OOP composition with mixins in D

I have recently given a very small talk for a Berlin D meetup about approaches to make OOP style designs more robust with tools available in D. Normally I am quite quite skeptical about OOP in general and prefer to write code in a mix of functional and generic style but often there is that big chunk of old code that works just fine and you are not going to rewrite it just to switch the paradigm. Tweaking slightly for improved maintainability totally makes sense though.

Common Approach

Looking through existing source of what is called "OOP-style code" you most commonly find design revolving about two basic principles:

  1. Defining reused code in base class methods
  2. Tweaking behavior by overriding some of those methods

This is dominating approach in Java and very common in C++ projects. It was also relatively common design style in D during old days of D1, affecting also Tango library we still use in our (Sociomantic) projects.

Funny thing about this approach is that it has been discouraged as bad OOP style for a very long time.


Favor 'object composition' over 'class inheritance' (Gang of Four, 1994)

This a quote from a famous "Design Patterns" book, still one of most respected works about practical application of OOP principles. And of all patterns and advices this has probably been the least recognized one in real projects. Sadly.

There is even a dedicated wiki page for that.

What makes inheritance-driven code reuse bad?

  1. It results in bunch of base classes that don't represent any meaningful abstraction, violating OOP concept of "object". Usually when you see a class named "SomethingBase" in your code, this is a clear sign something has gone wrong.
  2. It doesn't naturally scale unless your language allows multiple class inheritance (which will cause you suffering for other reasons). Adding new reusable utilities to derived classes becomes impossible without refactoring whole involved class hierarchy.
  3. Much less flexible than it may seem from simple cases. You are inherently limited to only overriding methods that are not final and which have been defined as distinct separate methods from the very beginning. If the base class author was not good enough at predicting all use cases (and nobody is perfect), you are pretty much doomed into a lot of copy-paste.

Sadly, those issues tend to only come up when applications reach maintenance cycle when reconsidering is not really an option anymore. Which starts a usual painful process of moving classes around and trying to reason with incredibly complicated hierarchies people so commonly associate with OOP.


Composition is an alternative approach for code reuse which implies that you define necessary utilities as small independent objects (as small as possible) and embed them within target class as building blocks, usually as private fields. Thus the name -- the programmer is supposed to compose such blocks into any set of features the target class needs.

I have a feeling that one of reasons the old advice of GoF was ignored so often in mainstream languages is that those simply don't provide any useful tools to minimize the involved boilerplate. Fortunately, D has something to offer here:

I will focus on template mixins here because it is a tool also available in D1 and thus more applicable to my daily job :)

Template Mixins : Basics

Essentially template mixins are a more hygienic and controlled way to copy declarations:

mixin template InjectMe(T)
    T var;

class C
    mixin InjectMe!int;

static assert (is(typeof(C.var) == int));

(declaring the template as a mixin template is not necessary but it's useful to convey the usage intentions)

To resolve naming conflicts, one can use named mixins:

class C
    mixin InjectMe!int name;

static assert (is(typeof(C.name.var) == int));
static assert (is(typeof(C.var) == int));

Usage of named mixins is highly recommended because, as you can see from this example, in the absence of name conflict, "short" version also works - thus you don't lose anything but it makes the code more forward-compatible if new fields get added later.

Template Mixins : Alias

However what makes template mixins truly shine as a composition tool is the template alias parameter.

import std.range.primitives : isInputRange;
import std.traits : isArray;

// makes host aggregate an input range which
// consumes target array when iterated but does
// not allow to modify its elements
mixin template ConsumeAsRORange(alias array)
    static assert (isArray!(typeof(array)));

    bool empty() { return array.length == 0; }
    auto front() { return cast(const) array[0]; }
    void popFront() { array = array[1 .. $]; }

class C
    private int[] secret_array;
    mixin ConsumeAsRORange!secret_array;

static assert (isInputRange!C);

alias parameter is how D does "pass by name" semantics. All template arguments must be compile-time entities so you can't pass secret_array itself like that as it is a field of runtime object. Yet you can pass its symbol name and alias will make use the of context pointer to resolve a field for a proper object, acting as certain form of a delegate.

Of course you could just drop any parameters completely and use this.secret_array directly - as whole mixin body gets pasted into the aggregate, it would resolve properly. However, this a terrible coding style I heavily discourage from, one that will make your code completely unmaintainable.

The beauty of alias parameters is that they allow you to define generic reusable building blocks that hold no silent assumptions about their mixin context - all dependencies can be expressed explicitly and have full compile-time verification with nice error messages from static assert.

The main limitation of this feature is that right now it is limited to only one context pointer and is sometimes not smart enough to recognize transitivity:

class C
    struct S { int[] secret_array; }
    S s;
    // won't compile
    // need context pointer for both C and S and compiler
    // isn't smart enough yet to see latter can be retrieve
    // from the former
    mixin ConsumeAsRORange!(s.secret_array);

Design Principles

With the above-mentioned features in mind I suggest following design principles when doing composition-driven OOP with D programming language:

  • provide reusable functionality as a small building blocks
  • define all expectations for the mixin context via template parameters
  • static assert a lot (including typeof(this) verification)
  • compose actual classes from those building blocks as necessary
  • reserve inheritance to useful abstractions

Primary Candidate : Exceptions

Exceptions have their own hierarchy which, once defined, is almost impossible to change without major code breakage (on the catching side).

That makes reusing code by inheritance simply impossible. For example, we often need exceptions that build messages in a persistent mutable buffer (to avoid GC allocations each time) and this is quite a bunch of fields and methods to copy:

class MyException : IOException
    mixin ReusableExceptionImpl!();

Yet mixins make it trivial without compromising the existing exception hierarchy.


With inheritance partial tweaking of provided utilities is achieved by overriding methods. With mixin based approach there are two main options:


mixin template Inject()
    void foo() { writeln("foo injected"); }
    void bar() { writeln("bar injected"); }

struct S
    mixin Inject!();
    void bar() { writeln("bar overriden"); }

void main()
    S s; s.foo(); s.bar();

Using a named mixin allows to still refer to original version, same as with using super when overriding.


If the provided building blocks are small enough, the simplest approach is to not mixin at all and provide your own implementation of method with same signature:

struct S1
    mixin FooImpl!(); // provides `this.foo`
    mixin BarImpl!(foo);

struct S2
    void my_foo() {}
    mixin BarImpl!(my_foo);

This is better suited for cases when the mixed in functionality is mostly unrelated.

Abrupt Ending

And this where is suddenly ends because I didn't have enough time to prepare more content for the talk :) Hope it gives at least some basic insight into the topic.