Add Pass A Struct To A Function as a Go TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1551 TILs and counting..._
+_1552 TILs and counting..._
 
 ---
 
@@ -415,6 +415,7 @@ _1551 TILs and counting..._
 - [Not So Random](go/not-so-random.md)
 - [Parse A String Into Individual Fields](go/parse-a-string-into-individual-fields.md)
 - [Parse Flags From CLI Arguments](go/parse-flags-from-cli-arguments.md)
+- [Pass A Struct To A Function](go/pass-a-struct-to-a-function.md)
 - [Produce The Zero Value Of A Generic Type](go/produce-the-zero-value-of-a-generic-type.md)
 - [Redirect File To Stdin During Delve Debug](go/redirect-file-to-stdin-during-delve-debug.md)
 - [Replace The Current Process With An External Command](go/replace-the-current-process-with-an-external-command.md)

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}
+```