Can a class with all private members be a POD class?

I've heard before that POD types cannot have private data

In C++03 POD types cannot have private data (see AndreyT's answer).

However the definition of POD has been changed in C++0x (See 9/10).

As per n3225

A POD struct is a class that is both a trivial class and a standard-layout class, and has no non-static data members of type non-POD struct, non-POD union (or array of such types).
... ...

A POD class is a class that is either a POD struct or a POD union.

That means 

struct demo
{
   private:
      int a, b;
};

is POD in C++0x because demo is both trivial and standard layout.

The definition of Standard layout is in section 9/7

A standard-layout class is a class that:

  • has no non-static data members of type non-standard-layout class (or array of such types) or reference,
  • has no virtual functions (10.3) and no virtual base classes (10.1),
  • has the same access control (Clause 11) for all non-static data members,
  • has no non-standard-layout base classes,
  • either has no non-static data members in the most-derived class and at most one base class with non-static data members, or has no base classes with non-static data members, and
  • has no base classes of the same type as the first non-static data member.11

.

Would then, WindowsApi::Uuid be a POD class?

Nopes! WindowsApi::Uuid is neither POD in C++03 nor in C++0x. A trivial class is a class that has a trivial default constructor (12.1) and is trivially copyable. WindowsApi::Uuid has a non trivial default constructor.

So this rule got relaxed in C++0x then?

Yes! (Considering Clause 11)

Also check out the FAQ entry on Aggregates and PODs


C++03 still does not allow non-static private or protected data in POD classes. This requirement is specified in the definition of aggregate

An aggregate is an array or a class (clause 9) with no user-declared constructors (12.1), no private or protected non-static data members (clause 11), no base classes (clause 10), and no virtual functions (10.3).

and POD class must be an aggregate first.

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