I have used unions earlier comfortably; today I was alarmed when I read this post and came to know that this code
pixel.colour = 0xff040201; // ARGB::colour is the active member from now on
// somewhere down the line, without any edit to pixel
if(pixel.components.a) // accessing the non-active member ARGB::components
If a standard-layout union contains several standard-layout structs that share a common initial sequence, and if an object of this standard-layout union type contains one of the standard-layout structs, it is permitted to inspect the common initial sequence of any of standard-layout struct members. §9.2/19: Two standard-layout structs share a common initial sequence if corresponding members have layout-compatible types and either neither member is a bit-field or both are bit-fields with the same width for a sequence of one or more initial members.
This is the purpose of a union - a single variable that can legitimately hold any of one of several types. [...] so long as the usage is consistent: the type retrieved must be the type most recently stored. It is the programmer's responsibility to keep track of which type is currently stored in a union; the results are implementation-dependent if something is stored as one type and extracted as another.
Use of unions can be essential for compatness of data [...] sometimes misused for "type conversion".
The purpose of unions is rather obvious, but for some reason people miss it quite often.
The purpose of union is to save memory by using the same memory region for storing different objects at different times. That's it.
It is like a room in a hotel. Different people live in it for non-overlapping periods of time. These people never meet, and generally don't know anything about each other. By properly managing the time-sharing of the rooms (i.e. by making sure different people don't get assigned to one room at the same time), a relatively small hotel can provide accomodations to a relatively large number of people, which is what hotels are for.
That's exactly what union does. If you know that several objects in your program hold values with non-overlapping value-lifetimes, then you can "merge" these objects into a union and thus save memory. Just like a hotel room has at most one "active" tenant at each moment of time, a union has at most one "active" member at each moment of program time. Only the "active" member can be read. By writing into other member you switch the "active" status to that other member.
For some reason, this original purpose of the union got "overriden" with something completely different: writing one member of a union and then inspecting it through another member. This kind of memory reinterpretation (akd "type punning") is
not a valid use of unions. It generally leads to undefined behavior. is decribed as producing implemenation-defined behavior in C89/90.
EDIT: Using unions for the purposes of type punning (i.e. writing one member and then reading another) was given a more detailed definition in one of the Technical Corrigendums to C99 standard (see DR#257 and DR#283). However, keep in mind that formally this does not protect you from running into undefined behavior by attempting to read a trap representation.