Question

Is it possible to make a C++ header file (.h) that declares a class, and its public methods, but does not define the private members in that class? I found a few pages that say you should declare the class and all its members in the header file, then define the methods separately in you cpp file. I ask because I want to have a class that is defined in a Win32 DLL, and I want it to be properly encapsulated: the internal implementation of that class might change, including its members, but these changes should not affect code that uses the class.

I guess that if I had this, then it would make it impossible for the compiler to know the size of my objects ahead of time. But that should be fine, as long as the compiler is smart enough to use the constructor and just pass around pointers to the location in memory where my object is stored, and never let me run “sizeof(MyClass)”.

Update: Thanks to everyone who answered! It seems like the pimpl idiom is a good way to achieve what I was talking about. I’m going to do something similar:

My Win32 DLL file will have a bunch of separate functions like this:

void * __stdcall DogCreate();
int __stdcall DogGetWeight(void * this);
void __stdcall DogSetWeight(void * this, int weight);

This is the typical way the Microsoft writes their DLL files so I think there is probably good reason for it.

But I want to take advantage of the nice syntax C++ has for classes, so I’ll write a wrapper class to wrap up all of these functions. It will have one member, which will be “void * pimpl”. This wrapper class will be so simple that I might as well just declare it AND define it in the header file. But this wrapper class really has no purposes other than making the C++ code look pretty as far as I can tell.

Answer 1

I think what you are looking for is something called the “pimpl idiom”. To understand how this works, you need to understand that in C++ you can forward declare something like so.

class CWidget; // Widget will exist sometime in the future
CWidget* aWidget;  // An address (integer) to something that 
                   // isn't defined *yet*

// later on define CWidget to be something concrete
class CWidget
{
     // methods and such 
};

So to forward declare means to promise to fully declare a type later. Its saying “there will be this thing called a CWidget, I promise. I’ll tell you more about it later.”.

The rules of forward declaration say that you can define a pointer or a reference to something that has been forward declared. This is because pointers and references are really just addresses-a number where this yet-to-be-defined thing will be. Being able to declare a pointer to something without fully declaring it is convenient for a lot of reasons.

Its useful here because you can use this to hide some of the internals of a class using the “pimpl” method. Pimpl means “pointer to implementation”. So instead of “widget” you have a class that is the actual implementation. The class you are declaring in your header is just a pass-through to the CImpl class. Here’s how it works:

// Thing.h

class CThing
{
public:
    // CThings methods and constructors...
    CThing();
    void DoSomething();
    int GetSomething();
    ~CThing();
private:
    // CThing store's a pointer to some implementation class to 
    // be defined later
    class CImpl;      // forward declaration to CImpl
    CImpl* m_pimpl;  // pointer to my implementation
};

Thing.cpp has CThing’s methods defined as pass-throughs to the impl:

// Fully define Impl
class CThing::CImpl
{
private:
     // all  variables
public:
     // methods inlined
     CImpl()
     {
          // constructor
     }

     void DoSomething()
     {
          // actual code that does something
     }
     //etc for all methods     
};

// CThing methods are just pass-throughs
CThing::CThing() : m_pimpl(new CThing::CImpl());
{
}  

CThing::~CThing()
{
    delete m_pimpl;
}

int CThing::GetSomething()
{
    return m_pimpl->GetSomething();
}

void CThing::DoSomething()
{
    m_impl->DoSomething();
}

tada! You’ve hidden all the details in your cpp and your header file is a very tidy list of methods. Its a great thing. The only thing you might see different from the template above is that people may use boost::shared_ptr<> or other smart pointer for the impl. Something that deletes itself.

Also, keep in mind this method comes with some annoyances. Debugging can be a tad bit annoying (extra level of redirection to step through). Its also a lot of overhead for creating a class. If you do this for every class, you’ll get tired of all the typing :).