test_subscript.cpp

#include <stdlib.h>
#include <inttypes.h>

union Bla {
	int a;
	char b;
	inline uint8_t &operator[](size_t i) { return reinterpret_cast<uint8_t *>(this)[i]; }
	inline uint8_t operator[](size_t i) const { return reinterpret_cast<const uint8_t *>(this)[i]; }
};

union Bli {
	int a;
	char b;
	uint8_t bytes[0];
};

void foo(int p)
{
	Bla bla{};
	bla.a = p;
	printf("%#.2x %#.2x %#.2x %#.2x\n", bla[0], bla[1], bla[2], bla[3]);
}

void bar(int p)
{
	Bli bli{};
	bli.a = p;
	printf("%#.2x %#.2x %#.2x %#.2x\n", bli.bytes[0], bli.bytes[1], bli.bytes[2], bli.bytes[3]);
}

int main(int argc, const char *argv[])
{
	int p = atoi(argv[1]);

	foo(p);
	bar(p);

	return 0;
}

test_subscript.o

0000000000400630 <foo(int)>:
  400630:       89 f8                   mov    %edi,%eax
  400632:       41 89 c0                mov    %eax,%r8d
  400635:       0f b6 f0                movzbl %al,%esi
  400638:       0f b6 d4                movzbl %ah,%edx
  40063b:       c1 e8 10                shr    $0x10,%eax
  40063e:       41 c1 e8 18             shr    $0x18,%r8d
  400642:       0f b6 c8                movzbl %al,%ecx
  400645:       bf 90 07 40 00          mov    $0x400790,%edi
  40064a:       31 c0                   xor    %eax,%eax
  40064c:       e9 9f fe ff ff          jmpq   4004f0 <printf@plt>
  400651:       66 66 66 66 66 66 2e    data16 data16 data16 data16 data16 nopw %cs:0x0(%rax,%rax,1)
  400658:       0f 1f 84 00 00 00 00 
  40065f:       00 

0000000000400660 <bar(int)>:
  400660:       89 f8                   mov    %edi,%eax
  400662:       41 89 c0                mov    %eax,%r8d
  400665:       0f b6 f0                movzbl %al,%esi
  400668:       0f b6 d4                movzbl %ah,%edx
  40066b:       c1 e8 10                shr    $0x10,%eax
  40066e:       41 c1 e8 18             shr    $0x18,%r8d
  400672:       0f b6 c8                movzbl %al,%ecx
  400675:       bf 90 07 40 00          mov    $0x400790,%edi
  40067a:       31 c0                   xor    %eax,%eax
  40067c:       e9 6f fe ff ff          jmpq   4004f0 <printf@plt>
  400681:       66 66 66 66 66 66 2e    data16 data16 data16 data16 data16 nopw %cs:0x0(%rax,%rax,1)
  400688:       0f 1f 84 00 00 00 00 
  40068f:       00