gj5 gj5 - 21 days ago 7
C++ Question

error: Class has not been declared despite header inclusion, and the code compiling fine elsewhere

So I have a class included in another class that keeps throwing a compile error of the form "error: 'ProblemClass' has not been declared. The files are set up thusly:

#ifndef PROBLEMCLASS_H
#define PROBLEMCLASS_H

#include <iostream>
#include <cmath>

class ProblemClass
{
public:

virtual void Init() = 0;
};

#endif


and the class where the error occurs looks like this:

#ifndef ACLASS_H
#define ACLASS_H

#include "problemclass.h"

class AClass : public Base
{
public:

void DoSomething(ProblemClass* problem);

};

#endif


The compile error occurs at void Dosomething();

I'm aware the code here isn't enough to solve the problem. I've been unable to create a minimal example that can reproduce it. So my question is much more general; what sort of things might cause this? Is there anything in particular I should look for, or some line of enquiry I should be following to track it down?

This code compiles fine in an almost identical version of the project.

Help of any sort would be greatly appreciated, no matter how vague. I'm using codeblocks 10.05 with mingw4.4.1 in win 7 64 bit.

Answer

You seem to be saying that the code you are showing doesn't actually produce the compiler error that you are having a problem with. So we can only guess. Here are some possibilities:

  • You could have forgot to include problemclass.h from the file where you are using ProblemClass.
  • You could have misspelled the name of ProblemClass either in its own header file or in the place where you are using it. This can be hard to spot if it is a capitalization error such as writing Problemclass or problemClass instead of ProblemClass.
  • You could have copy-pasted your inclusion guard #defines from one header file to another and then forgot to change the defined names. Then only the first of those two included header files would take effect.
  • You could have placed ProblemClass in a namespace A, in which case you must refer to ProblemClass as A::ProblemClass if you are referring to it from outside the namespace A.
  • You may be using templates and not expecting two-phase lookup to work the way it does.
  • You could have misspelled the file name in your include. The compiler would not report an error on that if you also have an old version of that file under the misspelled name.
  • You could have made ProblemClass a macro that only gets defined after you include problemclass.h, in which case what you see as ProblemClass gets replaced by something else by the macro preprocessor.
  • You could have defined ProblemClass in a header file other than problemclass.h and then problemclass.h actually defines something else.