The preprocessor inserts the contents of the files A.h and B.h exactly where the include statement occurs (this is really just copy/paste). When the compiler then parses A.cpp, it finds the declaration of class A before it knows about class B. This causes the error you see. There are two ways to solve this:
Include B.h in A.h. It is generally a good idea to include header files in the files where they are needed. If you rely on indirect inclusion though another header, or a special order of includes in the compilation unit (cpp-file), this will only confuse you and others as the project gets bigger.
If you use member variable of type B in class A, the compiler needs to know the exact and complete declaration of B, because it needs to create the memory-layout for A. If, on the other hand, you were using a pointer or reference to B, then a forward declaration would suffice, because the memory the compiler needs to reserve for a pointer or reference is independent of the class definition. This would look like this:
class B; // forward declaration
class A {
public:
A(int id);
private:
int _id;
B & _b;
};
This is very useful to avoid circular dependencies among headers.
I hope this helps.
error ‘Class’ does not name a type
Just in case someone does the same idiotic thing I did …
I was creating a small test program from scratch and I typed Class instead of class (with a small C). I didn’t take any notice of the quotes in the error message and spent a little too long not understanding my problem.
My search for a solution brought me here so I guess the same could happen to someone else.