Force C++ structure to pack tightly (part two) [closed]

I have the exact same problem as discussed in this question:
Force C++ structure to pack tightly

None of the solutions worked besides this (very tedious) one:
Just write code to decode the serialized data into your in-memory representation. @DavidRodríguez-dribeas

I must read a COFF object file that has a tightly packed fixed binary structure. This works fine by default using the Microsoft compiler. It does not seem to work at all using any of the compiler directives for the g++ compiler:
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.12)