size of a class instance

Bill Baxter dnewsgroup at
Tue Nov 14 00:13:02 PST 2006

Lionello Lunesu wrote:
> Sean Kelly wrote:
>> Bill Baxter wrote:
>>> Is this really the best way to get the size of a class instance given 
>>> the type?
>>>   MyClass.classinfo.init.length;
>> Yes.  And this only works at run time.  I proposed a new .isizeof 
>> property to simplify things, but it didn't seem terribly popular.  See:
>> Sean
> I suppose sizeof(MyClass) could simply return the right value. I don't 
> see how the size of a class' handle can be useful.
> L.

I suspect the reason is that for doing various low-level things you 
might actually need to know how much space the thing is occupying on the 
stack.  Like a varargs, for instance.  You need to know how big the 
thing is on the stack to advance the _argptr properly.  It doesn't 
matter how big the instance data on the heap is.

I just now did a web search to see what other GC languages that hide 
pointers from you do.

Java just doesn't have sizeof apparently.

C# has it.  From the spec:
   "For all other (non predefined types), the result of the sizeof 
operator is implementation-defined and is classified as a value, not a 
When applied to an operand that has struct type, the result is the total 
number of bytes in a variable of that type, including any padding."

So basically it sounds like they say you can't count on sizeof(aclass) 
returning anything in particular.

I guess when it comes down to it, finding the size of a class is really 
only useful for satisfying one's curiosity and optimizing member orders 
to get better alignment.  And for that purpose I guess it doesn't really 
have to be that intuitive a name.


More information about the Digitalmars-d-learn mailing list