forward declaration

This is a discussion on forward declaration within the C++ Programming forums, part of the General Programming Boards category; Hello I'm having problems with compiling.. The error I'm getting is: error C2079: 'A::m_b' uses undefined class 'B' header1: Code: ...

  1. #1
    l2u
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    forward declaration

    Hello

    I'm having problems with compiling..
    The error I'm getting is: error C2079: 'A::m_b' uses undefined class 'B'


    header1:

    Code:
    #include "header2.hpp"
    class B;
    class A {
    public:
    	B m_b;
    };
    header2:

    Code:
    #include "header1.hpp"
    class A;
    class B {
    public:
    	A &m_a;
    };
    What am I doing wrong?

  2. #2
    C++ Witch laserlight's Avatar
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    I think that it should be:
    Code:
    #include "header2.hpp"
    
    class A {
    public:
    	B m_b;
    };
    Code:
    class A;
    class B {
    public:
    	A &m_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

  3. #3
    Cat without Hat CornedBee's Avatar
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    A has a value member of type B. Therefore, it needs the entire definition. The declaration is only enough for the reference member, as B has.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  4. #4
    l2u
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    Now I get 3 errors:
    error C2146: syntax error : missing ';' before identifier 'm_b'
    error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
    error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

  5. #5
    l2u
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    Quote Originally Posted by CornedBee View Post
    A has a value member of type B. Therefore, it needs the entire definition. The declaration is only enough for the reference member, as B has.
    What should I do then?

  6. #6
    C++ Witch laserlight's Avatar
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    Now I get 3 errors:
    What code did you try? Note that you did not include header guards in your original example, so I omitted them as well.

    What should I do then?
    CornedBee basically explained what I illustrated.
    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. #7
    l2u
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    Thanks for help..
    I found out there was another class (C) that was causing the problem when I changed it to what you said.

    I had to change class C in the same way, and now it works.
    Thanks again!

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