What special considerations are needed when forward declarations are used with member objects?, C++ FAQ

C++ FAQ

From Marshall Cline: Bjarne Stroustrup, Herb Sutter, Andrei Alexandrescu, Pearson / Addison-Wesley Publishers and I collaborated to create a new C++ Super-FAQ! It's a team effort, with huge contributions from each of us and with amazing support from dozens of brilliant editors. The result is "awesomer" than ever! I originally wrote/published the FAQ in 1991 and now look forward to this new phase - and to continue working with it for another 20 years!

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FAQ: What special considerations are needed when forward declarations are used with member objects? ←(in the new Super-FAQ)

It's in Section: Miscellaneous technical issues:

FAQ: How do I convert a value (a number, for example) to a std::string?
FAQ: How do I convert a std::string to a number?
FAQ: Can I templatize the above functions so they work with other types?
FAQ: What should be done with macros that contain if?
FAQ: What should be done with macros that have multiple lines?
FAQ: What should be done with macros that need to paste two tokens together?
FAQ: Why can't the compiler find my header file in #include "c:\test.h" ?
FAQ: What are the C++ scoping rules for for loops?
FAQ: Why can't I overload a function by its return type?
FAQ: What is "persistence"? What is a "persistent object"?
FAQ: How can I create two classes that both know about each other?
FAQ: What special considerations are needed when forward declarations are used with member objects? (this FAQ)
FAQ: What special considerations are needed when forward declarations are used with inline functions?
FAQ: Why can't I put a forward-declared class in a std::vector<>?
FAQ: Why do some people think x = ++y + y++ is bad?
FAQ: What's the deal with "sequence points"?