Skip to main content

The Policy Clone idiom

Modern C++ library design is heavily based on templates. Highly reusable, flexible and extensible classes can be built using policy based class design techniques. (See "Modern C++ Design" by Andrei Alexandrescu.) Sometimes, the host class of the policies need to make an exact replica of one of its policies but instantiated with different type parameter. Unfortunately, the writer of the host class template does not know the template name to instantiate beforehand. This situation is quite analogous to the situation in the Factory Method (GoF) pattern where type of the object to be created is not known apriori and therefore, the object creation is simply delegated to the derived object.

In the world of templates, although the problem is conceptually is quite similar, the mechanism to solve it is a different because we are interested in a "clone" type and not an object. The Policy Clone idiom comes handy here. A member template struct (called rebind) is used to pass a different type parameter to the policy class template. For example,

template <typename T>
class NiftyAlloc
{
public:
template <typename Other>
struct rebind { /// The Policy Clone idiom
typedef NiftyAlloc<Other> other;
};
//...
};

template <typename T, class Alloc = AllocationPolicy<T> >
class Vector {
public:
typedef typename Alloc::template rebind<long>::other ClonePolicy;
};

Here, the Container template needs a replica of the allocation policy it is instantiated with. Therefore, it uses the rebind mechanism exposed by the NiftyAlloc policy. The type Alloc::template rebind<long>::other is same as NiftyAlloc<long>. Essentially, it says, "I don't know what kind of allocator this type is, and I don't know what it allocates, but I want an allocator just like it that allocates longs." To keep the compiler happy, we have to use both the keywords typename and template in the ClonePolicy typedef

The rule is as follows: If the name of a member template specialization appears after a ., ->, or :: operator, and that name has explicitly qualified template parameters, prefix the member template name with the keyword template. The Keyword typename is also necessary in the typedef because "other" is a type, not an variable. Hence the keywords!

As some of you out there might have guessed, this mechanism is used in standard STL containers and the Allocators used with them.
Labels:

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