problem_26.md

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Problem 26: Collecting Stats

2017-11-23

I want to know how many Students I create.

class Student {
        int assns, mt, finals;
        static int count;   // Associated with the class, not one per object
    public:
        Student(...) { ++count; }
        static int getCount() { return count; } // static methods
};

• static methods have no this parameter
  • Thus not really a method, more like scoped function

.cc

int Student::count = 0; // must define the variable

Now

Student s1{...}, s2{...}, s3{...};

std::cout << Student::getCount() << std::endl;

Now I want to count objects in other classes. How do we abstract the solution into reusable code.

template<typename T> struct Count {
        static int count;
        Count() { ++count; }
        Count(const Count &) { ++count; }
        Count(const &&) { ++count; }
        ~Count() { --count; }
        static int getCount() { return count; }
};

template<typename T> int Count<t>::count = 0;

class Student: count<Student> {
        int assns, mt, final;
    public:
        Student(...): ...
        // accessors
        using Count<Student>::getCount; // Make this function visible 
}

• Private Inheritance
  • inherits Count's implementation without creating an "is-a" relationship
  • Members of Count become private in Student

Now we can easily add it to other classes:

class Book:Count<Book> {
        ...
    public:
        using Count<Book>::getCount;
};

Why is Count a template?

• So that for each class C, class C: Count<C> creates a new unique, instantiation of Count for each C. This gives C its own counter vs. sharing one counter over all subclasses

This technique (inheriting from a template specialized by yourself)

• Looks weird, but happens enough to have its own name: The Curiously Recurring Template Pattern (CRTP)

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