Sizeof class instance

Jeremie Pelletier jeremiep at
Sat Oct 3 17:06:15 PDT 2009

Justin Johansson wrote:
> Jarrett Billingsley Wrote:
>> On Sat, Oct 3, 2009 at 5:50 PM, Justin Johansson <no at> wrote:
>>> How does one determine the sizeof (in bytes) of an instance of a class in D?
>>> .sizeof works as advertised for structs, but for reference types,
>>> .sizeof yields the sizeof the referencing variable (effectively same as size of a pointer)
>>> and not the size of the underlying instance.
>>> I did try scanning the NG and read spec_D1.00.pdf.  Perhaps I missed it in the latter.
>>> btw. I was poking under the hood of std.xml and though, wow, instances of Element
>>> class look humongous, and so I'm interested to how exactly how humongous.
>> There's no way to get it at compile-time in D1. The best you can do is
>> Class.classinfo.init.length.
>> In D2, you can use __traits(classInstanceSize, Class).
> Thanks Jeremie and Jarrett for answers.
> For investigative purposes (rather than adding up class member sizes in my head),
> would I get a fair answer if I copied the class data members into a struct, did a struct
> sizeof and added 4 bytes to allow for a virtual function table pointer (in the class and
> assuming the class has a VFT)?

You forgot the monitor pointer of the class, so thats (size_t.sizeof * 
2) to add to the size of the struct.

I wasn't aware of the traits method either, I just made this helper 
template to simplify its syntax:

template SizeOf(alias C) if(is(C == class)) {
	enum ClassSizeof = __traits(classInstanceSize, C);

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