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This primer covers the declaration and use of classes.

A class is a type that can contain variables and constants, called fields, as well as functions and iterators called methods. A new class type is declared using the class keyword.

class C {
  var a, b: int;
  proc printFields() {
    writeln("a = ", a, " b = ", b);

The new keyword creates an instance of a class by calling an initializer. The class C above does not declare any initializers, and so the compiler-generated one is used. The compiler-generated initializer has an argument for each field in the class. Once a class has been initialized, its methods can be called.

Classes have various memory management strategies that determine how they are freed. We’ll discuss these more below, but for now, know that new C(...) is equivalent to writing out new owned C(...) where owned is one of these memory management strategies.

A class variable can refer to an instance of a class.

var foo = new C(1, 3);

Default output is supported so a class can be written by making a call to write or writeln. Default input is also supported.


A class variable can refer to an instance of a class. Different class variables can refer to the same instance. For example, alias below refers to the same memory that stores the fields of foo.

We’ll talk more about borrow below.

var alias = foo.borrow();

now alias.b and foo.b refer to the same field, so the next line also modifies foo.b

alias.b -= 1;

Methods can also be defined outside of the class body by prefixing the method name with the class name. All methods have an implicit this argument that is a reference to the class instance, or object. The this argument can be used to access a field explicitly. For example, in the method below, the this argument is used to access the b field which is otherwise shadowed by the b argument.

proc C.sum_a_b_b(b: int) {
  return a + b + this.b;

Here, a class named D is declared as a derived class from C. This new class has all of the fields and methods from C, plus any additional fields or methods it declares. The printFields method has the same signature as a method from C – it is overridden.

class D: C {
  var c = 1.2, d = 3.4;
  override proc printFields() {
    writeln("a = ", a, " b = ", b, " c = ", c, " d = ", d);

The static type of the variable foo, declared above, is C. Because the class D is derived from C, the variable foo can reference an object of type D. If an overridden method such as printFields is called, it is dynamically dispatched to the method with the most specific dynamic type.

Note that since foo is an owned C, assigning to it will delete the previous instance “owned” by that variable.

foo = new D(3, 4);

A class type includes a memory management strategy. The currently supported strategies are owned, shared, unmanaged, and borrowed.

var unm: unmanaged C = new unmanaged C();

unm refers to a manually managed instance. It needs to have delete called on it to free the memory.

delete unm;

var own: owned C = new owned C(1, 10);

The instance referred to by own is deleted when it is no longer in scope. Only one owned C can refer to a given instance at a time, but the ownership can be transferred to another variable.

var own2 = new C(1, 10);
assert(own.type == own2.type);

The example above shows that new C(...) can be used as a shorthand for new owned C(...) because owned is the default memory management strategy for classes.

var share: shared C = new shared C(1, 10);

The instance referred to by share is reference counted – that is, several shared C variables can refer to the same instance and will be reclaimed when the last one goes out of scope.

It is possible to borrow from another class pointer. One way to do that is by calling the borrow() method directly:

var b1 = own.borrow();

now b1 and own refer to the same instance it is illegal to:

  • use the borrow after whatever it is borrowed from goes out of scope

  • use the borrow after the instance is deleted (for example if own is assigned to)

A class type without a decorator, such as C, has generic management. The this argument of a method is generally borrowed C.

The compiler automatically adds conversion from owned, shared, or unmanaged in the process of resolving a function call, method call, or variable initialization.

var b2: borrowed C = own; // same as b2 = own.borrow();
own.printFields(); // same as own.borrow().printFields();
proc printSum(arg: borrowed C) {
  var sum = arg.a + arg.b;
printSum(own); // same as printSum(own.borrow())

A variable of class type cannot store nil unless it is declared to have nilable class type. To create a nilable class type, apply the ? operator to another class type

var x: borrowed C?; // default-initializes to ``nil``

Non-nilable class types can be implicitly converted to the corresponding nilable class type.

x = b2; // converting from borrowed C to borrowed C?

The method printFields is available on borrowed C, but not on borrowed C?

As a result, the call x.printFields() needs adjustment. The ! operator is available to assert that an expression is not nil and return it as a non-nilable type. This operator will halt if the value is actually nil.

Note that when applied to an owned or shared variable, ! will result in a borrow from that variable.


There are a few method names that cause the method to have special meaning. Please see Special Methods for details.