Sizeof class instance

Jeremie Pelletier jeremiep at
Sun Oct 4 08:13:57 PDT 2009

Justin Johansson wrote:
> Jeremie Pelletier Wrote:
>> 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);
>> }
> Okay so PODO alone is 8 bytes.
>    writefln( "Object:      %d", Object.classinfo.init.length);
> Object:      8
> And we're talking 9 bytes for a simple boxed bool and 12 bytes for a simple boxed 32-bit integer
> by the looks of things.
> class Foo {
>    bool value;
> }
> class Bar {
>    int value;
> }
>    writefln( "Foo:         %d", Foo.classinfo.init.length);
>    writefln( "Bar:         %d", Bar.classinfo.init.length);
> Foo:         9
> Bar:         12
> On top of that there is possibly alignment in the heap so my 9 byte Foo would occupy 12 or 16 bytes
> of address space.  Would that be correct?
> Thanks for answers.  Justin.

The GC allocates memory in blocks using sizes which are powers of 2, 
starting at 16 bytes and going up to 4096 bytes, this alignment is used 
for various optimizations.

Objects should be used for their inheritance features, if you don't need 
inheritance you should stick to POD structs, although they require a 
different handling since they're value types (vs objects which are 
reference types) and don't always get allocated on the heap.

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