In this recipe, we will learn how to define structures that do not have any padding bytes between their data members. This may significantly reduce the amount of memory that's used by your application if it works with a large number of objects.
Note, though, that this has a cost. Unaligned memory access is slower, which results in sub-optimal performance. For some architectures, unaligned access is forbidden, thus requiring the C++ compiler to generate much more code to access the data fields than for aligned access.
Although packing your structs may result in more efficient memory usage, avoid using this technique unless it's really necessary. It has too many implied limitations that may lead to obscure, hard-to-find issues in your application later.
Consider packed structures as transport encoding and only use them to store, load...