Mastering Rust - Second Edition

Trait objects

Now, up until this point, we have mostly seen traits being used in a static dispatch context, where we specified trait bounds in generic APIs. However, we also have another way to create polymorphic APIs, where we can specify parameters as something that implements a trait rather than a generic or a concrete type. This form of type, specified as implementing a trait API, is known as a trait object. Trait objects are similar to C++ virtual methods. A trait object is implemented as a fat pointer and is an unsized type, which means that they can only be used behind references (&). We explain unsized types in Chapter 7, Advanced Concepts. A trait object fat pointer has the first pointer pointing points to the actual data associated ...