Заголовочный файл C++, который объявляет класс и методы, но не участников?

Google "идиома прыща" или "ручка C ++".

The pimpl idiom adds a void* private data member to your class, and this is a useful technique if you need something quick & dirty. It has its drawbacks however. Main among those is it makes it difficult to use polymorphism on the abstract type. Sometimes you might want an abstract base class and subclasses of that base class, collect pointers to all the different types in a vector and call methods on them. In addition, if the purpose of the pimpl idiom is to hide the implementation details of the class then it only almost succeeds: the pointer itself is an implementation detail. An opaque implementation detail, perhaps. But an implementation detail nonetheless.

An alternative to the pimpl idiom exists which can be used to remove all of the implementation details from the interface while providing a base type that can be used polymorphically, if needed.

In your DLL's header file (the one #included by client code) create an abstract class with only public methods and concepts which dictate how the class is to be instantiated (eg, public factory methods & clone methods):

kennel.h

/****************************************************************
 ***
 ***    The declaration of the kennel namespace & its members
 ***    would typically be in a header file.
 ***/

// Provide an abstract interface class which clients will have pointers to.
// Do not permit client code to instantiate this class directly.

namespace kennel
{
    class Animal
    {
    public:
        // factory method
        static Animal* createDog(); // factory method
        static Animal* createCat(); // factory method

        virtual Animal* clone() const = 0;  // creates a duplicate object
        virtual string speak() const = 0;   // says something this animal might say
        virtual unsigned long serialNumber() const = 0; // returns a bit of state data
        virtual string name() const = 0;    // retuyrns this animal's name
        virtual string type() const = 0; // returns the type of animal this is

        virtual ~Animal() {};   // ensures the correct subclass' dtor is called when deleteing an Animal*
    };
};

...Animal is an abstract base class and so cannot be instantiated; no private ctor needs to be declared. The presence of the virtual dtor ensures that if someone deletes an Animal*, the proper subclass' dtor will also be called.

In order to implement different subclasses of the base type (eg dogs & cats), you would declare implementation-level classes in your DLL. These classes derive ultimately from the abstract base class you declared in your header file, and the factory methods would actually instantiate one of these subclasses.

dll.cpp:

/****************************************************************
 ***
 ***    The code that follows implements the interface
 ***    declared above, and would typically be in a cc
 ***    file.
 ***/   

// Implementation of the Animal abstract interface
// this implementation includes several features 
// found in real code:
//      Each animal type has it's own properties/behavior (speak)
//      Each instance has it's own member data (name)
//      All Animals share some common properties/data (serial number)
//

namespace
{
    // AnimalImpl provides properties & data that are shared by
    // all Animals (serial number, clone)
    class AnimalImpl : public kennel::Animal    
    {
    public:
        unsigned long serialNumber() const;
        string type() const;

    protected:
        AnimalImpl();
        AnimalImpl(const AnimalImpl& rhs);
        virtual ~AnimalImpl();
    private:
        unsigned long serial_;              // each Animal has its own serial number
        static unsigned long lastSerial_;   // this increments every time an AnimalImpl is created
    };

    class Dog : public AnimalImpl
    {
    public:
        kennel::Animal* clone() const { Dog* copy = new Dog(*this); return copy;}
        std::string speak() const { return "Woof!"; }
        std::string name() const { return name_; }

        Dog(const char* name) : name_(name) {};
        virtual ~Dog() { cout << type() << " #" << serialNumber() << " is napping..." << endl; }
    protected:
        Dog(const Dog& rhs) : AnimalImpl(rhs), name_(rhs.name_) {};

    private:
        std::string name_;
    };

    class Cat : public AnimalImpl
    {
    public:
        kennel::Animal* clone() const { Cat* copy = new Cat(*this); return copy;}
        std::string speak() const { return "Meow!"; }
        std::string name() const { return name_; }

        Cat(const char* name) : name_(name) {};
        virtual ~Cat() { cout << type() << " #" << serialNumber() << " escaped!" << endl; }
    protected:
        Cat(const Cat& rhs) : AnimalImpl(rhs), name_(rhs.name_) {};

    private:
        std::string name_;
    };
};

unsigned long AnimalImpl::lastSerial_ = 0;


