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C3

Interfaces and Any Type

Working with the type of any at runtime.

The any type is recommended for writing code that is polymorphic at runtime where macros are not appropriate. It can be thought of as a typed void*.

An any can be created by assigning any pointer to it. You can then query the any type for the typeid of the enclosed type (the type the pointer points to) using the type field.

This allows switching over the typeid, either using a normal switch:

switch (my_any.type)
{
    case Foo.typeid:
        ...
    case Bar.typeid:
        ...
}

Or the special any-version of the switch:

switch (my_any)
{
    case Foo:
        // my_any can be used as if it was Foo* here
    case Bar:
        // my_any can be used as if it was Bar* here
}

Sometimes one needs to manually construct an any-pointer, which is typically done using the any_make function: any_make(ptr, type) will create an any pointing to ptr and with typeid type.

Since the runtime typeid is available, we can query for any runtime typeid property available at runtime, for example the size, e.g. my_any.type.sizeof. This allows us to do a lot of work on with the enclosed data without knowing the details of its type.

For example, this would make a copy of the data and place it in the variable any_copy:

void* data = malloc(a.type.sizeof);
mem::copy(data, a.ptr, a.type.sizeof);
any any_copy = any_make(data, a.type);

Variable argument functions with implicit any

Regular typed varargs are of a single type, e.g. fn void abc(int x, double... args). In order to take variable functions that are of multiple types, any may be used. There are two variants:

Explicit any vararg functions

This type of function has a format like fn void vaargfn(int x, any... args). Because only pointers may be passed to an any, the arguments must explicitly be pointers (e.g. vaargfn(2, &b, &&3.0)).

While explicit, this may be somewhat less user-friendly than implicit vararg functions:

Implicit any vararg functions

The implicit any vararg function has instead a format like fn void vaanyfn(int x, args...). Calling this function will implicitly cause taking the pointer of the values (so for example in the call vaanyfn(2, b, 3.0), what is actually passed are &b and &&3.0).

Because this passes values implicitly by reference, care must be taken not to mutate any values passed in this manner. Doing so would very likely break user expectations.

Interfaces

Most statically typed object-oriented languages implements extensibility using vtables. In C, and by extension C3, this is possible to emulate by passing around structs containing list of function pointers in addition to the data.

While this is efficient and often the best solution, but it puts certain assumptions on the code and makes interfaces more challenging to evolve over time.

As an alternative there are languages (such as Objective-C) which instead use message passing to dynamically typed objects, where the availability of a certain functionality may be queried at runtime.

C3 provides this latter functionality over the any type using interfaces.

Defining an interface

The first step is to define an interface:

interface MyName
{
    fn String myname();
}

While myname will behave as a method, we declare it without type. Note here that unlike normal methods we leave out the first “self”, argument.

Implementing the interface

To declare that a type implements an interface, add it after the type name:

struct Baz (MyName)
{
    int x;
}


// Note how the first argument differs from the interface.
fn String Baz.myname(Baz* self) @dynamic
{
    return "I am Baz!";
}

If a type declares an interface but does not implement its methods, then that is compile time error. A type may implement multiple interfaces, by placing them all inside of () e.g. struct Foo (VeryOptional, MyName) { ... }

A limitation is that only user-defined types may declare they are implementing interfaces. To make existing types implement interfaces is possible but does not provide compile time checks.

One of the interfaces available in the standard library is Printable, which contains to_format and to_new_string. If we implemented it for our struct above it might look like this:

fn String Baz.to_new_string(Baz baz, Allocator allocator) @dynamic
{
    return string::printf("Baz(%d)", baz.x, allocator: allocator);
}

“@dynamic” methods

A method must be declared @dynamic to implement an interface, but a method may also be declared @dynamic without the type declaring it implements a particular interface. For example, this allows us to write:

// This will make "int" satisfy the MyName interface
fn String int.myname(int*) @dynamic
{
    return "I am int!";
}

@dynamic methods have their reference retained in the runtime code and can also be searched for at runtime and invoked from the any type.

Referring to an interface by pointer

An interface e.g. MyName is can be cast back and forth to any, but only types which implement the interface completely may implicitly be cast to the interface.

So for example:

Bob b = { 1 };
double d = 0.5;
int i = 3;
MyName a = &b;          // Valid, Bob implements MyName.
// MyName c = &d;       // Error, double does not implement MyName.
MyName c = (MyName)&d;  // Would break at runtime as double doesn't implement MyName
// MyName z = &i;       // Error, implicit conversion because int doesn't explicitly implement it.
MyName* z = (MyName)&i; // Explicit conversion works and is safe at runtime if int implements "myname"

Calling dynamic methods

Methods implementing interfaces are like normal methods, and if called directly, they are just normal function calls. The difference is that they may be invoked through the interface:

fn void whoareyou(MyName a)
{
    io::printn(a.myname());
}

If we have an optional method we should first check that it is implemented:

fn void do_something(VeryOptional z)
{
    if (&z.do_something)
    {
        z.do_something(1, null);
    }
}

We first query if the method exists on the value. If it does we actually run it.

Here is another example, showing how the correct function will be called depending on type, checking for methods on an any:

fn void whoareyou2(any a)
{
    // Query if the function exists
    if (!&a.myname)
    {
        io::printn("I don't know who I am.");
        return;
    }
    // Dynamically call the function
    io::printn(((MyName)a).myname());
}


fn void main()
{
    int i;
    double d;
    Bob bob;


    any a = &i;
    whoareyou2(a); // Prints "I am int!"
    a = &d;
    whoareyou2(a); // Prints "I don't know who I am."
    a = &bob;
    whoareyou2(a); // Prints "I am Bob!"
}

Reflection invocation

This functionality is not yet implemented and may see syntax changes

It is possible to retrieve any @dynamic function by name and invoke it:

def VoidMethodFn = fn void(void*);


fn void* int.test_something(&self) @dynamic
{
    io::printfn("Testing: %d", *self);
}


fn void main()
{
    int z = 321;
    any a = &z;
    VoidMethodFn test_func = a.reflect("test_something");
    test_func(a); // Will print "Testing: 321"
}

This feature allows methods to be linked up at runtime.