tardy - runtime polymorphism without inheritance

What?

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);
}

Why?

Traditional inheritance-based runtime polymorphism has a few drawbacks:

• Classes must inherit the memory layout of 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 one of "mymod0" or "mymod1",
// this constructs an `int` "instance"

auto t = Transformer.create!("mymod0", "mymod1")(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;
}