Question

I’m having a bit of trouble with a method I’m trying to write for a class. I have class symbol and class terminal. class terminal extends class symbol, but one of the methods of class symbol needs to return a vector. E.g.:

#ifndef SYMBOL_H
#define SYMBOL_H

#include "terminal.h"
#include <vector>

using namespace std;

class symbol {
   public:
        vector<terminal> first();
        virtual void polymorphable();
};

#endif

With class terminal defined:

#ifndef TERMINAL_H
#define TERMINAL_H

#include "symbol.h"

using namespace std;

class terminal: public symbol {
    // ...
};

#endif

However, when doing this, I get two errors when building, with one or the other coming first: “‘terminal’ : undeclared identifier” on the line that defines the vector function, and “‘symbol’ : base class undefined” on the line with the terminal class definition.

How do I solve this ‘a requires b’, ‘b requires a’ issue?

Answer 1

Avoid circular dependencies by using Forward Declarations.

class terminal;

class symbol
{
  std::vector<terminal> first();
  // ...
};

There is a speculation of this approach being Undefined as per C++ standard.
@Ben Voight points out:

C++03 standard Section 17.6.4.8 says:

“In particular, the effects are undefined in the following cases: … if an incomplete type is used as a template argument when instantiating a template component, unless specifically allowed for that component…

Whether std::vector<X> f(); is std::vector<X> an instantiation, is being discussed here. If the answer there proves it is then this answer holds no good and I will delete the same, or else this stays valid.