Correct conventional Zig reference vs value passing re #89

build.zig

@@ -240,7 +240,8 @@ const exercises = [_]Exercise{
     },
     .{
         .main_file = "043_pointers5.zig",
-        .output = "Wizard (G:10 H:100 XP:20)",
+        .output = "Wizard (G:10 H:100 XP:20)\n  Mentor: Wizard (G:10000 H:100 XP:2340)",
+
     },
     .{
         .main_file = "044_quiz5.zig",

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");
 

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);
+    }
 }

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".)

patches/gollum.sh

@@ -22,7 +22,9 @@ p=patches/patches/$f.patch
 if [ ! -f $b ]; then echo "No $f! We hates it!"; exit 1; fi
 if [ ! -f $a ]; then echo "No $a! Where is it? Where is the answer, precious?"; exit; fi
 
-echo "Hissss!\tbefore: '$b'\n\t after: '$a'\n\t patch: '$p'\n"
+echo "Hissss!    before: '$b'"
+echo "            after: '$a'"
+echo "            patch: '$p'"
 
 diff $b $a > $p
 

patches/patches/042_pointers4.patch

@@ -1,4 +1,4 @@
-31c31
+40c40
 <     ??? = 5; // fix me!
 ---
 >     x.* = 5; // fix me!

patches/patches/043_pointers5.patch

@@ -1,4 +1,4 @@
-60c60
+71c71
 <     printCharacter(???);
 ---
 >     printCharacter(&glorp);