tune explanation of PhantomData ownership annotation

[rust-101.git] / workspace / src / part09.rs

diff --git a/workspace/src/part09.rs b/workspace/src/part09.rs
index d6b8bbae1eca6410e7f4a4c67b7940a3abdcf7b6..61cc70b8ddb6b678030b4f57892717668ad36d55 100644 (file)
--- a/workspace/src/part09.rs
+++ b/workspace/src/part09.rs
@@ -1,26 +1,10 @@
-// Rust-101, Part 09: Iterators (WIP)
-// ==================================
+// Rust-101, Part 09: Iterators
+// ============================

 
 use part05::BigInt;
 
 
 use part05::BigInt;
 

-// In the following, we will look into the iterator mechanism of Rust and make our `BigInt` compatible
-// with the `for` loops. Of course, this is all about implementing particular traits again. In particular,
-// an iterator is something that implements the `Iterator` trait. As you can see in [the documentation](http://doc.rust-lang.org/beta/std/iter/trait.Iterator.html),
-// this trait mandates a single function `next` returning an `Option<Self::Item>`, where `Item` is an
-// associated type chosen by the implementation. (There are many more methods provided for `Iterator`,
-// but they all have default implementations, so we don't have to worry about them right now).
-// 
-// For the case of `BigInt`, we want our iterator to iterate over the digits in normal, notational order: The most-significant
-// digit comes first. So, we have to write down some type, and implement `Iterator` for it such that `next` returns the digits
-// one-by-one. Clearly, the iterator must somehow be able to access the number it iterates over, and it must store its current
-// location. However, it cannot *own* the `BigInt`, because then the number would be gone after iteration! That'd certainly be bad.
-// The only alternative is for the iterator to *borrow* the number.
-
-// In writing this down, we again have to be explicit about the lifetime of the borrow: We can't just have an
-// `Iter`, we must have an `Iter<'a>` that borrowed the number for lifetime `'a`. <br/>
-// `usize` here is the type of unsigned, pointer-sized numbers. It is typically the type of "lengths of things",
-// in particular, it is the type of the length of a `Vec` and hence the right type to store an offset into the vector of digits.
-struct Iter<'a> {
+
+pub struct Iter<'a> {

     num: &'a BigInt,
     idx: usize, // the index of the last number that was returned
 }
     num: &'a BigInt,
     idx: usize, // the index of the last number that was returned
 }


@@ -33,10 +17,10 @@ impl<'a> Iterator for Iter<'a> {
     fn next(&mut self) -> Option<u64> {
         // First, check whether there's any more digits to return.
         if self.idx == 0 {
     fn next(&mut self) -> Option<u64> {
         // First, check whether there's any more digits to return.
         if self.idx == 0 {

-            // We already returned all the digits.
+            // We already returned all the digits, nothing to do.

             unimplemented!()
         } else {
             unimplemented!()
         } else {

-            // Decrement, and return next digit.
+            // Otherwise: Decrement, and return next digit.

             unimplemented!()
         }
     }
             unimplemented!()
         }
     }


@@ -44,9 +28,6 @@ impl<'a> Iterator for Iter<'a> {
 
 // All we need now is a function that creates such an iterator for a given `BigInt`.
 impl BigInt {
 
 // All we need now is a function that creates such an iterator for a given `BigInt`.
 impl BigInt {

-    // Notice that when we write the type of `iter`, we don't actually have to give the lifetime parameter of `Iter`. Just as it is
-    // the case with functions returning borrowed data, you can elide the lifetime. The rules for adding the lifetimes are exactly the
-    // same. (See the last section of [part 06](part06.html).)

     fn iter(&self) -> Iter {
         unimplemented!()
     }
     fn iter(&self) -> Iter {
         unimplemented!()
     }


@@ -60,3 +41,45 @@ pub fn main() {
     }
 }
 
     }
 }
 

+// Of course, we don't have to use `for` to apply the iterator. We can also explicitly call `next`.
+fn print_digits_v1(b: &BigInt) {
+    let mut iter = b.iter();
+    loop {
+        // Each time we go through the loop, we analyze the next element presented by the iterator - until it stops.
+        unimplemented!()
+    }
+}
+
+fn print_digits_v2(b: &BigInt) {
+    let mut iter = b.iter();
+    while let Some(digit) = iter.next() {
+        println!("{}", digit)
+    }
+}
+
+// **Exercise 09.1**: Write a testcase for the iterator, making sure it yields the corrects numbers.
+// 
+// **Exercise 09.2**: Write a function `iter_ldf` that iterators over the digits with the least-significant
+// digits coming first. Write a testcase for it.
+
+// ## Iterator invalidation and lifetimes
+
+fn iter_invalidation_demo() {
+    let mut b = BigInt::new(1 << 63) + BigInt::new(1 << 16) + BigInt::new(1 << 63);
+    for digit in b.iter() {
+        println!("{}", digit);
+        /*b = b + BigInt::new(1);*/                                 /* BAD! */
+    }
+}
+
+// ## Iterator conversion trait
+
+impl<'a> IntoIterator for &'a BigInt {
+    type Item = u64;
+    type IntoIter = Iter<'a>;
+    fn into_iter(self) -> Iter<'a> {
+        self.iter()
+    }
+}
+// With this in place, you can now replace `b.iter()` in `main` by `&b`. Go ahead and try it! <br/>
+