Best way to read binary data in C++ using classes without padding - c++

Question

I am reading simple binary data, without pointers, using C++ classes without padding with the following code:
#include <fstream>
#include <iostream>
using namespace std;
class Data {
public:
int a;
int b;
short int c;
double d;
}__attribute__((packed));
int main() {
Data myData;
ifstream ifs("test.bin", ios::binary);
ifs.read((char *)&myData, sizeof(myData));
ifs.close();
}
I am using this method because the data might have 20+ different formats and I want to write 20+ different classes to cover all the formats that might show up. I also read that other options include using bit-fields, pragma directives, and even the boost serialization routines (I can't because I have to use std). My question is: is this the best way to read simple binary data using classes without padding? Do you suggest any other alternative? I would like to learn what is the safest and most widely used method out there.

Answer 1

Typically, one would use a struct instead of a class, but yes, the same concept applies to both.

Answer 2

I've used these macros to allow packed structs to compile on both gcc and VC:
#ifdef _MSC_VER
#define BEGIN_PACK __pragma( pack(push, 1) )
#define END_PACK __pragma( pack(pop) )
#else
#define BEGIN_PACK
#define END_PACK __attribute__((packed))
#endif
So then you'd use them like this:
BEGIN_PACK
struct Data {
int a;
int b;
short int c;
double d;
} END_PACK;
But yes, that's usually how it's done. Note that these are non-standard extensions.
C++11 has defined packing directives, but I don't know if they're supported by compilers yet.