Dictionaries and Custom Classes

One benefit of the dictionary data type is the ability to accept almost any data type, including custom classes. In some cases however, the behavior of custom classes affects how they interact with dictionaries. This example shows how to overload two key functions for custom classes to make sure that dictionaries behave as expected.

Hash Equivalence

Dictionaries are hash maps, meaning that they convert keys into uint64 scalar hash codes. Each hash code represents a unique key and is used during lookup to quickly find the requested value. In MATLAB^®, these scalars are generated using the keyHash function, which uses input property information to generate a uint64 scalar. The keyMatch function can be used to determine whether hashed keys are equivalent. For a dictionary to function properly, when keyMatch(A,B) is true, then keyHash(A) and keyHash(B) must be equal. For most data types, this relationship holds true without any extra steps. However, some custom classes can have properties that you do not want to include as part of the comparison.

For example, create a class that is used to collect data and record the time that the data was collected. This class has two properties dataValue and timestamp.

classdef myDataClass
    properties
        dataValue double = 0
        timestamp datetime = datetime
    end
end

For the purposes of comparing data, only dataValue is important. However, keyHash uses both properties when generating a hash code.

Overload keyHash and keyMatch for Custom Classes

To make myDataClass work as intended with dictionaries, overload keyHash and keyMatch. Add keyHash and keyMatch methods to myDataClass that use only the dataValue property to generate and compare hash values.

classdef myDataClass
    properties
        dataValue double = 0
        timestamp datetime = datetime
    end
    methods
        function h = keyHash(obj)
            h = keyHash(obj.dataValue);
        end
        function tf = keyMatch(objA,objB)
            tf = keyMatch(objA.dataValue,objB.dataValue);
        end
    end
end