Skip to main content

Boost C++ Idioms

This time lets take a brief look at some nifty C++ idioms in the Boost peer-reviewed libraries. We will talk about Boost Base-from-Member idiom, Boost Safe bool idiom, Boost Named External Argument idiom, Boost Non-member get() idiom, Boost Meta-function wrapper idiom, Boost Iterator pair idiom, and the Boost Mutant idiom.

  • Boost Base-from-Member idiom

  • This idiom is used to initialize a base class from a data-member of the derived class. It sounds contradictory to the rules of C++ language and that is the reason why this idiom is present. It basically boils down to pushing the parameter data member in a private base class and put that private base class before the dependent base class in the derivation order. A generalization of the technique can be found here.

  • Boost Safe bool idiom

  • Many authors have talked about the evils of type conversion functions defined in a class. Such functions allow the objects of that type to participate in nonsensical expressions. One good example is in standard C++:

    std::cout << std::cin << std::cerr; // Compiles just fine!

    The safe bool idiom invented by Peter Dimov eliminates these problems. It is used in std::auto_ptr, boost::shared_ptr. Bjorn Karlsson, the author of the book Beyond the C++ Standard Library, tell us about this nifty technique called the safe bool idiom in his article on Artima.

  • Boost Named External Argument idiom

  • It is a technique to pass parameters to included header files! Quickest way to learn about this idiom is to read this one paragraph.

  • Boost Non-member get() idiom

  • In Cheshire Cat idiom or pimpl like idioms, accessing non-const functions of the pointee wrapped inside a const wrapper object is a problem. I discussed a technique to have const-overloaded arrow operator to avoid such an accident. This is important because the user of the wrapper does not (and should not) know that it is in-fact using pimpl idiom. The Boost non-member get() idiom deals with the same problem, albeit differently, in the context of value_initialized objects. It also used const-overloaded versions of get() functions.

  • Boost Meta-function wrapper idiom

  • The compilers that don't support template template parameters, meta-function wrapper (a.k.a. Policy Clone idiom) is useful to instantiate a clone of template parameter. Essentially, it says, "I don't know what type you are, and I don't know how you were parameterized, but I want an exact clone of you, which is parameterized with type T (T is known)."
    Note that it is one rare place where we have to use both the keywords typename and template in the typedef. For compilers that support template template parameters, there is no need to use this idiom. Arguably this idiom is more general than template template parameters.

    Not much information other than simple googling is available about the remaining two idioms: Boost Iterator pair idiom and Boost Mutant idiom. Any inputs are more than welcome!

    Comments

    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

    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

    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