Struct Types

Declare and initialize

Declare a variable of type example set to its zero value.

var e1 example

How much memory do we allocate for example?

• a bool is 1 byte
• int16 is 2 bytes
• float32 is 4 bytes

Putting together, we have 7 bytes. However, the actual answer is 8. That leads us to a new concept of padding and alignment.

Padding and alignment

The padding byte is sitting between the bool and the int16. The reason is because of alignment.

The idea of alignment: It is more efficient for this piece of hardware to read memory on its alignment boundary. We will take care of the alignment boundary issues so the hardware people don’t.

Rule 1: Depending on the size a particular value, Go determines the alignment we need. Every 2 bytes value must follow a 2 bytes boundary. Since the bool value is only 1 byte and start at address 0, then the next int16 must start on address 2. The byte at address that get skipped over becomes a 1 byte padding. Similarly, if it is a 4 bytes value then we will have a 3 bytes padding value.

Rule 2: The largest field represents the padding for the entire struct. We need to minimize the amount of padding as possible. Always lay out the field from highest to smallest. This will push any padding down to the bottom.

Example:

// In this case, the entire struct size has to follow a 8 bytes value because int64 is 8 bytes.
type example struct {
    counter int64
    pi      float32
    float   bool
}

Name type versus anonymous type

If we have two name type identical struct, we can’t assign one to another.

For example, example1 and example2 are identical struct, var ex1 example1, var ex2 example2. ex1 = ex2 is not allowed. We have to explicitly say that ex1 = example2(ex2) by performing a conversion.

// Declare a variable of an anonymous type and init using a struct literal.
e3 := struct {
    flag    bool
    counter int16
    pi      float32
}{
    flag:    true,
    counter: 10,
    pi:      3.141592,
}

However, if ex is a value of identical anonymous struct type (like e3 above), then it is possible to assign ex1 = ex

var e4 example
e4 = e3 // OK