Subclass B Inherits from template class A <B>

Question

I recently came across a code that looks like this, and I can't wrap it around:

template<typename T>
class A
{
}

class B: A<B>
{
}

---

So my general questions are:

• Why does this not give a compilation error? In particular, how does a class Binherit from a template class A<B>, if Bnot already defined?
• When will this structure ever be needed?

Answer 1

One of the features: this template template will help you avoid using vtable. This is called "Static Polymorphism" - http://en.m.wikipedia.org/wiki/Curiously_recurring_template_pattern

, :

class Item {
public:
    virtual void func() = 0;
}

class A : public Item {
// …
}

class B : public Item {
// …
}

Item *item = new A();
item->func();

:

template<typename T>
class Item {
public:
    void func() {
        T::func();
    }
}

class A : public Item<A> {
// …
}

class B : public Item<B> {
// …
}

Item<A> *item = new A();
item->func();

, . ...