Difference between get(), getproperty(), and getfield()

1. get
   • Scope：Generally used to retrieve values from indexable data structures (such as Dict, Array) or other containers with customizable interfaces.
   • Function：get(collection, key, default) will attempt to retrieve the value for key from collection. If the value is not found, default will be returned.
   • Example：

     d = Dict(:a => 1, :b => 2)
     println(get(d, :a, 0))  # 1
     println(get(d, :c, 0))  # 0 (Due to :c not existing)
     

   • Application：When there is no guarantee that a key will always exist, you can gracefully provide a default value and avoid throwing an error.
2. getproperty
   • Scope：This is the underlying mechanism for attribute access* in Julia, which is mainly related to dot notation syntax (.).
   • Function：When we use the obj.someproperty syntax on an object obj, Julia will call getproperty(obj, :someproperty) to get the property. You can also define/override the Base.getproperty method in your custom types to execute some custom logic when accessing properties (such as lazy loading, redirecting to other fields, calculating virtual properties, etc.).
   • Example：

     struct MyType
         x::Int
     end
     
     function Base.getproperty(m::MyType, sym::Symbol)
         if sym === :y
             return m.x + 1
         else
             return getfield(m, sym)  # fallback to default field access
         end
     end
     
     m = MyType(10)
     println(m.x)  # Actual call getproperty(m, :x) -> getfield(m, :x)
     println(m.y)  # Actual call getproperty(m, :y) -> Custom logic returns 11
     

   • Application：Intercept and dynamically handle access to the properties of structures/objects. For example, properties that are “calculated” rather than actually stored can be made to be accessed like normal fields by overriding getproperty.
3. getfield
   • Scope：This is the function for directly accessing the fields of a structure in Julia.
   • Function：getfield(obj, :fieldname) is low-level access to a field of a structure and does not trigger the overriding logic of getproperty. In other words, it just “gets the actual field in the object” and does not execute additional custom methods.
   • Example：

     struct MyType
         x::Int
     end
     
     m = MyType(10)
     println(getfield(m, :x))  # 10
     

   • Application：Use getfield when you need to bypass any possible getproperty overloads or when you explicitly want to access the fields of the underlying structure directly. For example, in a custom implementation of getproperty, getfield is often used to get the actual field value.

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Summary

• get is a high-level “safe” getter function for data structures that provides default values to handle missing keys.
• getproperty is the mechanism behind the dot syntax (.), allowing users to customize property access behavior through overloading.
• getfield is the lowest-level function that directly accesses the fields of a structure without any overloading. It is generally only explicitly called when writing low-level code or customizing getproperty.