// Implementation of interface-level functions
//  In this case, just the factory functions.
kennel::Animal* kennel::Animal::createDog()
{
    static const char* name [] = {"Kita", "Duffy", "Fido", "Bowser", "Spot", "Snoopy", "Smkoky"};
    static const size_t numNames = sizeof(name)/sizeof(name[0]);

    size_t ix = rand()/(RAND_MAX/numNames);

    Dog* ret = new Dog(name[ix]);
    return ret;
}

kennel::Animal* kennel::Animal::createCat()
{
    static const char* name [] = {"Murpyhy", "Jasmine", "Spike", "Heathcliff", "Jerry", "Garfield"};
    static const size_t numNames = sizeof(name)/sizeof(name[0]);

    size_t ix = rand()/(RAND_MAX/numNames);

    Cat* ret = new Cat(name[ix]);
    return ret;
}


// Implementation of base implementation class
AnimalImpl::AnimalImpl() 
: serial_(++lastSerial_) 
{
};

AnimalImpl::AnimalImpl(const AnimalImpl& rhs) 
: serial_(rhs.serial_) 
{
};

AnimalImpl::~AnimalImpl() 
{
};

unsigned long AnimalImpl::serialNumber() const 
{ 
    return serial_; 
}

string AnimalImpl::type() const
{
    if( dynamic_cast<const Dog*>(this) )
        return "Dog";
    if( dynamic_cast<const Cat*>(this) )
        return "Cat";
    else
        return "Alien";
}

Now you have the interface defined in the header & the implementation details completely seperated out where client code can't see it at all. You would use this by calling methods declared in your header file from code that links to your DLL. Here's a sample driver:

main.cpp:

std::string dump(const kennel::Animal* animal)
{
    stringstream ss;
    ss << animal->type() << " #" << animal->serialNumber() << " says '" << animal->speak() << "'" << endl;
    return ss.str();
}

template<class T> void del_ptr(T* p)
{
    delete p;
}

int main()
{
    srand((unsigned) time(0));

    // start up a new farm
    typedef vector<kennel::Animal*> Animals;
    Animals farm;

    // add 20 animals to the farm
    for( size_t n = 0; n < 20; ++n )
    {
        bool makeDog = rand()/(RAND_MAX/2) != 0;
        if( makeDog )
            farm.push_back(kennel::Animal::createDog());
        else
            farm.push_back(kennel::Animal::createCat());
    }

    // list all the animals in the farm to the console
    transform(farm.begin(), farm.end(), ostream_iterator<string>(cout, ""), dump);

    // deallocate all the animals in the farm
    for_each( farm.begin(), farm.end(), del_ptr<kennel::Animal>);

    return 0;
}

Use pimpl idiom.

Yes, this can be a desireable thing to do. One easy way is to make the implementation class derive from the class defined in the header.

The downside is that the compiler won't know how to construct your class, so you'll need some kind of factory method to get instances of the class. It will be impossible to have local instances on the stack.

Вы должны объявить все члены в заголовке, чтобы компилятор знал, насколько велик объект, и так далее.

Но вы можете решить эту проблему с помощью интерфейса:

ext.h:

class ExtClass
{
public:
  virtual void func1(int xy) = 0;
  virtual int func2(XYClass &param) = 0;
};

int.h:

class ExtClassImpl : public ExtClass
{
public:
  void func1(int xy);
  int func2(XYClass&param);
};

int.cpp:

  void ExtClassImpl::func1(int xy)
  {
    ...
  }
  int ExtClassImpl::func2(XYClass&param)
  {
    ...
  }

Check out the class The Handle-Body Idiom in C++

Is it possible to make a C++ header file (.h) that declares a class, and its public methods, but does not declare the private members in that class?

Самый близкий ответ - идиома PIMPL.

См. это Идиома быстрого Pimpl от Herb Sutter.

IMO Pimpl действительно полезен на начальных этапах разработки, где ваш заголовочный файл буду менять много раз. Pimpl имеет свою стоимость из-за размещения \ освобождения внутреннего объекта в куче.