How to unpack numbers in cpp if they are pack at python using struct.pack(fmt, v1, v2, …)

Question

In python, I encoded numbers using struct

struct.pack("<2q", 456, 123)

It returns

'\xc8\x01\x00\x00\x00\x00\x00\x00{\x00\x00\x00\x00\x00\x00\x00'

In cpp, how could I decode such string into a tuple of integers?

Answer 1

It should be fairly simple to unpack that string, you can just copy the bytes out into the appropriate sized integer:

#include <iostream>
#include <string>
#include <cstring>

int main()
{
  std::string input = std::string("\xc8\x01\x00\x00\x00\x00\x00\x00{\x00\x00\x00\x00\x00\x00\x00", 16);
  for ( size_t i = 0; i < input.size() / 8; i++ )
  {
      int64_t value;
      memcpy(&value, &input[i*8], 8);
      std::cout << value << "\n";
  }
}

Answer 2

q is long long so 64 bit signed integers. From https://docs.python.org/3/library/struct.html :

Format  C Type      Python type     Standard size
q      long long    integer         8

You could read this buffer and copy into an array of 2 long long (64 bit using stdint.h define)

#include <iostream>
#include <strings.h>
#include <stdint.h>

int main()
{
 // you're supposed to get that when reading the buffer from a file for instance:
 const unsigned char buffer[] = {0xc8,0x01,0x00,0x00,0x00,0x00,0x00,0x00,'{',0x00,0x00,0x00,0x00,0x00,0x00,0x00};
 int64_t array[2];
 memcpy(array,buffer,sizeof(array));
 std::cout << array[0] << "," << array[1] << '\n';
}

prints:

456,123

I didn't handle endianness here. Just assumed they were the same. But if you want that, just swap the bytes using the size of the type and you're good to go.