Thread: Can a inner class have a member of outer class type?

  1. 10-06-2009 #1
    pheres
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    Can a inner class have a member of outer class type?

    Hi,

    for example

    Code:
    class A
{
    class B
    {
          A mA;    // VS complains that A is undefined
    }
};
    Is there any way around?

    Thank you in advance!
  2. 10-06-2009 #2
    laserlight
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    Why does B need an A object as a member?
    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
  3. 10-06-2009 #3
    pheres
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    A defines a structure and B defines some presentation, read- and write functions to it. Therefore B uses some state (member) variables which shall not be member of A, so it needs to be an own class.

    I could of course just do

    Code:
    class A
{

};

class B
{
      A mA;    // VS complains that A is undefined
}
    ...which would rather be the same, at least for my. Why not for the compiler?

    I would like to use the inner class because this way it's in the namespace defined by the outer class.
  4. 10-06-2009 #4
    anon
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    This still doesn't explain why B needs an instance of A, rather than storing (or being passed) a reference/pointer to an A.
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
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  5. 10-06-2009 #5
    pheres
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    The data is stored on a server, B need to receive it. You can't hold remote data per reference.
  6. 10-06-2009 #6
    laserlight
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    Quote Originally Posted by pheres
    A defines a structure and B defines some presentation, read- and write functions to it. Therefore B uses some state (member) variables which shall not be member of A, so it needs to be an own class.
    It sounds as if B is redundant and you should move the member functions of B to A.
    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
  7. 10-06-2009 #7
    pheres
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    Quote Originally Posted by pheres View Post
    B uses some state (member) variables which shall not be member of A, so it needs to be an own class.
    That means B's members functions depend on each other, so B needs it's own member variables. But the can't be part of A because A is needed by legacy code as well and this code relies on the memory layout of A.

    I guess there is no solution to the original problem and I've to split A and B.
  8. 10-06-2009 #8
    laserlight
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    Quote Originally Posted by pheres
    I guess there is no solution to the original problem and I've to split A and B.
    Yeah, perhaps as a workaround you can put them both in the same namespace.
    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
  9. 10-06-2009 #9
    quzah
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    Short answer: No. If A has B as a member, B cannot have A as a member. Why? It would be infinite recursion. B can have a pointer to type A, but it cannot have an instance of A.


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  10. 10-06-2009 #10
    pheres
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    B is no member of A. B is an inner class to A.
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