tardy 0.0.1

Runtime polymorphism

To use this package, run the following command in your project's root directory:

tardy - runtime polymorphism without inheritance

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import tardy;

interface ITransformer {
    int transform(int) @safe pure const;
alias Transformer = Polymorphic!ITransformer;

int xform(Transformer t) {
    return t.transform(3);

struct Adder {
    int i;
    int transform(int j) @safe pure const { return i + j; }

struct Plus1 {
    int transform(int i) @safe pure const { return i + 1; }

unittest {
    assert(xform(Transformer(Adder(2))) == 5);
    assert(xform(Transformer(Adder(3))) == 6);

    assert(xform(Transformer(Plus1())) == 4);


Traditional inheritance-based runtime polymorphism has a few drawbacks:

  • Classes must inherit the memory layout or their parent classes.
  • Bakes in reference semantics.
    • Must be careful with "copies" (actually references)
    • All instances are nullable
  • User-defined classes must be written with inheritance in mind so using third-party code is sometimes not possible without a wrapper. In other words, the set of types that can participate is closed.
  • Mandatory heap allocations for the virtual table and the instance...
  • ... which means ownership issues if not using the GC
  • Doesn't work well with algorithms expecting values (consider sorting)

Louis Dionne explained it better and in more detail in his talk.

Tardy makes it so:

  • Structs, classes, and other values (ints, arrays, etc. via UFCS) can implement an interface.
  • The resulting instances have value semantics.
  • An allocator can be specified for storage allocation (defaults to the GC).
  • Function attributes can be specified for the generated copy constructor and destructor for instances.

Creating instances

Instances may be created by passing a value to Polymorphic's constructor or by emplacing them and having Polymorphic call the instance type's contructor itself. The examples above show how to construct using a value. To explicitly instantiate a particular type:

auto t = Polymorphic!MyInterface.create!MyType(arg0, arg1, arg2, ...);

One can also pass modules to create where Polymorphic should look for UFCS candidates:

// Using the `Transfomer` example above, and assuming there's a UFCS function in "mymod",
// this constructs an `int` "instance"

auto t = Transformer.create!("mymod")(42);

Specifying function attributes for the copy constructor and destructor

The vtable type is constructed at compile-time by reflecting on the interface passed as the first template parameter passed to Polymorphic. To not overly constrain what users may do with their types (@safe, pure, ...), the default is @safe, but attributes can be specified for each of these compiler-generated member functions:

interface MyInterface {
    import std.traits: FA = FunctionAttribute;
    enum CopyConstructorAttrs = FA.safe | FA.pure_;
    enum DestructorAttrs = FA.pure_ | FA.nogc;
  • Atila Neves
0.0.1 2020-Jun-13
~master 2020-Jul-01
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