Header class file to be used by others

Question

I would like to know if there is a way to put only protected and public stuff on the header file .h, and all private stuff in the compile unit .cpp I need this because the library is going to be used by others, and I wouldn't like to have to copy and edit all .h files to remove private declarations and implementations. I tried but got the duplicate declaration error

another question is about private static stuff can I declare them on the header file and implement them on the .cpp unit? a private variable and a public get method I tried but couldn't implement the method on the unit, it only worked with the declaration and implementation on the header

[]s, Joe

Answer 1

The proper way to deal with this is to implement the pimpl idiom: Create a class or struct for all private data and put a pointer to such an object in the header file, together with a forward declaration. Now nothing of the private data and methods is visible from the header file.

Answer 2

The correct answer to this is to use a Pimpl (via a pointer, as Pavel points out). There's also a crazy but potentially correct way described in Matthew Wilson's Imperfect C++, where you can forward declare an internal structure and include an opaque block of memory in your class, and then in-place construct the internal structure (whose definition is made in the implementation file) in the main class constructor in the implementation file.

I should point out that Wilson shows this in an appendix where he confesses to several such "crimes against programming" as a caution to programmers trying to be too clever. He says, and I say, that you shouldn't use this. However, if you have some overriding exacting performance requirements it's possible that it might be of some use.

Answer 3

Andreas has the answer but note that this will make your code somewhat more obtuse for yourself:

 // header file

 struct hidden_structure;

 class Foo {
     hidden_structure* hidden_data;
 public:
     Foo();
     ~Foo();

     void doStuff();
 };

 // your cpp file

 struct hidden_structure;
     int stuff;
     hidden_structure() : stuff(0) {}
 }

 Foo::Foo() : hidden_data(new hidden_structure) {}
 Foo::~Foo() { delete hidden_data; }

 void Foo::doStuff() { hidden_data->stuff += 34; } // hey, it does a lot of stuff

As you can see, the more data inside hidden_structure the more complex this can become.

Answer 4

Couldn't you declare the full class name for users, but put a class definition in the .h file that includes only the publics and privates, then in the .cpp file subclass from the public definition, and define only privates?

Like this:

.h file--

class useThis;   // users actually use this class

class publics {  //this is the interface they see
public:
    int foo;
};

.cpp file--

#include "foo.h"

class useThis: publics {

private:
    void add(int b);

};

void useThis::add( int b )
{
    foo+= b;
}

Answer 5

Same as John's code, but I am using a pointer instead of references:

// file.h
class TheClass_p;
class TheClass{
  public: 
    TheClass();
    ~TheClass();
  private
    TheClass_p *d;

};

// file.cpp
class TheClass_p {
  int foo;
  float: bar;
};

TheClass::TheClass(){
   d = new TheClass_p;
}

TheClass::~TheClass(){
   delete d;
}

Edit: added destructor to free a memory leak