects?
      There are different schemes for that, you should look on the Web
      for the available solutions.

      V
      --
      Please remove capital 'A's when replying by e-mail
      I do not respond to top-posted replies, please don't ask


        Comment

        • Martin Drautzburg
          #3
          Re: saving &quot;this&quot ; in base class

          Victor Bazarov wrote:

          >the pointer that arrives at RCObj::init() is not the same as "this"
          >inside Part::Part()
          >
          Of course it isn't. So?
          I guess you are trying to tell me that the "this" pointer that arrives
          at the base class is good enough for all practical issues as virtual
          methods will always be dispatched correctly. So the actual value of
          this is of no concern.
          >(2) how can I possibly keep track of all my created objects?
          >
          There are different schemes for that, you should look on the Web
          for the available solutions.
          Haven't found anything yet. What exactly should I look for.

          Currently I am having problems calling a method that was not declared in
          the base class. So whenever I fetch an object from the
          static std::list<RCObj *theObjects;
          object pool I will only get an RCObj which knows nothing about methods
          implemented in its derived classes, even though the object *is* really
          of a derived class.

          I am thinking about providing virtual conversion functions in the RCObj
          base class like asMidiByte(), asNoteOn() and whatever derived classes I
          have. So the base class would only have to know about derived classes
          and not about each and every method in them. But I haven't tried this
          yet. What do you think?



            Comment

            • BobR
              #4
              Re: saving &quot;this&quot ; in base class


              Martin Drautzburg wrote in message...
              >
              Currently I am having problems calling a method that was not declared in
              the base class. So whenever I fetch an object from the
              static std::list<RCObj *theObjects;
              object pool I will only get an RCObj which knows nothing about methods
              implemented in its derived classes, even though the object *is* really
              of a derived class.
              The object is 'sliced'.
              Do you know about 'downcasting'?

              RCObj *basepointer( /* init it */ );

              MyDerived *Md = dynamic_cast<My Derived*>( basepointer );
              if( Md ){
              Md->FuncOnlyInDeri ved();
              }
              else{
              std::cerr<<"Not a derived object"<<std::e ndl;
              }

              --
              Bob R
              POVrookie


                Comment

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