Additional features

Default values

Sometimes, you may want to provide default values for user defined types. This can be achieved by setting the default value in the IDL to {}, like in this example:

dictionary SubConfig
{
    int answer = 42;
};

dictionary Config
{
    SubConfig sub = {};
};

This will create a new instance of SubConfig in Config. Note, that this is only really possible if SubConfig has no required fields.

As an extension to this, this approach can also be used with polymorphic types. Here, there might not be a single concrete type to instantiate, but rather a family of types that share a common base. In this case you can define the desired default type in the IDL via the extra attribute Default like so:

dictionary Base
{};

dictionary A : Base
{};

dictionary B : Base
{};

dictionary Container
{
    [Default=A] Base sub = {};
}