Add Pass A Struct To A Function as a Go TIL

go/pass-a-struct-to-a-function.md

@@ -0,0 +1,65 @@
+# Pass A Struct To A Function
+
+Go operates as _pass-by-value_ which means that when we pass a struct to a
+function, the receiving function gets a copy of the struct. Two things worth
+noticing about that are 1) an extra memory allocation happens when calling the
+function and 2) altering the struct does not affect the original in the calling
+context.
+
+On the other hand, we can have a function that takes a pointer to a struct.
+When we call that function, we have a reference to the memory location of the
+struct instead of a copy of the struct. That means no additional allocation and
+modifications to the dereferenced struct are modifications to the original in
+the calling context.
+
+Here is an example that demonstrates both of these. Notice the printed output
+that is included in comments at the end which shows memory locations and
+contents of the struct at various points.
+
+```go
+package main
+
+import "fmt"
+
+type Order struct {
+	Item string
+	Quantity int
+	DineIn bool
+}
+
+func main() {
+	order := Order{Item: "taco", Quantity: 3, DineIn: true}
+
+	fmt.Println("Order:", order)
+	fmt.Printf("main - Loc: %p\n", &order)
+
+	doubledOrder := doubleOrder(order)
+
+	fmt.Println("Double Order:", doubledOrder)
+	fmt.Println("Original Order:", order)
+
+	doubleOrderPtr(&order)
+
+	fmt.Println("Double Order Ptr:", order)
+}
+
+func doubleOrder(order Order) Order {
+	fmt.Printf("doubleOrder - Loc: %p\n", &order)
+	order.Quantity *= 2
+
+	return order
+}
+
+func doubleOrderPtr(order *Order) {
+	fmt.Printf("doubleOrderPtr - Loc: %p\n", order)
+	(*order).Quantity *= 2
+}
+
+// Order: {taco 3 true}
+// main - Loc: 0xc0000b4000
+// doubleOrder - Loc: 0xc0000b4040
+// Double Order: {taco 6 true}
+// Original Order: {taco 3 true}
+// doubleOrderPtr - Loc: 0xc0000b4000
+// Double Order Ptr: {taco 6 true}
+```