What is padding in C?
All modern CPUs expect that the fundamental types — int’s, float’s and long’s — are stored in the memory at their natural boundary; typically, at addresses that are multiples of their length.  Some CPU work efficiently if the memory is properly aligned, and some can work in either case. 
For the [...]

Default access to a union is public, it can contain member functions and member data.
Other than that there are some more specialties for unions in C++.

 
It can’t have a static data members or a member of reference type.
It can’t have virtual functions.
It can’t be used as a base class nor it can have [...]

to access a structure member fastly, structure need to be padded

Padding will be done for each and every member, if required, not for the entire structure

Alignment : data storing boundary address starts at aligned address

Examples:
Size of int = 4 bytes
size of char = 1byte
alignment 64 bit (4 bytes)
Struct
{
int a ;
char [...]