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Bidirectionally Speaking

Given a design that requires a bidirectional association between two peer classes:


here is how it can be implemented in C++:


Note: I’m using structs here instead of classes to keep the code in this post less verbose

The key to implementing a bidirectional association in C++ is including a forward struct declaration statement in each of the header files. If you try to code the bidirectional relationship with #include directives instead of forward declarations:


you’ll have unknowingly caused a downstream problem because you’ve introduced a circular dependency into the compiler’s path. To show this, let’s implement the following larger design:




Now, if we try to create a Controller object in a .cpp file like this:


the compiler will be happy with the forward class declarations implementation. However, it will barf on the circularly dependent #include directives implementation. The compiler will produce a set of confusing errors akin to this:


I can’t remember the exact details of the last time I tried coding up a design that had a bidirectional relationship between two classes, but I do remember creating an alternative design that did not require one.

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