An interesting simple code of forward declaration

This is a discussion on An interesting simple code of forward declaration within the C++ Programming forums, part of the General Programming Boards category; There are 2 classes, A and B: Code: --------------------------------- FileA.h --------------------------------- #include <FileB.h> class B; //Why forward declaration here class ...

  1. #1
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    An interesting simple code of forward declaration

    There are 2 classes, A and B:
    Code:
    ---------------------------------
    FileA.h
    ---------------------------------
    #include <FileB.h>
    class B; //Why forward declaration here
    class A{
    ...
    B objB;
    ...
    };
    ---------------------------------
    FileA.cc
    ---------------------------------
    #include <FileA.h>
    ...implementation of class A...
    
    
    
    
    ---------------------------------
    FileB.h
    ---------------------------------
    class A; //  I understand this is forward declaration 
    class B{
    ...
    A objA;
    ...
    };
    
    ---------------------------------
    FileB.cc
    ---------------------------------
    ...Implementation of class B
    What I don't understand is that why forward declare class B in FileA.h? Seems FileA.h already included FileB.h.

  2. #2
    Frequently Quite Prolix dwks's Avatar
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    You're right, it wouldn't technically be required to have that forward declaration, because including FileA.h would result in the following code:
    Code:
    class A; //  I understand this is forward declaration 
    class B{
    ...
    A objA;
    ...
    };
    
    class B; //Why forward declaration here
    class A{
    ...
    B objB;
    ...
    };
    However, spurious forward declarations never hurt anyone (that I've ever heard of). It might at least make it clearer to the reader that B is a class, without them having to delve into FileB.h to find out.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
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  3. #3
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    Note that your example is a little off, since the forward declaration of A in FileB.h is not enough to have a member variable of type A inside B.

    One possible reason for this in real code would be if the programmer was afraid of recursive includes. However, if a forward declaration is good enough with the #include, then the #include should be removed anyway, and if it is not, then it won't help in a situation of recursive includes, so I would argue it isn't a good idea anyway.

  4. #4
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    Hi, Daved. I don't quite understand you. Why "forward declaration of A in FileB.h is not enough to have a member variable of type A inside B." But the code can run correctly, can't it?

  5. #5
    Frequently Quite Prolix dwks's Avatar
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    If all the compiler has of a class (or structure) is a forward declaration, you can't declare an instance of that class. The best you can do is to declare a pointer to it.

    Imagine what could happen otherwise.
    Code:
    class two;
    class one { two t; };
    class two { one o; };
    
    int main() {}
    Oops. Dinkumware's online compiler complains with
    Code:
    sourceFile.cpp(2) : error C2079: 'one::t' uses undefined class 'two'
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


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    Unofficial Wiki FAQ: cpwiki.sf.net

    My website: http://dwks.theprogrammingsite.com/
    Projects: codeform, xuni, atlantis, nort, etc.

  6. #6
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    dwks explained what I was referring to. In your example, class B has a member of type A, but if a cpp file includes FileB.h without including FileA.h before it, then the compiler won't have enough information about A to compile B properly.

    No matter how the includes are done, your example cannot compile anyway. The class B has a member of type A, so to know how big B is the compiler needs to know how big A is. But A has a member of type B, so to know how big A is the compiler needs to know how big B is. But B has a member of type A... and so on.

    If you have a pointer or reference to A in B (or B in A), then it is fine, because the compiler doesn't need to know the size of A, it just needs the size of a pointer or reference and a forward declaration will do.

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