Unions

[Aceix]

Why were unions created? what are their uses!?!

[o3hC5Di1]

Unions are useful when you don't know beforehand which datatype a variable will be holding.

For instance:

union Number
{
    int i;
    double d;
};

In this case memory will be reserved for the largest datatype in the union to make sure that whatever value a variable of type Number has, it will fit in memory.

This is a simple example of course, when you create your own objects (which might be inherited from one and other) this could seem more useful.

Hope that helps.

All the best,
NwN

[Script Coder]

They are not used much anymore as far as I know, plus if you you know what they do, when the time comes you will know to use them.

[zb0S216C]

I've seen them used for multiple implementations. Here's a few:

• Conversions. Because C++'s conversion operators do not allow conversion from a void pointer (with reinterpret_cast), a union may be used to perform this cast. For instance:

template <typename T>
void *convert_to_voidp(T *pointer)
{
    union
    {
        T *in;
        void *out;
    } result;
    
    result.in = pointer;
    return(result.out);
}

C++ restricts conversions to a void pointer for a reason, so this sort of conversion isn't exactly safe, nor does it make much sense. You'd best have a good reason for using this conversion method. Be sure to favour static_cast to convert from a void*.

• CPU Registers. This sort of union usage is found in kernel implementations. Here's a sample:

union cpu_registers
{
    short ax; // 16-bit register
    int eax;  // 32-bit register
    long long int rax; // 64-bit register
};

• Variants. Commonly found in scripting languages such as Lua, and Python. Variants allow a variable to store 1 value at a time, but that value can be of multiple types. This usage of unions allow variables to be dynamic, and possibly polymorphic. Here's a sample:

union variant 
{
    double double_value;
    int int_value;
    char char_value;

    /* operators... */
};

enum variant_tag 
{
    DOUBLE,
    INT,
    CHAR
};

struct dynamic_variable 
{
    variant value;
    variant_tag tag; // Identifies the current value type
};

• I/O Ports. Similar to the CPU registers implementation of the union.

Since unions only reserve enough memory to hold the largest member, it saves memory, unlike a structure which allocates memory for members + alignment padding. There's no other structure in the C/C++ programming language that behaves like a union. Because of this, it's considered unique. Unions are by far one of the most flexible structures in the language. The flexibility of a union depends on the types it supports, and the operators it overloads.

Edit: Typo.

_Wazzak

[o3hC5Di1]

Thanks for that Framework - informative as always :)

All the best,
NwN

[ne555]

Because C++'s conversion operators do not allow conversion to a void pointer

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