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embedding part 1.

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Rob Pike 2009-10-16 11:13:40 -07:00
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doc/effective_go.html
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@ -1,6 +1,6 @@
<!-- Effective Go -->
<!-- interfaces; cast,conversion, type assertion; embedding; errors; testing; initialization -->
<!-- testing?; concurrency; initialization-->
<h2 id="introduction">Introduction</h2>
@ -34,7 +34,7 @@ should read first.
<h3 id="read">Examples</h3>
<p>
The <a href="http://s2/?dir=//depot2/go/src/pkg">Go package sources</a>
The <a href="/src/pkg/">Go package sources</a>
are intended to serve not
only as the core library but also as examples of how to
use the language.
@ -1551,7 +1551,7 @@ type Handler interface {
<p>
For brevity, let's ignore POSTs and assume HTTP requests are always
GETs; that simplification does not affect the way the handlers are
made. Here's a trivial but complete implementation of a handler to
set up. Here's a trivial but complete implementation of a handler to
count the number of times the
page is visited.
</p>
@ -1568,7 +1568,7 @@ func (ctr *Counter) ServeHTTP(c *http.Conn, req *http.Request) {
</pre>
<p>
(Keeping with our theme, note how <code>Fprintf</code> can print to an HTTP connection.)
For reference, here's how to set up such a server.
For reference, here's how to attach such a server to a node on the URL tree.
<pre>
import "http"
...
@ -1595,17 +1595,17 @@ has been visited? Tie a channel to the web page.
<pre>
// A channel that sends a notification on each visit.
// (Probably want the channel to be buffered.)
type Chan chan int
type Chan chan *http.Request
func (ch Chan) ServeHTTP(c *http.Conn, req *http.Request) {
ch <- 1;
ch <- req;
fmt.Fprint(c, "notification sent");
}
</pre>
<p>
Finally, let's say we wanted to present on <code>/args</code> the arguments
used when invoking the server binary.
It's easy to write a function to print the arguments:
It's easy to write a function to print the arguments.
</p>
<pre>
func ArgServer() {
@ -1617,8 +1617,8 @@ func ArgServer() {
<p>
How do we turn that into an HTTP server? We could make <code>ArgServer</code>
a method of some type whose value we ignore, but there's a cleaner way.
Since we can write a method for (almost) any type, we can write a method
for a function.
Since we can define a method for any type except pointers and interfaces,
we can write a method for a function.
The <code>http</code> package contains this code:
</p>
<pre>
@ -1641,8 +1641,8 @@ calls <code>f</code>. That may seem odd but it's no different from, say,
the receiver being a channel and the method sending on the channel.
</p>
<p>
To make <code>ArgServer</code> into an HTTP server, we first give it the right
signature.
To make <code>ArgServer</code> into an HTTP server, we first modify it
to have the right signature.
</p>
<pre>
// Argument server.
@ -1653,30 +1653,134 @@ func ArgServer(c *http.Conn, req *http.Request) {
}
</pre>
<p>
<code>ArgServer</code> has same signature as <code>HandlerFunc</code>,
so the function can be converted to that type to access its methods,
just as we converted <code>Sequence</code> to <code>[]int</code> earlier.
The code to set it up is short:
<code>ArgServer</code> now has same signature as <code>HandlerFunc</code>,
so it can be converted to that type to access its methods,
just as we converted <code>Sequence</code> to <code>IntArray</code>
to access <code>IntArray.Sort</code>.
The code to set it up is concise:
</p>
<pre>
http.Handle("/args", http.HandlerFunc(ArgServer));
</pre>
<p>
When someone visits the page <code>/args</code>,
the handler installed at that page has type
<code>HandlerFunc</code> and value <code>ArgServer</code>.
the handler installed at that page has value <code>ArgServer</code>
and type <code>HandlerFunc</code>.
The HTTP server will invoke the method <code>ServeHTTP</code>
of that type, with that receiver, which will in turn call
of that type, with <code>ArgServer</code> as the receiver, which will in turn call
<code>ArgServer</code> (via the invocation <code>f(c, req)</code>
inside <code>HandlerFunc.ServeHTTP</code>) and the arguments
will be displayed.
inside <code>HandlerFunc.ServeHTTP</code>).
