class structure question

This is a discussion on class structure question within the C++ Programming forums, part of the General Programming Boards category; Is there a sensible way to achieve something like this: An instance of class M instantiates one of class S-1 ...

  1. #1
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    class structure question

    Is there a sensible way to achieve something like this:

    An instance of class M instantiates one of class S-1 and one of class S-2, and...
    I want S-1 to be able to call methods of M,
    S-2 to be able to call methods of S-1,
    and M to be able to call methods of both S-1 and S-2.

    These classes are otherwise unrelated -- no inheritance.

    Alternatively, I could combine S-1 into M (just replace the S-1 methods and data with M methods and data) but then I would need S-2 (which is declared by M) to be able to call M methods (and M to be able to call S-2 methods).

  2. #2
    C++ Witch laserlight's Avatar
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    Well, obviously M can call the (public) methods (or rather, member functions) of S-1 and S-2. To allow S-1 to call the (public) member functions of M, you may need to pass a reference or pointer to the M object to the S-1 object instantiated. For S-2 to call the (public) member functions of S-1, you may need to pass a reference or pointer to the S-1 object.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    Quote Originally Posted by laserlight View Post
    ... you may need to pass a reference or pointer to the S-1 object.
    Yes, I forgot to mention that I was hoping to avoid that. Is there any way to avoid that?

    Or can you think of any other (non-class?) structure to replace the S-1 class that would serve as an interface making member functions of M accessible from other classes that are instantiated by M.

    The point is that everything in M is (robot) platform-specific and provides access to the hardware. S-2, etc, would contain code that should be portable to other platforms (given appropriate interfaces). The OS requires that M be the primary object which instantiates S-2, and not the other way around.

    Any ideas?

  4. #4
    C++ Witch laserlight's Avatar
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    The point is that everything in M is (robot) platform-specific and provides access to the hardware. S-2, etc, would contain code that should be portable to other platforms (given appropriate interfaces).
    What's S-1 for?

    The OS requires that M be the primary object which instantiates S-2, and not the other way around.
    That's fine though. As I mentioned, you can pass a reference or pointer to M to S-2.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    S-1 (e.g.) is just to encapsulate a particular subset of M functions -- to help avoid having individual developers continually stepping on each other's work.

    As to the pointers, it seemed a little 'inelegant' but I guess we can live with it.

  6. #6
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    Quote Originally Posted by R.Stiltskin View Post
    Is there a sensible way to achieve something like this:

    An instance of class M instantiates one of class S-1 and one of class S-2, and...
    I want S-1 to be able to call methods of M,
    S-2 to be able to call methods of S-1,
    and M to be able to call methods of both S-1 and S-2.

    These classes are otherwise unrelated -- no inheritance.

    Alternatively, I could combine S-1 into M (just replace the S-1 methods and data with M methods and data) but then I would need S-2 (which is declared by M) to be able to call M methods (and M to be able to call S-2 methods).
    you could use pointers to member functions
    or declare M *m as a member of S-1
    and S-1 *s1 as a member of S-2
    and S-1 *s1 and S-2 *s2 as members of M

    and make sure the methods you want to access are public.

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    Hmmm ... it seemed easy until I tried to give s_1 a pointer to m. g++ gives
    classtest1.h:17: "error: 'm' was not declared in this scope".

    which makes sense, since each header file refers to the other. Can you see a way to get around this?


    classtest.h:
    Code:
    #ifndef _classtest_h
    #define _classtest_h
    
    #include "classtest1.h"
    #include "classtest2.h"
    #include <iostream>
    using namespace std;
    
    
    class m {
    public:
      m() { one.setnum(6); two.setone(&one); one.setbody(this)}
      void print1() {
        one.print();
      }
      void print2() {
        two.print();
      }
      void print3() {  
        cout << "hello from m" << endl;
      }
    private:
      s_1 one;
      s_2 two;
      int i;
    };
    
    #endif

    classtest1.h:
    Code:
    #ifndef _classtest1_h
    #define _classtest1_h
    
    #include "classtest.h"
    #include <iostream>
    using namespace std;
    
    class s_1 {
    public:
      void print () {
        cout << "s_1 secretnum = " << secretnum << endl;
        body->print3();
      }
      void setnum(int x){
        secretnum = x;
      }
      void setbody(m* _body) {
        body = _body;
      }
    private:
      int secretnum;
      m* body;
    };
    
    #endif
    classtest2.h:
    Code:
    #ifndef _classtest2_h
    #define _classtest2_h
    
    #include "classtest1.h"
    #include <iostream>
    using namespace std;
    
    
    class s_2 {
    public:
      void setone(s_1* _one) {
        one = _one;
      }
      void print() {
        cout << "s_2 calling s_1" << endl;
        one->print();
      }
    private:
      s_1* one;
    };
    
    #endif
    classtest.cc
    Code:
    #include "classtest.h"
    #include "classtest1.h"
    #include "classtest2.h"
    #include <iostream>
    using namespace std;
    
    //slave1 one;
    
    int main () {
      m rex;
      rex.print1();
      rex.print2();
      return 0;
    }

  8. #8
    "I Win!" by U. Lose vart's Avatar
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    use forward declarations to avoid unneded dependencies like

    Code:
    #ifndef _classtest2_h
    #define _classtest2_h
    #include <iostream>
    
    class s_1;
    
    class s_2 {
    public:
      void setone(s_1* _one) {
        one = _one;
      }
      void print() {
        std::cout << "s_2 calling s_1" << std::endl;
        one->print();
      }
    private:
      s_1* one;
    };
    
    #endif
    never put using directive into header
    you will probably need to move the print function implementaion into cpp file
    To be or not to be == true

  9. #9
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    a good way to avoid header file cross referencing is to separate your classes into header files and cpp files and use forward declarations

    in header file:


    Code:
    class S_1;
    class M
    {
    // declarations...
        S_1 *s1;
    };
    then in the cpp file, where you delcare the method bodies, do: #include "S_1.h"
    Last edited by m37h0d; 03-20-2008 at 03:41 PM.

  10. #10
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    I still had to #include the s_1 and s_2 headers in m's header file. When I tried with forward declarations I got "incomplete type" errors for m's 'one' and 'two' data members.

    But the other two headers were ok with just forward declarations, and now the whole thing compiles & runs correctly.

    Thanks for the help.

  11. #11
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    Quote Originally Posted by R.Stiltskin View Post
    I still had to #include the s_1 and s_2 headers in m's header file. When I tried with forward declarations I got "incomplete type" errors for m's 'one' and 'two' data members.

    But the other two headers were ok with just forward declarations, and now the whole thing compiles & runs correctly.

    Thanks for the help.
    it sounds like you're delcaring S_1 s1 and not S_1 *s1.


    you have to use a pointer in forward declarations. this works because pointers are always 32bts, but when declaring an actual instance of the class as a member, it has to know exactly how much memory to allocate.

  12. #12
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    Quote Originally Posted by m37h0d View Post
    it sounds like you're delcaring S_1 s1 and not S_1 *s1.
    Exactly.

    Thanks for the explanation.

  13. #13
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    Quote Originally Posted by R.Stiltskin View Post
    Exactly.

    Thanks for the explanation.
    be sure to split your classes into h files, containing only declarations, and cpp files containing the implementations, and do your #includes to any non-descendant (you can't do a forward declaration for a base class) classes in the cpp file.

    this will avoid a lot of headaches with header conflicts

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