How do you declare an interface in C++?

Question

How do I setup a class that represents an interface? Is this just an abstract base class?

Answer 1

There is no concept of "interface" per se in C++. AFAIK, interfaces were first introduced in Java to work around the lack of multiple inheritance. This concept has turned out to be quite useful, and the same effect can be achieved in C++ by using an abstract base class.

An abstract base class is a class in which at least one member function (method in Java lingo) is a pure virtual function declared using the following syntax:

class A
{
  virtual void foo() = 0;
};

An abstract base class cannot be instantiated, i. e. you cannot declare an object of class A. You can only derive classes from A, but any derived class that does not provide an implementation of foo() will also be abstract. In order to stop being abstract, a derived class must provide implementations for all pure virtual functions it inherits.

Note that an abstract base class can be more than an interface, because it can contain data members and member functions that are not pure virtual. An equivalent of an interface would be an abstract base class without any data with only pure virtual functions.

And, as Mark Ransom pointed out, an abstract base class should provide a virtual destructor, just like any base class, for that matter.

Answer 2

A little addition to what's written up there:

First, make sure your destructor is also pure virtual

Second, you may want to inherit virtually (rather than normally) when you do implement, just for good measures.

Answer 3

Make a class with pure virtual methods. Use the interface by creating another class that overrides those virtual methods.

A pure virtual method is a class method that is defined as virtual and assigned to 0.

class IDemo
{
    public:
        virtual ~IDemo() {}
        virtual void OverrideMe() = 0;
};

class Child : public IDemo
{
    public:
        virtual void OverrideMe()
        {
            //do stuff
        }
};

Answer 4

All good answers above. One extra thing you should keep in mind - you can also have a pure virtual destructor. The only difference is that you still need to implement it.

Confused?

    --- header file ----
    class foo {
    public:
      foo() {;}
      virtual ~foo() = 0;

      virtual bool overrideMe() {return false;}
    };

    ---- source ----
    foo::~foo()
    {
    }

The main reason you'd want to do this is if you want to provide interface methods, as I have, but make overriding them optional.

To make the class an interface class requires a pure virtual method, but all of your virtual methods have default implementations, so the only method left to make pure virtual is the destructor.

Reimplementing a destructor in the derived class is no big deal at all - I always reimplement a destructor, virtual or not, in my derived classes.

Answer 5

Here is the definition of abstract class in c++ standard

n4687

13.4.2

An abstract class is a class that can be used only as a base class of some other class; no objects of an abstract class can be created except as subobjects of a class derived from it. A class is abstract if it has at least one pure virtual function.

Answer 6

As far I could test, it is very important to add the virtual destructor. I'm using objects created with new and destroyed with delete.

If you do not add the virtual destructor in the interface, then the destructor of the inherited class is not called.

class IBase {
public:
    virtual ~IBase() {}; // destructor, use it to call destructor of the inherit classes
    virtual void Describe() = 0; // pure virtual method
};

class Tester : public IBase {
public:
    Tester(std::string name);
    virtual ~Tester();
    virtual void Describe();
private:
    std::string privatename;
};

Tester::Tester(std::string name) {
    std::cout << "Tester constructor" << std::endl;
    this->privatename = name;
}

Tester::~Tester() {
    std::cout << "Tester destructor" << std::endl;
}

void Tester::Describe() {
    std::cout << "I'm Tester [" << this->privatename << "]" << std::endl;
}


void descriptor(IBase * obj) {
    obj->Describe();
}

int main(int argc, char** argv) {

    std::cout << std::endl << "Tester Testing..." << std::endl;
    Tester * obj1 = new Tester("Declared with Tester");
    descriptor(obj1);
    delete obj1;

    std::cout << std::endl << "IBase Testing..." << std::endl;
    IBase * obj2 = new Tester("Declared with IBase");
    descriptor(obj2);
    delete obj2;

    // this is a bad usage of the object since it is created with "new" but there are no "delete"
    std::cout << std::endl << "Tester not defined..." << std::endl;
    descriptor(new Tester("Not defined"));


    return 0;
}

If you run the previous code without virtual ~IBase() {};, you will see that the destructor Tester::~Tester() is never called.