Rollup merge of #27163 - steveklabnik:doc_std_io_cursor, r=alexcrichton

Beef up the docs on the type, as well as adding examples for all
methods.

r? @alexcrichton

src/libstd/io/cursor.rs

@@ -15,16 +15,67 @@ use cmp;
 use io::{self, SeekFrom, Error, ErrorKind};
 use slice;
 
-/// A `Cursor` is a type which wraps a non-I/O object to provide a `Seek`
+/// A `Cursor` wraps another type and provides it with a [`Seek`][seek]
 /// implementation.
 ///
-/// Cursors are typically used with memory buffer objects in order to allow
-/// `Seek`, `Read`, and `Write` implementations. For example, common cursor types
-/// include `Cursor<Vec<u8>>` and `Cursor<&[u8]>`.
+/// [seek]: trait.Seek.html
 ///
-/// Implementations of the I/O traits for `Cursor<T>` are currently not generic
-/// over `T` itself. Instead, specific implementations are provided for various
-/// in-memory buffer types like `Vec<u8>` and `&[u8]`.
+/// Cursors are typically used with in-memory buffers to allow them to
+/// implement `Read` and/or `Write`, allowing these buffers to be used
+/// anywhere you might use a reader or writer that does actual I/O.
+///
+/// The standard library implements some I/O traits on various types which
+/// are commonly used as a buffer, like `Cursor<Vec<u8>>` and `Cursor<&[u8]>`.
+///
+/// # Examples
+///
+/// We may want to write bytes to a [`File`][file] in our production
+/// code, but use an in-memory buffer in our tests. We can do this with
+/// `Cursor`:
+///
+/// [file]: ../fs/struct.File.html
+///
+/// ```no_run
+/// use std::io::prelude::*;
+/// use std::io::{self, SeekFrom};
+/// use std::fs::File;
+///
+/// // a library function we've written
+/// fn write_ten_bytes_at_end<W: Write + Seek>(writer: &mut W) -> io::Result<()> {
+///     try!(writer.seek(SeekFrom::End(-10)));
+///
+///     for i in 0..10 {
+///         try!(writer.write(&[i]));
+///     }
+///
+///     // all went well
+///     Ok(())
+/// }
+///
+/// # fn foo() -> io::Result<()> {
+/// // Here's some code that uses this library function.
+/// //
+/// // We might want to use a BufReader here for efficiency, but let's
+/// // keep this example focused.
+/// let mut file = try!(File::create("foo.txt"));
+///
+/// try!(write_ten_bytes_at_end(&mut file));
+/// # Ok(())
+/// # }
+///
+/// // now let's write a test
+/// #[test]
+/// fn test_writes_bytes() {
+///     // setting up a real File is much more slow than an in-memory buffer,
+///     // let's use a cursor instead
+///     use std::io::Cursor;
+///     let mut buff = Cursor::new(vec![0; 15]);
+///
+///     write_ten_bytes(&mut buff).unwrap();
+///
+///     assert_eq!(&buff.get_ref()[5..15], &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+/// }
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(Clone, Debug)]
 pub struct Cursor<T> {
@@ -34,16 +85,50 @@ pub struct Cursor<T> {
 
 impl<T> Cursor<T> {
     /// Creates a new cursor wrapping the provided underlying I/O object.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    ///
+    /// let buff = Cursor::new(Vec::new());
+    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
+    /// # force_inference(&buff);
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new(inner: T) -> Cursor<T> {
         Cursor { pos: 0, inner: inner }
     }
 
     /// Consumes this cursor, returning the underlying value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    ///
+    /// let buff = Cursor::new(Vec::new());
+    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
+    /// # force_inference(&buff);
+    ///
+    /// let vec = buff.into_inner();
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn into_inner(self) -> T { self.inner }
 
     /// Gets a reference to the underlying value in this cursor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    ///
+    /// let buff = Cursor::new(Vec::new());
+    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
+    /// # force_inference(&buff);
+    ///
+    /// let reference = buff.get_ref();
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn get_ref(&self) -> &T { &self.inner }
 
@@ -51,14 +136,60 @@ impl<T> Cursor<T> {
     ///
     /// Care should be taken to avoid modifying the internal I/O state of the
     /// underlying value as it may corrupt this cursor's position.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    ///
+    /// let mut buff = Cursor::new(Vec::new());
+    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
+    /// # force_inference(&buff);
+    ///
+    /// let reference = buff.get_mut();
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn get_mut(&mut self) -> &mut T { &mut self.inner }
 
-    /// Returns the current value of this cursor
+    /// Returns the current position of this cursor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    /// use std::io::prelude::*;
+    /// use std::io::SeekFrom;
+    ///
+    /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
+    ///
+    /// assert_eq!(buff.position(), 0);
+    ///
+    /// buff.seek(SeekFrom::Current(2)).unwrap();
+    /// assert_eq!(buff.position(), 2);
+    ///
+    /// buff.seek(SeekFrom::Current(-1)).unwrap();
+    /// assert_eq!(buff.position(), 1);
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn position(&self) -> u64 { self.pos }
 
-    /// Sets the value of this cursor
+    /// Sets the position of this cursor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    ///
+    /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
+    ///
+    /// assert_eq!(buff.position(), 0);
+    ///
+    /// buff.set_position(2);
+    /// assert_eq!(buff.position(), 2);
+    ///
+    /// buff.set_position(4);
+    /// assert_eq!(buff.position(), 4);
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn set_position(&mut self, pos: u64) { self.pos = pos; }
 }