class declaration over different files

Question

I am not able to find a similar question else where on this site, but is it possible to declare a class over two different files.

for example, all public class components in a file and privates and others in a different file.

publics.h

    class test {
     public:
        int geta();
        void seta(int); 
    };

privates.h

    class test {
     private:
        int a;
    };

The above way is definitely wrong, but is there any such method.

["pImpl idiom"](https://stackoverflow.com/questions/60570/why-should-the-pimpl-idiom-be-used) — Mooing Duck, Sep 17 '19 at 00:21

Fred Foo · Accepted Answer · 2012-07-03T10:03:59.960

8

There is a way to get something quite similar: private inheritance.

// private.hpp
class test_details {
  protected:
    int a;
};

// public.hpp

#include "private.hpp"

class test : private test_details {
  public:
    int geta() const { return a; }
    void seta(int i) { a = i; }
};

Note that you will still need to (indirectly) include the private header in any module that uses the public class, so you're not really hiding anything this way.

edited Jul 03 '12 at 10:03

answered Jul 03 '12 at 09:49

Fred Foo

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haha. I like the solution very much. clever. clean. But I'll wait before marking yours as the best answer. – nagavamsikrishna Jul 03 '12 at 09:53
Well, just that the private data-members and functions are not portable. So we wanted them to be in a different location altogether. Your solution solves the problem. (I can't stop smiling at the solution!!) – nagavamsikrishna Jul 03 '12 at 10:57

score 6 · Answer 2 · answered Jul 03 '12 at 09:43

Not like that, but the pimpl idiom (or opaque pointer, or Chesshire cat) can help you achieve similar functionality - you can provide a public interface where all implementation details are hidden in an implementation member.

C++ doesn't support partial classes.

Also, note that what you have there are class definitions, not declarations. C++ mandates that if multiple definitions of a class are available, they must be identical, otherwise it's undefined behavior.

Michael Anderson · Answer 3 · 2019-09-17T00:18:59.643

1

This is a good use case for an abstract base class

 //File test.h
 class test {
     public:
        virtual ~test() {}
        virtual int geta()=0;
        virtual void seta(int)=0; 
    };

 //File test_impl.h
 class test_impl : public test {
     public:
        int geta() { return a; }
        void seta(int a ) { a = v; }
     private:
        int a;
    };

edited Sep 17 '19 at 00:18

answered Jul 03 '12 at 09:45

Michael Anderson

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You need pure `virtual` function(s) for an abstract base class. – hmjd Jul 03 '12 at 09:47
`test` should probably have a virtual destructor. – Frerich Raabe Jul 03 '12 at 09:48
@hmjd agreed - copy, paste oversite - fixed – Michael Anderson Jul 03 '12 at 09:58

class declaration over different files

3 Answers3