c++ - struct and class padding

Question

I always pad my structs in C to get the maximum performance (good memory alignment).

// on a x86_64 
struct A {

 int64_t z;
 int32_t x;
 int16_t y;
 // we don't care the uint8_t position, as they are 1 byte wide.
 uint8_t s[8];
 uint8_t t[4];

}

But if I decide to go the c++ route, creating an object for another purpose, I need a class:

class B {

   B(){}
  ~B(){}

 public:
   int64_t a;
   int8_t  b;

 private:
   int32_t c;

//methods...
}

Then, c is not aligned anymore.

Is there a way to avoid doing that (multiple labels):

class B {

  B(){}
  ~B(){}

 public:
   int64_t a;

 private:
   int32_t c;

 public:
   int8_t  b;

}

(on some cpus, alignment matters). Thanks

Answer 1

Yep. Put all the state in a struct, aligned and padded as you wish. Preferably no member functions on the struct, keep it trivial. The class holds a private instance of this struct. Class member functions act on this state directly.

That should suffice. Plus you get a clear separation between state and functions which is always nice. Tends to be used with set/get functions in the class, unless you're especially attached to using inconsistent syntax for function calls and state access.

Alignas may also be of interest.