Byte-for-byte copies of types in C++11?

Question

The C++11 standard guarantees that byte-for-byte copies are always valid for POD types. But what about certain trivial types?

Here's an example:

struct trivial
{
  int x;
  int y;
  trivial(int i) : x(2 * i) { std::cout << "Constructed." << std::endl; }
};

If I were to copy this struct, byte-for-byte, is it guaranteed to copy properly, even though it isn't technically a POD? When is the line drawn as to when it's not okay to byte-copy an object?

Answer 1

Yes, it is guaranteed to copy properly.

Quoting the FDIS, §3.9/2:

For any object (other than a base-class subobject) of trivially copyable type T, whether or not the object holds a valid value of type T, the underlying bytes making up the object can be copied into an array of char or unsigned char. If the content of the array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value.

And §3.9/3:

For any trivially copyable type T, if two pointers to T point to distinct T objects obj1 and obj2, where neither obj1 nor obj2 is a base-class subobject, if the underlying bytes making up obj1 are copied into obj2, obj2 shall subsequently hold the same value as obj1.

So the requirements you're asking about are, §3.9/9:

Arithmetic types, enumeration types, pointer types, pointer to member types, std::nullptr_t, and cv-qualified versions of these types are collectively called scalar types. Scalar types, POD classes, arrays of such types and cv-qualified versions of these types are collectively called POD types. Scalar types, trivially copyable class types, arrays of such types, and cv-qualified versions of these types are collectively called trivially copyable types.

And §9/6:

A trivially copyable class is a class that:

• has no non-trivial copy constructors,
• has no non-trivial move constructors,
• has no non-trivial copy assignment operators,
• has no non-trivial move assignment operators, and
• has a trivial destructor.