Question

I got what POD means, and I know that when I declare a structure in C++ as

struct f {};

there’s a default constructor, a default copy constructor, a default destructor, etc.. (if I got this correctly)

My question is: how can I declare a POD structure with just plain data (like 4 int values) without implicit constructors/destructors/etc.. getting in the way?

Did I miss something?

Answer 1

Every object type has a constructor—else how would you construct it?—and a destructor—else how would it be destroyed? They don’t “get in the way” of anything, because the default implementations are no-ops in the case of POD-only fields, just as if you had said:

struct f {
    f() {}
    ~f() {}
    int a, b, c, d;
};

However, if you write an empty constructor, the type becomes non-POD. C++11 solves this with defaulting:

f() = default;
~f() = default;

The situation is slightly different with copy constructors, where the implicitly generated one is just a convenience that does “the right thing” for POD types:

f(const f& other) : a(other.a), b(other.b), c(other.c), d(other.d) {}

There is no reason to rewrite this yourself. If you want to make a type noncopyable, you can mark the copy constructor as deleted with f(const f&) = delete; or declare it private.

It’s also important to note that member functions are not stored on the object as member variables are. You can think of a class or struct as simultaneously two things:

1. A description of a data layout

2. A namespace containing functions and types for manipulating that data

The C++ model of object-oriented programming simply couples these two things in one place.