look for python in the PATH

[rust-101.git] / src / part04.rs

diff --git a/src/part04.rs b/src/part04.rs
index 47177a7731d27172fd2d448f364415553fcfe7f9..e4c6a59e00140e7489feb5718b1ae9ae2c8700ba 100644 (file)
--- a/src/part04.rs
+++ b/src/part04.rs
@@ -13,10 +13,9 @@ use std::cmp;
       *first = 1337; // This is bad!
   }
 */
       *first = 1337; // This is bad!
   }
 */

-// What's going wrong here? `first` is a pointer into the vector `v`.
-// The operation `push_back` may re-allocate the storage for the vector,
-// in case the old buffer was full. If that happens, `first` is now
-// a dangling pointer, and accessing it can crash the program (or worse).
+// What's going wrong here? `first` is a pointer into the vector `v`. The operation `push_back`
+// may re-allocate the storage for the vector, in case the old buffer was full. If that happens,
+// `first` is now a dangling pointer, and accessing it can crash the program (or worse).

 // 
 // It turns out that only the combination of two circumstances can lead to such a bug:
 // *aliasing* and *mutation*. In the code above, we have `first` and the buffer of `v`
 // 
 // It turns out that only the combination of two circumstances can lead to such a bug:
 // *aliasing* and *mutation*. In the code above, we have `first` and the buffer of `v`


@@ -24,6 +23,7 @@ use std::cmp;
 // Therefore, the central principle of the Rust typesystem is to *rule out mutation in the presence
 // of aliasing*. The core tool to achieve that is the notion of *ownership*.
 
 // Therefore, the central principle of the Rust typesystem is to *rule out mutation in the presence
 // of aliasing*. The core tool to achieve that is the notion of *ownership*.
 

+// ## Ownership

 // What does that mean in practice? Consider the following example.
 fn work_on_vector(v: Vec<i32>) { /* do something */ }
 fn ownership_demo() {
 // What does that mean in practice? Consider the following example.
 fn work_on_vector(v: Vec<i32>) { /* do something */ }
 fn ownership_demo() {


@@ -37,7 +37,7 @@ fn ownership_demo() {
 // Passing a `Vec<i32>` to `work_on_vector` is considered *transfer of ownership*: Someone used
 // to own that vector, but now he gave it on to `take` and has no business with it anymore.
 // 
 // Passing a `Vec<i32>` to `work_on_vector` is considered *transfer of ownership*: Someone used
 // to own that vector, but now he gave it on to `take` and has no business with it anymore.
 // 

-// If you give a book to your friend, you cannot come to his place next day and get the book!
+// If you give a book to your friend, you cannot just come to his place next day and get the book!

 // It's no longer yours. Rust makes sure you don't break this rule. Try enabling the commented
 // line in `ownership_demo`. Rust will tell you that `v` has been *moved*, which is to say that ownership
 // has been transferred somewhere else. In this particular case, the buffer storing the data
 // It's no longer yours. Rust makes sure you don't break this rule. Try enabling the commented
 // line in `ownership_demo`. Rust will tell you that `v` has been *moved*, which is to say that ownership
 // has been transferred somewhere else. In this particular case, the buffer storing the data


@@ -45,6 +45,7 @@ fn ownership_demo() {
 // Essentially, ownership rules out aliasing, hence making the kind of problem discussed above
 // impossible.
 
 // Essentially, ownership rules out aliasing, hence making the kind of problem discussed above
 // impossible.
 

+// ## Shared borrowing

 // If you go back to our example with `vec_min`, and try to call that function twice, you will
 // get the same error. That's because `vec_min` demands that the caller transfers ownership of the
 // vector. Hence, when `vec_min` finishes, the entire vector is deleted. That's of course not what
 // If you go back to our example with `vec_min`, and try to call that function twice, you will
 // get the same error. That's because `vec_min` demands that the caller transfers ownership of the
 // vector. Hence, when `vec_min` finishes, the entire vector is deleted. That's of course not what


@@ -62,14 +63,13 @@ fn ownership_demo() {
 // introduces aliasing, so in order to live up to its promise of safety, Rust does not allow
 // mutation through a shared borrow.
 
 // introduces aliasing, so in order to live up to its promise of safety, Rust does not allow
 // mutation through a shared borrow.
 

-// So, let's re-write `vec_min` to work on a shared borrow of a vector. In fact, the only
-// thing we have to change is the type of the function. `&Vec<i32>` says that we need
-// a vector, but we won't own it. I also took the liberty to convert the function from
-// `SomethingOrNothing` to the standard library type `Option`.
+// So, let's re-write `vec_min` to work on a shared borrow of a vector, written `&Vec<i32>`.
+// I also took the liberty to convert the function from `SomethingOrNothing` to the standard
+// library type `Option`.

 fn vec_min(v: &Vec<i32>) -> Option<i32> {
     let mut min = None;
     for e in v {
 fn vec_min(v: &Vec<i32>) -> Option<i32> {
     let mut min = None;
     for e in v {

-        // In the loop, `e` now has type `&i32`, so we have to dereference it.
+        // In the loop, `e` now has type `&i32`, so we have to dereference it to obtain an `i32`.

         min = Some(match min {
             None => *e,
             Some(n) => cmp::min(n, *e)
         min = Some(match min {
             None => *e,
             Some(n) => cmp::min(n, *e)


@@ -95,18 +95,20 @@ fn shared_borrow_demo() {
 // borrow, Rust knows that it cannot mutate `v` in any way. Hence the pointer into the buffer of `v`
 // that was created before calling `vec_min` remains valid.
 
