I\'m having a hard time with this concept. What is Stroustrup getting at here? W

Question

I'm having a hard time with this concept. What is Stroustrup getting at here? What is special about a class whose "representation is part of its definition"? What does a "concrete type" contrast with? (I assume it contrasts with "abstract type", but since, AFAIK, you can't even bring an instance of an abstract type into existence, it seems obvious you couldn't place that on the stack, initialize it, etc.)

Is there such a thing as a class I could instantiate that would NOT fit this description of a "concrete class"? Normally I find BS very easy to follow, but I'm missing the point here.

The basic idea of concrete classes is that they behave

Explanation / Answer

You've pretty much got it right, a concrete type is something that you can create an instance of, while an abstract type is not. For example, consider a typical pedagogical hierarchy such as:

class animal {
virtual void noise() = 0;
}
class dog: public animal {
void noise() { cout << "bark "; }
public:
human *master;
}
class cat: public animal {
void noise() { cout << "meow "; }
public:
std::vector<human *> slaves;
}
You can create instances of dog or cat because they are concrete, and not abstract. On the other hand, you can't create an instance of an animal, but only hold a pointer (or refernce) to one. For example:

void hungry(animal *a) {
a->noise();
}
In terms that Stroustrup is using, the representation of an animal is not included in the definition of class animal. The representation for a particular kind of animal is described in a subclass, here either dog or cat.

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