1. Objects

Ymir is a object oriented language, with polymorph types. In this chapter, we assume that you are familiar with object oriented programming paradigm (if not, you will benefit from reading information on that paradigm first cf. object oriented programming.).

In Ymir, object instances are dynamically allocated on the heap. Indeed, because object types are polymorph, their size cannot be known at compile time, and consequently cannot be placed in the stack. To illustrate this point, the following figure presents the UML definition of a class A, and an heir class B. The class A contains a field x, of type i32, thus of size 4 bytes. The class B, contains another field y of size 4, leading to a class B of size 8.

Because we want to following source code to be accepted (basic principle of polymorphism), the size of A, cannot be statically set to 4 bytes, (nor to 8, the class B can come from a totally different part of the source code - from a library for example -, and unknown when using the type A, without talking of the huge loss of memory if only A values are used).

def foo (a : &(A)) // ...    

def main () {
    let b : &B = // ...
    foo (b); // calling foo with a B, heir of A
}

For that reason, object instances are aliasable types, and only contains a pointer to the values of the class. You may have notice the syntax &(A), in the above example. If this syntax is similar to pointer, this is because objects are basically pointers. However, unlike basic pointers, these cannot be used in pointer arithmetics, cannot be null, and does not need to be dereferenced to access the value. In other words, these are reference, more than pointers. Yes, unlike other object oriented language such as java or D, object instances cannot be null. This is a really important part of the object system in Ymir guaranteeing that every objects are pointing to a valid value, and that value is correctly initialized (constructor was called).

We will see in the coming chapters, that removing the possibility of null objects does not remove any capacity on the language, while adding strong safety, the number one error of java programs being NullPointerException (cf. Which Java exceptions are the most frequent?, The Top 10 Exception Types in Production Java Applications – Based on 1B Events).