Structs (Julia)

Structs in Julia provide users with ways to define their own data types. Users can then define functions that accept these user-defined types as appropriate.

Julia provides two different types of structs: a struct and a mutable struct. They can be defined as follows:

struct Dog
	name::String
	age::Integer
end
 
mutable struct DogTwo
	name::String
	age::Integer
end

When a struct is defined, Julia also automatically provides a constructor function where each field of the struct is an argument to the constructor, e.g.:

dog1 = Dog("Nala", 9)
dog2 = DogTwo("Darcy", 3)

But we can also create our own constructor functions as well, e.g.:

function Dog(name::String, age::AbstractFloat)
	age_f = Int(floor(age))
	return Dog(name, age_f)
end

The difference between a struct and a mutable struct is that fields in a struct cannot be modified after they’re defined, whereas fields in a mutable struct can.

dog1 = Dog("Nala", 9)
dog2 = DogTwo("Darcy", 3)
 
# this will error
dog1.age = 10
 
# this will work
dog2.age = 4

I tend to pretty much always use mutable structs because they tend to make more sense for my workflows, and I’d prefer the ability to change field values if necessary. But I assume there’s a performance benefit to using a regular struct

Further Reading

• Julia Manual Section on Types