Forward Declarations

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Both interfaces and classes can be forward declared. Forward declarations permit the creation of mutually dependent objects, for example:

Slice

module Family
{
    interface Child;            // Forward declaration

    sequence<Child*> Children;  // OK

    interface Parent
    {
        Children getChildren(); // OK
    }

    interface Child           // Definition
    {
        Parent* getMother();
        Parent* getFather();
    }
}

Without the forward declaration of Child, the definition could not compile because Child and Parent are mutually dependent interfaces and the Slice compiler is a one-pass compiler (after the pre-processing step).

You can use forward-declared interfaces and classes to define types (such as the Children sequence in the previous example). Forward-declared interfaces and classes are also legal as the type of a structure, exception, or class field, as the value type of a dictionary, and as the parameter and return type of an operation.

Finally, you cannot inherit from a forward-declared interface or class until after its definition has been seen by the compiler:

Slice

interface Base;                         // Forward declaration

interface Derived1 extends Base {}      // Error!

interface Base {}                       // Definition

interface Derived2 extends Base {}      // OK, definition was seen

Not inheriting from a forward-declared base interface or class until its definition is seen is necessary because, otherwise, the compiler could not enforce that derived interfaces must not redefine operations that appear in base interfaces.

Language Mapping

A forward declaration maps to a forward declaration in programming languages with forward declarations (such as C++). It maps to nothing in other languages.

When a forward declaration corresponds to a type defined in a separate Slice file, add the [js:defined-in:<file>] metadata to tell the Slice-to-JavaScript compiler which file to import the type from in the TypeScript declarations.

Slice

// Greeter.ice
module VisitorCenter
{
    ["js:defined-in:./Admin.ice"]
    interface GreeterAdmin;

    interface Greeter
    {
        GreeterAdmin* getAdminObject();
        string greet(string name);
    }
}

// Admin.ice
module VisitorCenter
{
    interface GreeterAdmin { ... }
}