CPU overhead for struct?

Question

In C/C++, is there any CPU overhead for acessing struct members in comparison to isolated variables?

For a concrete example, should something like the first code sample below use more CPU cycles than the second one? Would it make any difference if it were a class instead of a struct? (in C++)

1)

struct S {
    int a;
    int b;
};

struct S s;
s.a = 10;
s.b = 20;    
s.a++;
s.b++;

2)

int a;
int b;
a = 10;
b = 20;
a++;
b++;

Answer 1

"Don't optimize yet." The compiler will figure out the best case for you. Write what makes sense first, and make it faster later if you need to. For fun, I ran the following in Clang 3.4 (-O3 -S):

void __attribute__((used)) StructTest() {
  struct S {
      int a;
      int b;
  };

  volatile struct S s;
  s.a = 10;
  s.b = 20;    
  s.a++;
  s.b++;
}
void __attribute__((used)) NoStructTest() {
  volatile int a;
  volatile int b;
  a = 10;
  b = 20;    
  a++;
  b++;
}

int main() {
  StructTest();
  NoStructTest();
}

StructTest and NoStructTest have identical ASM output:

pushl   %ebp
movl    %esp, %ebp
subl    $8, %esp
movl    $10, -4(%ebp)
movl    $20, -8(%ebp)
incl    -4(%ebp)
incl    -8(%ebp)
addl    $8, %esp
popl    %ebp
ret