Question

I have a struct/class which is partiall Plain Old Data (POD).

struct S {
  // plain-old-data structs with only arrays and members of basic types (no pointers);
  Pod1 pod1;
  Pod2 pod2;
  Pod3 pod3;
  Pod4 pod4;
  vector<int> more;
};

I copy objects of class S a lot.
I would like to copy it with memcpy, but S::more prevents it.
I would like to avoid call to 4 memcpy’s and make it all with one for an extra bit of performance.
Should I do someting like this?

memcpy(s1, s2, sizeof(Pod1) + sizeof(Pod2) + sizeof(Pod3) + sizeof(Pod4);

I can’t pack them in separate struct since it would clobber all the code that uses pod1 – pod4.

What is the best solution?

Answer 1

The best solution is to rely on C++ automatic copy constructor and copy operator. The compiler has then a chance to understand your code and optimize it well. Try to avoid memcpy in C++ code.

If you need to copy only part of the structure, create a method for it:

struct S {
  // plain-old-data structs with only arrays and members of basic types (no pointers);
  Pod1 pod1;
  Pod2 pod2;
  Pod3 pod3;
  Pod4 pod4;
  vector<int> more;
};

void copyPartOfS(S& to, const S& from)
{
  s.pod1 = from.pod1;
  s.pod2 = from.pod2;
  s.pod3 = from.pod3;
  s.pod4 = from.pod4;
}

...

S one, two;
one = two; // Full copy
copyPartOfS(one, two); // Partial copy