c++ wrong values with pragma pack

Question

header1.h

#pragma pack(4)

header2.h

#include <iostream>

struct my_struct
{
  unsigned int a;
  double b;
};

__forceinline void show_h(my_struct* my_struct_ptr)
{
  std::cout << sizeof(my_struct) << '\t' << my_struct_ptr->b << '\n';
}

void show_cpp(my_struct*);

header2.cpp

#include "header2.h"

void show_cpp(my_struct* my_struct_ptr)
{
  std::cout << sizeof(my_struct) << '\t' << my_struct_ptr->b << '\n';
}

main.cpp

#include "header1.h"
#include "header2.h"
#include <iostream>

int main()
{
  my_struct my_struct;
  my_struct.b = 4.56;
  std::cout << sizeof(my_struct) << '\t' << my_struct.b << '\n';
  show_h(&my_struct);
  show_cpp(&my_struct);
  return 0;
}

main.cpp, header2.h and header2.cpp sees my_struct differently. Seems like it's about #pragma pack(4) which is defined in header1.h. Why it's affecting header2.h and main.cpp but not header2.cpp?

output

12 4.56
12 4.56
16 -9.25596e+061

Answer 1

Get rid of header1.h and do

#pragma pack(push, 4)
struct my_struct
{
  unsigned int a;
  double b;
};
#pragma pack(pop)

Without this, having the packing done via a separate header will lead to confusions, when it's added in one TU while not in another TU.