Unions

Unions are a special data type that, like a structure, describes stored fields, but unlike a structure, it can store only one of the fields at a time.

The size of a union is equal to the size of the largest field.

To understand what a union is, consider an example:

union IntOrString { mut: i int s string } fn main() { mut val := IntOrString{} val.i = 10 println(val.i) // 10 val.s = 'hello' println(val.s) // hello }

Unions are specified using the union keyword. Fields are set in the same way as in structures.

In this example, we have created a val variable of type IntOrString. At the beginning, val does not contain any data, so we should explicitly state what we want to store in it.

Since the size of the union is equal to the size of the largest field, in this case, the size of val is equal to the size of string, i.e. 16.

In memory, it will look like this:

          IntOrString union
+---------------+  +---------------+
| | | | | | | | |  | | | | | | | | |
+---------------+  +---------------+
      8 bytes           8 bytes

After creation, we assign val.i = 10. So val now stores an integer.

In memory, it would look like this (x marks the bytes that store some value):

          IntOrString union
+---------------+  +---------------+
|x|x|x|x| | | | |  | | | | | | | | |
^^^^^^^^^
   10
+---------------+  +---------------+
      8 bytes           8 bytes

Now, when we access val.i, the compiler accesses memory and interprets its value as an integer. Thus, we get the value 10.

Now we assign val.s = 'hello'. Now, x stores a string, not an integer. Previously stored data is overwritten with new.

In memory, it will look like this:

          IntOrString union
+---------------+  +---------------+
|x|x|x|x|x|x|x|x|  |x|x|x|x|x|x|x|x|
^^^^^^^^^^^^^^^^^  ^^^^^^^^ ^^^^^^^^
 pointer to data     len    additio-
                            nal info
+---------------+  +---------------+
      8 bytes           8 bytes

The first 8 bytes store a pointer to the character array, the first 4 bytes of the second 8 bytes store the length of the string, and the remaining 4 bytes store additional information.

Now try replacing println(val.s) with println(val.i) in the example above and run the example.

You will get 6829482 or some other number in the output.

This happened because the compiler accessed the first 4 bytes of the first 8 bytes and interpreted them as an integer.

          IntOrString union
+---------------+  +---------------+
|x|x|x|x|x|x|x|x|  |x|x|x|x|x|x|x|x|
^^^^^^^^^^^^^^^^^  ^^^^^^^^ ^^^^^^^^
 pointer to data     len    additio-
                            nal info
^^^^^^^^
  try access to this memory
+---------------+  +---------------+
      8 bytes           8 bytes

And since a pointer to an array of characters is stored there, the compiler interprets it as an integer and returns the result.

Embedding

Just like structs, unions support embedding.

struct Rgba32_Component { r byte g byte b byte a byte } union Rgba32 { Rgba32_Component value u32 } clr1 := Rgba32{ value: 0x008811FF } clr2 := Rgba32{ Rgba32_Component: Rgba32_Component{ a: 128 } } sz := sizeof(Rgba32) unsafe { println('Size: ${sz}B,clr1.b: ${clr1.b},clr2.b: ${clr2.b}') // Size: 4B, clr1.b: 136, clr2.b: 0 }

Union member access must be performed in an unsafe block.

Embedded struct arguments are not necessarily stored in the order listed.

Unions

# Embedding

Embedding