CRTP as an implementation technique
As we pointed out earlier, CRTP is often used as a purely implementation pattern; however, even in this role, it can influence design: some design choices are desirable but hard to implement, and, if a good implementation technique comes along, the design choices often change. So, let us see what problems can be solved with CRTP.
CRTP for code reuse
Let us start with a specific implementation issue: we have multiple classes that have some common code. Ordinarily, we would write a base class for them. But the common code is not really common: it does the same thing for all classes except it for the types it uses. What we need is not a common base class but a common base class template. And that brings us to CRTP.
An example is an object registry. It may be desirable, often for debugging purposes, to know how many objects of a certain type are currently in existence, and perhaps even to maintain a list of such objects. We definitely do not...
