From b8617bb7527d39874b507a4c25bdaa8b6c4f4c3f Mon Sep 17 00:00:00 2001
From: Dave Gauer <dave@ratfactor.com>
Date: Sun, 31 Jul 2022 15:57:20 -0400
Subject: Correct conventional Zig reference vs value passing re #89

---
 exercises/042_pointers4.zig | 11 ++++++++++-
 exercises/043_pointers5.zig | 40 +++++++++++++++++++++++++++++-----------
 exercises/047_methods.zig   |  6 +++---
 3 files changed, 42 insertions(+), 15 deletions(-)

(limited to 'exercises')

diff --git a/exercises/042_pointers4.zig b/exercises/042_pointers4.zig
index 261dbc1..359a2f1 100644
--- a/exercises/042_pointers4.zig
+++ b/exercises/042_pointers4.zig
@@ -1,6 +1,15 @@
 //
 // Now let's use pointers to do something we haven't been
-// able to do before: pass a value by reference to a function!
+// able to do before: pass a value by reference to a function.
+//
+// Why would we wish to pass a pointer to an integer variable
+// rather than the integer value itself? Because then we are
+// allowed to *change* the value of the variable!
+//
+//     +-----------------------------------------------+
+//     | Pass by reference when you want to change the |
+//     | pointed-to value. Otherwise, pass the value.  |
+//     +-----------------------------------------------+
 //
 const std = @import("std");
 
diff --git a/exercises/043_pointers5.zig b/exercises/043_pointers5.zig
index f3a9ca0..9e2fa6f 100644
--- a/exercises/043_pointers5.zig
+++ b/exercises/043_pointers5.zig
@@ -1,8 +1,7 @@
 //
-// Passing integer pointers around is generally not something you're going
-// to do. Integers are cheap to copy.
-//
-// But you know what IS useful? Pointers to structs:
+// As with integers, you can pass a pointer to a struct when you
+// will wish to modify that struct. Pointers are also useful when
+// you need to store a reference to a struct (a "link" to it).
 //
 //     const Vertex = struct{ x: u32, y: u32, z: u32 };
 //
@@ -16,7 +15,8 @@
 //     YES: pv.x
 //     NO:  pv.*.x
 //
-// We can write functions that take pointer arguments:
+// We can write functions that take pointers to structs as
+// arguments. This foo() function modifies struct v:
 //
 //     fn foo(v: *Vertex) void {
 //         v.x += 2;
@@ -24,13 +24,13 @@
 //         v.z += 7;
 //     }
 //
-// And pass references to them:
+// And call them like so:
 //
 //     foo(&v1);
 //
-//
 // Let's revisit our RPG example and make a printCharacter() function
-// that takes a Character pointer.
+// that takes a Character by reference and prints it...*and*
+// prints a linked "mentor" Character, if there is one.
 //
 const std = @import("std");
 
@@ -44,19 +44,30 @@ const Class = enum {
 const Character = struct {
     class: Class,
     gold: u32,
-    health: u8 = 100, // <--- You can also provide fields a default value!
+    health: u8 = 100, // You can provide default values
     experience: u32,
+
+    // I need to use the '?' here to allow for a null value. But
+    // I don't explain it until later. Please don't tell anyone.
+    mentor: ?*Character = null,
 };
 
 pub fn main() void {
-    var glorp = Character{
+    var mighty_krodor = Character{
+        .class = Class.wizard,
+        .gold = 10000,
+        .experience = 2340,
+    };
+
+    var glorp = Character{ // Glorp!
         .class = Class.wizard,
         .gold = 10,
         .experience = 20,
+        .mentor = &mighty_krodor, // Glorp's mentor is the Mighty Krodor
     };
 
     // FIX ME!
-    // Please pass our Character "glorp" to printCharacter():
+    // Please pass Glorp to printCharacter():
     printCharacter(???);
 }
 
@@ -78,4 +89,11 @@ fn printCharacter(c: *Character) void {
         c.health,
         c.experience,
     });
+
+    // Checking an "optional" value and capturing it will be
+    // explained later (this pairs with the '?' mentioned above.)
+    if (c.mentor) |mentor| {
+        std.debug.print("  Mentor: ", .{});
+        printCharacter(mentor);
+    }
 }
diff --git a/exercises/047_methods.zig b/exercises/047_methods.zig
index 048cfa0..a3ed220 100644
--- a/exercises/047_methods.zig
+++ b/exercises/047_methods.zig
@@ -10,7 +10,7 @@
 //         pub fn hello() void {
 //             std.debug.print("Foo says hello!\n", .{});
 //         }
-//     }
+//     };
 //
 // 2. A function that is a member of a struct is a "method" and is
 //    called with the "dot syntax" like so:
@@ -23,10 +23,10 @@
 //     const Bar = struct{
 //         number: u32,
 //
-//         pub fn printMe(self: *Bar) void {
+//         pub fn printMe(self: Bar) void {
 //             std.debug.print("{}\n", .{self.number});
 //         }
-//     }
+//     };
 //
 //    (Actually, you can name the first parameter anything, but
 //    please follow convention and use "self".)
-- 
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