struct.go

package main

import "fmt"

// example represents a type with different fields.
type example struct {
	flag    bool
	counter int16
	pi      float32
}

func main() {
	// ----------------------
	// Declare and initialize
	// ----------------------

	// Declare a variable of type example set to its zero value.
	// 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.
	// 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.
	var e1 example

	// Display the value.
	fmt.Printf("%+v\n", e1)

	// 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.

	// 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
	// }

	// Declare a variable of type example and init using a struct literal.
	// Every line must end with a comma.
	e2 := example{
		flag:    true,
		counter: 10,
		pi:      3.141592,
	}

	// Display the field values.
	fmt.Println("Flag", e2.flag)
	fmt.Println("Counter", e2.counter)
	fmt.Println("Pi", e2.pi)

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

	fmt.Println("Flag", e3.flag)
	fmt.Println("Counter", e3.counter)
	fmt.Println("Pi", e3.pi)

	// ---------------------------
	// Name type vs 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.
	// 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

	fmt.Printf("%+v\n", e4)
}