The arguments will then be displayed.
</p>
<p>
In summary, we have made an HTTP server from a struct, an integer,
In this section we have made an HTTP server from a struct, an integer,
a channel, and a function, all because interfaces are just sets of
methods, which can be defined for (almost) any type.
</p>
<h2 id="embedding">Embedding</h2>
<p>
Go does not provide the typical, type-driven notion of subclassing,
but it does have the ability to &ldquo;borrow&rdquo; pieces of an
implementation by <em>embedding</em> types within a struct or
interface.
</p>
<p>
Interface embedding is very simple.
We've mentioned the <code>io.Reader</code> and <code>io.Writer</code> interfaces before;
here are their definitions.
</p>
<pre>
type Reader interface {
Read(p []byte) (n int, err os.Error);
}
type Writer interface {
Write(p []byte) (n int, err os.Error);
}
</pre>
<p>
The <code>io</code> package also exports several other interfaces
that specify objects that can implement several such methods.
For instance, there is <code>io.ReadWriter</code>, an interface
containing both <code>Read</code> and <code>Write</code>.
We could specify <code>io.ReadWriter</code> by listing the
two methods explicitly, but it's easier and more evocative
to embed the two interfaces to form the new one, like this:
</p>
<pre>
// ReadWrite is the interface that groups the basic Read and Write methods.
type ReadWriter interface {
Reader;
Writer;
}
</pre>
<p>
This says just what it looks like: A <code>ReadWriter</code> can do
what a <code>Reader</code> does <em>and</em> what a <code>Writer</code>
does; it is a union of the embedded interfaces (which must be disjoint
sets of methods).
Only interfaces can be embedded within interfaces.
<p>
The same basic idea applies to structs, but with more far-reaching
implications. The <code>bufio</code> package has two struct types,
<code>bufio.Reader</code> and <code>bufio.Writer</code>, each of
which of course implements the analogous interfaces from package
<code>io</code>.
And <code>bufio</code> also implements a buffered reader/writer,
which it does by combining a reader and a writer into one struct
using embedding: it lists the types within the struct
but does not give them field names.
</p>
<pre>
// ReadWriter stores pointers to a Reader and a Writer.
// It implements io.ReadWriter.
type ReadWriter struct {
*Reader;
*Writer;
}
</pre>
<p>
This struct could be written as
</p>
<pre>
type ReadWriter struct {
reader *Reader;
writer *Writer;
}
</pre>
<p>
but then to promote the methods of the fields and to
satisfy the <code>io</code> interfaces, we would also need
to provide forwarding methods, like this:
</p>
<pre>
func (rw *ReadWriter) Read(p []byte) (n int, err os.Error) {
return rw.reader.Read(p)
}
</pre>
<p>
By embedding the structs directly, we avoid this bookkeeping.
The methods of embedded types come along for free, which means that <code>bufio.ReadWriter</code>
not only has the methods of <code>bufio.Reader</code> and <code>bufio.Writer</code>,
it also satisfies all three interfaces:
<code>io.Reader</code>,
<code>io.Writer</code>, and
<code>io.ReadWriter</code>.
</p>
<p>
There's one important way in which embedding differs from subclassing. When we embed a type,
the methods of that type become methods of the out type
<but when they are invoked the receiver of the method is the inner type, not the outer one.
In our example, when the <code>Read</code> method of a <code>bufio.ReadWriter</code> is
invoked, it has the exactly the same effect as the forwarding method written out above;
the receiver is the <code>reader</code> field of the <code>ReadWriter</code>, not the
<code>ReadWriter</code> itself.
</p>
<h2 id="errors">Errors</h2>
<p>
@ -1735,7 +1839,7 @@ field for recoverable failures.
<pre>
for try := 0; try < 2; try++ {
file, err := os.Open(filename, os.O_RDONLY, 0);
file, err = os.Open(filename, os.O_RDONLY, 0);
if err == nil {
return
}