Plain Old Data types with private members?

6
votes

Is Demo a POD type in C++03?

struct Demo
{
     private:
       int x;
       int y;
};

C++03, §9p4:

A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-defined copy assignment operator and no user-defined destructor.

After reading Steve Jessop's post, I believe Demo is a non-POD because the members are private. However the Standard doesn't say anything about the relation between POD types and access modifiers.

In C++0x Demo is POD because §9p9 (n3126) says:

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).

Demo is trivial1 as well as a standard-layout class so it is a POD. Is my interpretation correct?

1 A trivial class is a class that has a trivial default constructor (12.1) and is trivially copyable. [9p5, n3126]

2
c++c++11
I'd agree that it's POD. It seems that parts of the definition are contradicting each other in that an aggregate can't have private members, but then nothing notes private members elsewhere. Whether the members are public or private will not affect default compare and copy construction, so I'd agree that it's trivial and therefore POD. – JoshD

2 Answers

13
votes

In C++03, it's definitely not a POD. According to §9/4, "A POD-struct is an aggregate class ...", and according to §8.5.1/1:

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)."

Under C++0x, at least as of N3090/3092, I believe it is a POD. These require only that all non-static members have the same access, not that the access is necessarily public. This is to fix a problem that I believe I was the first to point out -- in C++98/03, a vacuous access specifier leads to a problem:

struct x { 
    int a;
public:
    int b;
public:
   int c;
};

This fits the requirements of a POD struct -- but the standard still gives permission for the relative positions of b and c to be swapped because of the intervening access specifier. As a result, being a POD struct doesn't provide the layout guarantees that were intended to ensure compatibility with C structs (for the obvious example).

4
votes

From C++11 on, the easiest by far is to ask the compiler with std::is_pod:

#include <iostream>
#include <type_traits>

struct Demo
{
     private:
       int x;
       int y;
};

int main()
{
    std::cout << std::boolalpha;
    std::cout << std::is_pod<Demo>::value << std::endl;
}

true