doc/classes.md: explain instantiation better

doc/classes.md

@@ -35,8 +35,18 @@ In reality, instances may be bigger than expected due to
 [padding](https://stackoverflow.com/questions/4306186/structure-padding-and-packing),
 but this can be almost always ignored.
 
-You can use e.g. `Point{x=12, y=34}` to instantiate the class,
-and the usual `.` syntax to access its fields:
+To create an instance of `Point`,
+we simply need enough memory to hold the two `int`s,
+and we need to tell the compiler to treat it as a `Point` instance.
+In other words, we simply create a variable whose type is `Point`:
+
+```python
+p: Point
+```
+
+Because the memory is [uninitialized](tutorial.md#undefined-behavior-ub),
+we still need to assign values to the fields, which we can access as `p.x` and `p.y`.
+Like this:
 
 ```python
 import "stdlib/io.jou"
@@ -46,30 +56,26 @@ class Point:
     y: int
 
 def main() -> int:
-    p = Point{x=12, y=34}
+    p: Point
+    p.x = 12
+    p.y = 34
     printf("%d, %d\n", p.x, p.y)  # Output: 12, 34
     p.y++
     printf("%d, %d\n", p.x, p.y)  # Output: 12, 35
     return 0
 ```
 
-This does not allocate any heap memory (TODO: document heap allocations).
-In fact, it's basically same as creating two variables `x` and `y` in the `main()` function:
+Alternatively, we could create the instance in one line with `p = Point{x = 12, y = 34}`.
+This syntax is explained in detail [below](#instantiating).
 
-```python
-import "stdlib/io.jou"
 
-def main() -> int:
-    x = 12
-    y = 34
-    printf("%d, %d\n", x, y)  # Output: 12, 34
-    return 0
-```
+## Pointers
 
-This means that if you pass an instance of a class to a function, you get a copy,
-as if you had just passed the two integers:
+Instances of classes are often passed around as pointers.
+To understand why, let's try to make a function
+that increments the `x` coordinate of a `Point`:
 
-```python
+ ```python
 import "stdlib/io.jou"
 
 class Point:
@@ -86,6 +92,27 @@ def main() -> int:
     return 0
 ```
 
+The problem is that when we do `increment_y()`,
+we simply pass the 64 (or more) bytes of the struct to the `increment_y()` method.
+This is very similar to creating two variables `x` and `y` in the `main()` function:
+
+```python
+import "stdlib/io.jou"
+
+def increment_y(x: int, y: int) -> None:
+    y++  # Doesn't work as expected
+
+def main() -> int:
+    x = 12
+    y = 34
+    increment_y(x, y)
+    printf("%d, %d\n", x, y)  # Output: 12, 34
+    return 0
+```
+
+In either case, the `increment_y()` function gets a copy of the coordinates,
+so `instance.y++` or `y++` only increments the `y` coordinate of the copy.
+
 For this reason, instances of classes are often passed around as [pointers](tutorial.md#pointers).
 This way the `increment_y()` function knows where the original instance is in the computer's memory,
 so that it can place the new value there instead of its own copy of the instance.
@@ -110,6 +137,9 @@ def main() -> int:
 
 Here `ptr->y` does the same thing as `(*ptr).y`:
 it accesses the `y` member of the instance located wherever the pointer `ptr` is pointing.
+This arrow syntax feels weird at first,
+but it's convenient once you get used to it,
+and the C programming language uses the same syntax.
 
 
 ## Methods
@@ -181,8 +211,13 @@ so `self` is not a pointer.
 
 ## Instantiating
 
-As we have seen, the instantiating syntax is `ClassName{field=value}`.
-The curly braces are used to distinguish instantiating from function calls.
+As we have seen, "instantiating" simply means taking a chunk of memory of the correct size,
+but it's often done with the `ClassName{field=value}` syntax.
+Let's look at this syntax in more detail.
+
+The curly braces are used to distinguish instantiating syntax from function calls.
+This makes the Jou compiler simpler, but if you don't like this syntax,
+feel free to [create an issue](https://github,com/Akuli/jou/issues/new) to discuss it.
 
 If you omit some class fields, they will be initialized to zero.
 Specifically, the memory used for the fields will be all zero bytes.
@@ -222,38 +257,23 @@ def main() -> int:
 You can achieve the same thing by setting the memory used by the instance to zero bytes.
 This is often done with the `memset()` function from [stdlib/mem.jou](../stdlib/mem.jou).
 It takes in three parameters, so that `memset(ptr, 0, n)` sets `n` bytes starting at pointer `ptr` to zero.
-To calculate the correct `n`, you can use the `sizeof` operator (TODO: document sizeof):
 
-```python
-import "stdlib/io.jou"
-import "stdlib/mem.jou"
-
-
-class Person:
-    name: byte*
-    country: byte[50]
+To calculate the correct `n`, you can use `sizeof(instance)`, where `instance` is any instance of the class.
+It doesn't matter which instance you use, because all instances of the class are of the same size.
+In general, the value of `sizeof(x)` only depends on the type of `x`,
+and it doesn't even evaluate `x` when the program runs.
 
-    def introduce(self) -> None:
-        if self->name == NULL:
-            printf("I'm an anonymous person from '%s'\n", self->country)
-        else:
-            printf("I'm %s from '%s'\n", self->name, self->country)
-
-
-def main() -> int:
-    akuli = Person{name="Akuli", country="Finland"}
-    memset(&akuli, 0, sizeof(akuli))
-    akuli.introduce()  # Output: I'm an anonymous person from ''
-    return 0
-```
-
-This works the same way if you have multiple instances next to each other in memory,
-such as in an array, and you want to zero-initialize all of them:
+For example, the following program creates an array of three uninitialized instances of `Person`,
+and then zero-initializes all of them using `memset()`.
+Array elements are simply next to each other in memory,
+so it's enough to do one `memset()` that is big enough to set all of them to zero.
+Like this:
 
 ```python
 import "stdlib/io.jou"
 import "stdlib/mem.jou"
 
+
 class Person:
     name: byte*
     country: byte[50]
@@ -264,29 +284,21 @@ class Person:
         else:
             printf("I'm %s from '%s'\n", self->name, self->country)
 
-def main() -> int:
-    contributors = [
-        Person{name="Akuli", country="Finland"},
-        Person{name="littlewhitecloud", country="China"},
-        Person{name="Moosems", country="USA"},
-    ]
-
-    # Output: I'm Akuli from 'Finland'
-    # Output: I'm littlewhitecloud from 'China'
-    # Output: I'm Moosems from 'USA'
-    for i = 0; i < 3; i++:
-        contributors[i].introduce()
 
-    memset(&contributors, 0, sizeof(contributors))
+def main() -> int:
+    people: Person[3]
+    memset(&people, 0, sizeof(people[0]) * 3)
 
     # Output: I'm an anonymous person from ''
     # Output: I'm an anonymous person from ''
     # Output: I'm an anonymous person from ''
     for i = 0; i < 3; i++:
-        contributors[i].introduce()
+        people[i].introduce()
 
     return 0
 ```
 
-Here `sizeof(contributors)` is the size of the entire array in bytes,
-which is 3 times the size of a `Person`.
+Instead of `sizeof(people[0]) * 3`, you could just as well use `sizeof(people)`.
+The size of an array of 3 elements is simply 3 times the size of one element.
+You could also use `people = [Person{}, Person{}, Person{}]` to create and zero-initialize the array,
+but this becomes annoying if the array contains many instances.