That is of course not Kosher at all. Compilers are generally free to layout and pad structs in whichever way they prefer, meaning that two compilers for the same platform might very well use incompatible layouts. It can lead to major problems if structs saved by say a 64-bit version of the program is to be loaded by the 32-bit version.
Compilers (for C and C++) typically provide mechanisms to control padding and other layout requirements as necessary to make this work.
It's not that unusual in C and C++ code to define a struct that has a specific and well-defined memory layout. It's kosher, as long as you accept that you're working with a specific set of real compilers and use the appropriate #pragmas or other controls as needed to avoid undefined behaviour.
Yep, but then you force the compiler to choose a specific struct-layout which may be suboptimal if, for example, coding for simd. You also can't control the values of the struct padding bytes which may cause very interesting bugs.
