Skip to main content

Inheritance vs std::variant

C++17 added std::variant and std::visit in its repertoire. They are worth a close examination. I've been wondering about whether they are always better than inheritance for modeling sum-types (fancy name for discriminated unions) and if not, under what circumstances they are not. We'll compare the two approaches in this blog post. So here it goes.

Inheritancestd::variant
Need not know all the derived types upfront (open-world assumption)Must know all the cases upfront (closed-world assumption)
Dynamic Allocation (usually)No dynamic allocation
Intrusive (must inherit from the base class)Non-intrusive (third-party classes can participate)
Reference semantics (think how you copy a vector of pointers to base class?)Value semantics (copying is trivial)
Algorithm scattered into classesAlgorithm in one place
Language supported (Clear errors if pure-virtual is not implemented)Library supported (poor error messages)
Creates a first-class abstractionIt’s just a container
Keeps fluent interfacesDisables fluent interfaces. Repeated std::visit
Supports recursive types (Composite)Must use recursive_wrapper and dynamic allocation. Not in the C++17 standard.
Adding a new operation (generally) boils down to implementing a polymorphic method in all the classesAdding a new operation simply requires writing a new free function
Use the Visitor design pattern to get some of the benefits of the right hand side

std::variant and std::visit idiom resembles closely to pattern matching in functional languages. However, functional languages are leaps and bounds ahead of the current capabilities of the pattern matching idiom.
  1. Generally, pattern matching makes it easy to dissect a variant type (or even a structured type) by the "shape" of the type and its content. I.e., a more powerful pattern matching in C++ would use structured bindings in some way. 
  2. Conditional structured bindings would match only if the condition is satisfied. 
  3. Matching on the dynamic type of the variable would be really nice but without paying excessive performance cost.
For all these reasons, I think this is best done as a language feature as opposed to a library supported capability. At the moment, it's great to have some basic pattern matching capabilities in C++ that allows you to use both of the styles depending on the problem you're trying to solve.

See a detailed example on slideshare.


Comments

Vittorio Romeo said…
Nice comparison. I created a "pattern-matching like" library to simplify variant/optional declaration/visitation: https://github.com/SuperV1234/scelta

You might find it useful if you work with variants/optionals a lot.
Tue Feb 20, 04:43:00 AM PST
Hassan Raza said…
Nice Information very useful for us..
Tue Mar 27, 05:46:00 AM PDT
Post a Comment

Popular Content

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 annoyin
24 comments
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
17 comments
Read more

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