size of a class instance

Lionello Lunesu lio at
Thu Nov 16 08:56:03 PST 2006

Bill Baxter wrote:
> 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 
> constant.
> ...
> 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.
> --bb

Uh, according to the D spec, the order of the members of a _class_ may 
be changed by the compiler, so optimizing manually would have no effect.


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