Re: Java-like interfaces in c++

From:

dave_mikesell@fastmail.fm

Newsgroups:

comp.lang.c++

Date:

30 Mar 2007 06:08:59 -0700

Message-ID:

<1175260139.358447.265970@r56g2000hsd.googlegroups.com>

On Mar 30, 8:31 am, Morten Lemvigh <lemv...@studentergaarden.dk>
wrote:

I'm trying to make something like a Java interface. I have a class, that
needs a distance measure. The distance measure is itself a class passed
to the constructor:

MyClass::MyClass(vector v, DistanceMeasure dm)...

Then I have different specific distance measures inheriting from
DistanceMeasure (fx. CosDistance), and I would like to be able to do fx.

CosDisance cs;
Vector v;
MyClass ms(v, cs);

If I make the methods in DistanceMeasure virtual, I'm told by the
linker, that it's an undefined reference. If I make them pure virtual,
I'm told that the constructor declaration above is illegal, because dm
is abstract. If I provide a dummy implementation in DistanceMeasure it's
the DistanceMeasure methods that are called from MyClass and not the
overriding methods from CosDistance.

How can I specify an interface for a distance measure, and make MyClass
indifferent to the class implementing the interface, and use the methods
from the class provided in the constructor???

Use pointers.

class DistanceMeasure {
public:
    virtual long getDistance() = 0;
};

class CosDistance : public DistanceMeasure {
public:
    virtual long getDistance() { return 1; }
};

class MyClass {
public:
    MyClass::MyClass(DistanceMeasure * dm) {}
};

int main()
{
    DistanceMeasure * cs = new CosDistance();
    MyClass myClass(cs);

    return 0;
}