Question:
I wrote:

 class Adjective {   Sentence adjective_sentence; };   class Noun {   Adjective a; };   class NounPhrase {   Noun n; };   class Sentence {   NounPhrase np; };  

Of course it doesn’t work, because I try to create an object before its class is defined. The opposite order (Sentence-NounPhrase-Noun-Adjective) also doesn’t work now because in Sentence I declare a nounphrase object.

It must be a simple problem to solve, because I tried this in Java and worked perfectly.

Answer:
Instead of creating complete objects as data members, forward declare all classes, and then use pointers or references as data members:

class Sentence; // fwd declarationclass Noun; // ditto class Adjective {   Sentence * padjective_sentence; //fine };   class Noun {   Adjective & radj; // reference, also fine };   class NounPhrase {   Noun * pnoun; };

]

In a separate .cpp file, where the member functions of these classes are defined, you can allocate the object members dynamically and assign their address to the pointers or references:

#include "sentence.h"#include "adjective.h"#include "noun.h"#include "nounphrase.h"Adjective::Adjective {   padjective_sentence new Sentence;  }Adjective::~Adjective {   delete padjective_sentence;}