Size of empty class and empty function ??

Question

Hi Friends,

What will be solution in the fallowing code

Class A{}
void func(){}

printf("%d,%d",sizeof(A),sizeof(func));

Thanks in advance

Prashant Dubey

Answer 1

Why don't you just test it out? The function will be probably be promoted to a pointer.

Answer 2

You are taking the size of a function which cannot be done. You need to preceede the name with a & to take the size of a pointer to that function. You also need to cast the sizeof value to be of type int, which is what printf expects for the d specifier

printf("%d,%d", (int)sizeof(A), (int)sizeof(&func));

As for the concrete values ---- It's not known what they are beyond that are greater or equal to 1. It depends on the compiler.

Answer 3

Size of an empty class is non zero(most probably 1), so as to have two objects of the class at different addresses.

http://www2.research.att.com/~bs/bs_faq2.html#sizeof-empty explains it better

 class A{};
 void func(){}

 std::cout<<sizeof(A)<<std::endl<<sizeof(&func));// prints 1 and 4 on my 32 bit system

Answer 4

By sizeof(func), you probably mean sizeof(&func), which is the size of a function pointer in bytes.

As far as what the size of an A object is, the standard only says:

Complete objects and member subobjects of class type shall have nonzero size.

which is qualified by footnote 94:

Base class subobjects are not so constrained.

The reason for the footnote is to allow for a compiler optimization known as empty member optimization, which is commonly used in template libraries to access routines of a class type (e.g. allocator routines of an allocator class type) without needing a member object of that class type.