Thread: circular header dependencies

  1. #1
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    circular header dependencies

    Here is a problem of circular dependency. I want to allow a class B ctor friend access to private members of a class A object. But in order to tell class A this, class A needs to know about the existence of the class B::B(const A&) ctor, so a simple forward declaration will not work.
    Code:
    //A.h:
    #ifndef A_H
    #define A_H
    #include "B.h"
    
    class A {
    public:
        A(int i);
        friend B::B(const A &);
    private:
        int _i;
    };
    
    #endif
    
    //B.h:
    #ifndef B_H
    #define B_H
    #include "A.h"
    
    class B {
    public:
        B(const A &);
    private:
        int deg;
    };
    
    #endif
    
    //A.cpp:
    #include "A.h"
    A::A(int i) : _i(i) { }
    
    //B.cpp:
    #include "B.h"
    B::B(const A &a) : deg(a._i) { }
    My basic approach was to have each class header include the other class header so that they each know of the other's entire structure. But inevitably one header comes before the other (of course) so it seems that it is simply impossible to have each class know of the other classes' complete definition as it is being compiled.

    So there is no main() here. Errors are displayed with just:
    Code:
    gcc B.cpp

  2. #2
    C++ Witch laserlight's Avatar
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    Instead of including the header for class A in the header for class B, forward declare class A in the header for class B, just before the definition of class B.
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  3. #3
    and the hat of int overfl Salem's Avatar
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    What's wrong with A having a public accessor function, instead of messy friendship of your private parts?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  4. #4
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    Quote Originally Posted by laserlight View Post
    Instead of including the header for class A in the header for class B, forward declare class A in the header for class B, just before the definition of class B.
    Yes, that exposes the name A so that the B(const A &) ctor can be declared. I actually came across that solution after I posted my question.

    Then, in order to compile B.cpp, one must still #include "A.h" either in B.h after class B definition, or in B.cpp before the definition of B::B(const A &a) ctor, so that a._i is known of. (actually, B.cpp could just #include "A.h" as that #includes "B.h", but i just let the hdr guards do their thing.)

  5. #5
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    Quote Originally Posted by Salem View Post
    What's wrong with A having a public accessor function, instead of messy friendship of your private parts?
    Yes, that is a better solution, e.g, define a public:
    Code:
    int get_deg() const { return _i; }
    method for class A. At this point, I'm just using this code to study about dependencies in class definitions. The real code, from which I distilled this example, was corrected as you suggest.
    Last edited by dslowik; 05-31-2017 at 03:57 PM. Reason: made method get_deg const.

  6. #6
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    Quote Originally Posted by Salem View Post
    messy friendship of your private parts
    Actually, it would be easier to allow the even messier friendship of declaring the entire class B to be a friend of class A. That would only require a forward declaration of class B prior to the class A definition, which would then include the statement: friend class B; (but still require a complete definition of class A prior to referring to members of A in B.cpp implementation.)

    By restricting only a particular method or ctor of B as friend of A, as I did, the complete class definition of B must be available at the point of class A definition. Thus began the dance of headers. [you, in particular, probably already know this, just being explicit since that's generally a good thing ]
    Last edited by dslowik; 05-31-2017 at 04:36 PM.

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