Question

This is purely hypothetical, but I’m not sure whether the following code will result in undefined behavior according to the C++ spec. I’d like to make a copy of the bytes in an object, blast the object by overwriting it with zeros, then copy the old bytes back. Can I do so without causing undefined behavior?

Sample code:

NonPODType o;
char bytes[sizeof(o)];

memcpy(bytes, &o, sizeof(o));
memset(&o, 0, sizeof(o));
memcpy(&o, bytes, sizeof(o));

Answer 1

In general, no. There’s an explicit guarantee that this works for trivially copyable types on §3.9/2, but there’s no such thing for other types.

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 (1.7) 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. [Example:

#define N sizeof(T)
char buf[N];
T obj;
// obj initialized to its original value
std::memcpy(buf, &obj, N);
// between these two calls to std::memcpy,
// obj might be modified
std::memcpy(&obj, buf, N);
// at this point, each subobject of obj of scalar type
// holds its original value

—end example ]