Question

Is there a way to get the actual size of a class instance in Delphi?

I know about the InstanceSize method of the TObject class but that method does not recursively invokes itself for object members. For example, let’s say we have two classes:

type MyClass1 = class   private     myVar1 : integer;     myVar2 : integer; end;  type MyClass2 = class   private     myOtherVar1 : integer;     myOtherVar2 : MyClass1; end; 

for this segment of code, MyClass1 will be 12 bytes length (4 bytes for each integer plus 4 for the class overhead) and MyClass2 will be 24 bytes lengh (4 bytes for the class overhead, 12 bytes from myOtherVar2 and another 4 for the myOtherVar1 integer). Using InstanceSize will result on 12 bytes for each of them since myOtherVar2 is interpreted as a pointer (4 bytes) instead of as a class reference.

Is there a way to get the total size of the class including its reference to other class instances?

Answer 1

Construct one MyClass1 object and a million MyClass2 such that each MyClass2 points to the same MyClass1.

How much memory does each MyClass2 take? 12.000012 bytes?

How much memory does a circular list take? Infinity as you can keep chasing pointers for ever?

In languages with pointers, a naive recursive size-of algorithm isn’t useful in general. You need to write your own algorithm which embodies knowledge about the aggregation/composition, sharing and recursive references specific to how you’re using the objects.