Question

The following struct X has 3 bytes of payload and 1 byte of padding:

struct X
{
    short a;
    char b;
};

memory layout: aab.

The following struct Y has 4 bytes of payload and 2 byte of padding:

struct Y
{
    X x;
    char c;
};

memory layout: aab.c.

Is there any way to keep X nested inside of Y and have sizeof(X) == 4 && sizeof(Y) == 4?

memory layout: aabc

Ideally, I would want this kind of space optimization for all types X (think X as a template parameter).

Answer 1

For hopefully obvious reasons the size of both X and Y can’t be four (if it were possible, X would have multiple definitions, the normal padded one and one where the padding is used by the char of Y). So the only way Y could be 4 is if X were changed to eliminate its padding. This can be done by compiler specific pragmas or directives. However, since that could result in X being accessed unaligned it would effectively restrict possible portability of your program (some architectures will even crash if misaligned accesses are made).

Instead, can you explain more clearly the end result you’re hoping to achieve here?