Skip to main content

Non-Virtual Interface (NVI) idiom and the design intent

Assuming that the philosophy of Non-Virtual Interface (NVI) idiom is strictly adhered I came up with a table that summarizes the design intent in terms of access modifiers and virtuality. Strict adherence of NVI allows separation of the interface of a class into two distinct interfaces: client interface (public non-virtual) and subclass interface (protected virtual/non-virtual). Such a structure helps mitigate the Fragile Base Class (FBC) Interface problem if discipline is followed. Its only downside is a little bit of code bloat. More about this approach of resolving FBC can be found here.





























  non-virtual virtual but not pure pure virtual without body pure virtual with body
Public Clients expect substitutability. Extension points are
encapsulated.  Subclasses should stay away from this if an equivalent
protected function using NVI is given.		 Clients expect substitutability. Extension points are
visible. Subclasses can optionally extend  them but assuming NVI is
place, subclasses should not reuse it.		 Substitutability is mandatory as base class itself can't be
instantiated.		 Substitutability is mandatory as base class itself can't be
instantiated. Subclasses should call the method in the base. e.g.,
destructor.
Protected For the purpose of reuse only by subclasses. An interface
for subclasses. Beware of the Fragile Base Class (FBC) interface problem.		 An optional extension point for subclasses. FBC applies
here.		 A mandatory extension point for subclasses A mandatory extension point for subclasses and it should
call the method in the base. e.g., destructor.
Private Clients as well as subclasses have to live with it. Like
final keyword in Java. Not for reuse. Top secret
of a class.		 If you happen to know the secret, it is not for (re)use but
you can risk extending it. Don't scream if the the treasure evaporates
tomorrow.		 If you happen to know the secret, it is not for (re)use but
you must risk extending it! Surprises for tomorrow if base is updated.		 Ditto as for pure virtual without body


The structure of NVI is similar to the Thread-safe Interface pattern. The purpose of public methods in thread-safe interface is to acquire lock and let the helper functions do the real job without worrying about locking. In NVI, the public non-virtual methods do a simple job of dynamic dispatching of protected virtual helper functions.

Comments

Post a Comment

Popular Content

Unit Testing C++ Templates and Mock Injection Using Traits

Unit testing your template code comes up from time to time. (You test your templates, right?) Some templates are easy to test. No others. Sometimes it's not clear how to about injecting mock code into the template code that's under test. I've seen several reasons why code injection becomes challenging. Here I've outlined some examples below with roughly increasing code injection difficulty. Template accepts a type argument and an object of the same type by reference in constructor Template accepts a type argument. Makes a copy of the constructor argument or simply does not take one Template accepts a type argument and instantiates multiple interrelated templates without virtual functions Lets start with the easy ones. Template accepts a type argument and an object of the same type by reference in constructor This one appears straight-forward because the unit test simply instantiates the template under test with a mock type. Some assertion might be tested in...
Post a Comment
Read more

Covariance and Contravariance in C++ Standard Library

Covariance and Contravariance are concepts that come up often as you go deeper into generic programming. While designing a language that supports parametric polymorphism (e.g., templates in C++, generics in Java, C#), the language designer has a choice between Invariance, Covariance, and Contravariance when dealing with generic types. C++'s choice is "invariance". Let's look at an example. struct Vehicle {}; struct Car : Vehicle {}; std::vector<Vehicle *> vehicles; std::vector<Car *> cars; vehicles = cars; // Does not compile The above program does not compile because C++ templates are invariant. Of course, each time a C++ template is instantiated, the compiler creates a brand new type that uniquely represents that instantiation. Any other type to the same template creates another unique type that has nothing to do with the earlier one. Any two unrelated user-defined types in C++ can't be assigned to each-other by default. You have to provide a...
14 comments
Read more

Multi-dimensional arrays in C++11

What new can be said about multi-dimensional arrays in C++? As it turns out, quite a bit! With the advent of C++11, we get new standard library class std::array. We also get new language features, such as template aliases and variadic templates. So I'll talk about interesting ways in which they come together. It all started with a simple question of how to define a multi-dimensional std::array. It is a great example of deceptively simple things. Are the following the two arrays identical except that one is native and the other one is std::array? int native[3][4]; std::array<std::array<int, 3>, 4> arr; No! They are not. In fact, arr is more like an int[4][3]. Note the difference in the array subscripts. The native array is an array of 3 elements where every element is itself an array of 4 integers. 3 rows and 4 columns. If you want a std::array with the same layout, what you really need is: std::array<std::array<int, 4>, 3> arr; That's quite annoying for...
20 comments
Read more