 // borrow, Rust knows that it cannot mutate `v` in any way. Hence the pointer into the buffer of `v`
 // that was created before calling `vec_min` remains valid.
 

+// ## Mutable borrowing

 // There is a second kind of borrow, a *mutable borrow*. As the name suggests, such a borrow permits
 // mutation, and hence has to prevent aliasing. There can only ever be one mutable borrow to a
 // particular piece of data.
 
 // As an example, consider a function which increments every element of a vector by 1.
 // There is a second kind of borrow, a *mutable borrow*. As the name suggests, such a borrow permits
 // mutation, and hence has to prevent aliasing. There can only ever be one mutable borrow to a
 // particular piece of data.
 
 // As an example, consider a function which increments every element of a vector by 1.

+// The type `&mut Vec<i32>` is the type of mutable borrows of `vec<i32>`. Because the borrow is
+// mutable, we can change `e` in the loop.

 fn vec_inc(v: &mut Vec<i32>) {
     for e in v {
         *e += 1;
     }
 }
 fn vec_inc(v: &mut Vec<i32>) {
     for e in v {
         *e += 1;
     }
 }

-// The type `&mut Vec<i32>` is the type of mutable borrows of `vec<i32>`. Because the borrow is
-// mutable, we can change `e` in the loop. How can we call this function?
+// Here's an example of calling `vec_inc`.

 fn mutable_borrow_demo() {
     let mut v = vec![5,4,3,2,1];
     /* let first = &v[0]; */
 fn mutable_borrow_demo() {
     let mut v = vec![5,4,3,2,1];
     /* let first = &v[0]; */


@@ -114,33 +116,21 @@ fn mutable_borrow_demo() {
     vec_inc(&mut v);
     /* println!("The first element is: {}", *first); */
 }
     vec_inc(&mut v);
     /* println!("The first element is: {}", *first); */
 }

-// `&mut` is the operator to create a mutable borrow. We have to mark `v` as mutable in order
-// to create such a borrow. Because the borrow passed to `vec_inc` only lasts as
-// long as the function call, we can still call `vec_inc` on the same vector twice:
-// The durations of the two borrows do not overlap, so we never have more than one mutable borrow.
-// However, we can *not* create a shared borrow that spans a call to `vec_inc`. Just try
+// `&mut` is the operator to create a mutable borrow. We have to mark `v` as mutable in order to create such a
+// borrow. Because the borrow passed to `vec_inc` only lasts as long as the function call, we can still call
+// `vec_inc` on the same vector twice: The durations of the two borrows do not overlap, so we never have more
+// than one mutable borrow. However, we can *not* create a shared borrow that spans a call to `vec_inc`. Just try

 // enabling the commented-out lines, and watch Rust complain. This is because `vec_inc` could mutate
 // the vector structurally (i.e., it could add or remove elements), and hence the pointer `first`
 // enabling the commented-out lines, and watch Rust complain. This is because `vec_inc` could mutate
 // the vector structurally (i.e., it could add or remove elements), and hence the pointer `first`

-// could become invalid.
+// could become invalid. In other words, Rust keeps us safe from bugs like the one in the C++ snipped above.

 // 
 // Above, I said that having a mutable borrow excludes aliasing. But if you look at the code above carefully,
 // you may say: "Wait! Don't the `v` in `mutable_borrow_demo` and the `v` in `vec_inc` alias?" And you are right,
 // they do. However, the `v` in `mutable_borrow_demo` is not actually usable, it is not *active*: As long as there is an
 // 
 // Above, I said that having a mutable borrow excludes aliasing. But if you look at the code above carefully,
 // you may say: "Wait! Don't the `v` in `mutable_borrow_demo` and the `v` in `vec_inc` alias?" And you are right,
 // they do. However, the `v` in `mutable_borrow_demo` is not actually usable, it is not *active*: As long as there is an

-// outstanding borrow, Rust will not allow you to do anything with `v`. This is, in fact, what
-// prevents the creation of a mutable borrow when there already is a shared one, as you witnessed
-// when enabling `first` above.
-
-// This also works the other way around: In `multiple_borrow_demo`, there is already a mutable borrow
-// active in the `vec_min` line, so the attempt to create another shared borrow is rejected by the compiler.
-fn multiple_borrow_demo() {
-    let mut v = vec![5,4,3,2,1];
-    let first = &mut v[0];
-    /* vec_min(&v); */
-    *first += 1;
-    println!("The first element is now: {}", *first);
-}
+// outstanding borrow, Rust will not allow you to do anything with `v`.

 
 

-// So, to summarize - the ownership and borrowing system of Rust enforces the following three rules:
+// ## Summary
+// The ownership and borrowing system of Rust enforces the following three rules:

 // 
 // * There is always exactly one owner of a piece of data
 // * If there is an active mutable borrow, then nobody else can have active access to the data
 // 
 // * There is always exactly one owner of a piece of data
 // * If there is an active mutable borrow, then nobody else can have active access to the data


@@ -149,4 +139,4 @@ fn multiple_borrow_demo() {
 // As it turns out, combined with the abstraction facilities of Rust, this is a very powerful mechanism
 // to tackle many problems beyond basic memory safety. You will see some examples for this soon.
 
 // As it turns out, combined with the abstraction facilities of Rust, this is a very powerful mechanism
 // to tackle many problems beyond basic memory safety. You will see some examples for this soon.
 

-// [index](main.html) | [previous](part03.html) | [next](main.html)
+// [index](main.html) | [previous](part03.html) | [next](part05.html)