Union – useless anachronism or useful old school trick?

Asked 2009-05-13T13:47:06.333

I recently came across a great data structures book,"Data Structures Using C" (c) 1991, at a local Library book sale for only $2. As the book's title implies, the book covers data structures using the C programming language.

I got the book knowing it would be out-dated but would probably contain lots of advanced C topics that I wouldn't encounter elsewhere.

Sure enough within 5 minutes I found something I didn't know about C. I came across a section talking about the union keyword and I realized that I had never used it, nor ever seen any code that does. I was grateful for learning something interesting and quickly bought the book.

For those of you not knowledgeable about what a union is, the book uses a good metaphor to explain:

To fully understand the concept of a union, it is necessary to examine its implementation. A Structure may be regarded as a road map to an area of memory. It defines how the memory is to be interpreted. A union provides several different road maps for the same area of memory, and it is the responsibility of the programmer to determine which road map is in current use. In practice, the compiler allocates sufficient storage to contain the largest member of the union. It is the road map, however, that determines how that storage is to be interpreted.

I could easily come up with contrived situations or hacks where I would use a Union. (But I am not interested in contrived situations or hacks...)

Have you used or seen an implementation where using Union solved the problem **more elegantly** than not using a Union?

Added bonus if you include a quick explanation of why using union was better/easier than not using a union.

c++ c data-structures unions

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3 Answers

UNIONs implement some sort of polymorphism in a non-OOP world. Usually, you have a part which is common and depending on that part, you use the rest of the UNIONs. Therefore, in such cases where you do not have an OOP language and you want to avoid excessive pointer arithmetic, unions can be more elegant in some cases.

answered 2009-05-13T13:51:08.010

It's useful for setting bits in, say, registers instead of shift/mask operations:

typedef union {
    unsigned int as_int; // Assume this is 32-bits
    struct {
        unsigned int unused1 : 4;
        unsigned int foo : 4;
        unsigned int bar : 6;
        unsigned int unused2 : 2;
        unsigned int baz : 3;
        unsigned int unused3 : 1;
        unsigned int quux : 12;
    } field;
} some_reg;

Note: Which way the packing happens is machine-dependent.

some_reg reg;
reg.field.foo = 0xA;
reg.field.baz = 0x5;
write_some_register(some_address, reg.as_int);

I might have blown some syntax somewhere in there, my C is rusty :)

EDIT:

Incidentally, this works the opposite way also:

reg.as_int = read_some_register(some_address);
if(reg.field.bar == BAR_ERROR1) { ...

answered 2009-05-13T14:04:01.033

It's often used in the specification of data transmission protocols, where you'd want to avoid wasting space in your data structures. It allows memory space to be saved by using the same space for multiple mutually exclusive options.

For example:

enum PacketType {Connect, Disconnect};
struct ConnectPacket {};
struct DisconnectPacket {};
struct Packet
{
    // ...
    // various common data
    // ...
    enum PacketType type;
    union
    {
        ConnectPacket connect;
        DisconnectPacket disconnect;
    } payload;
};

The ConnectPacket and DisconnectPacket structures occupy the same space, but that's ok because a packet can't be both types at the same time. The enum value is used to determine which part of the union is in use. Using the union has allowed us to avoid duplicating the common parts of the Packet structure.

answered 2009-05-13T14:10:07.137

Union – useless anachronism or useful old school trick?

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