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doc/go_faq.html: add entry about pointer to interface

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R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7546050
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Rob Pike 2013-03-15 11:38:50 -07:00
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commit 09cd13c51d
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@ -974,6 +974,57 @@ struct. If the interface value holds a pointer, copying the interface value
makes a copy of the pointer, but again not the data it points to.
</p>
<h3 id="pointer_to_interface">
When should I use a pointer to an interface?</h3>
<p>
Almost never. Pointers to interface values arise only in rare, tricky situations involving
disguising an interface value's type for delayed evaluation.
</p>
<p>
It is however a common mistake to pass a pointer to an interface value
to a function expecting an interface. The compiler will complain about this
error but the situation can still be confusing, because sometimes a
<a href="#different_method_sets">pointer
is necessary to satisfy an interface</a>.
The insight is that although a pointer to a concrete type can satisfy
an interface, with one exception <em>a pointer to an interface can never satisfy a interface</em>.
</p>
<p>
Consider the variable declaration,
</p>
<pre>
var w io.Writer
</pre>
<p>
The printing function <code>fmt.Fprintf</code> takes as its first argument
a value that satisfies <code>io.Writer</code>—something that implements
the canonical <code>Write</code> method. Thus we can write
</p>
<pre>
fmt.Fprintf(w, "hello, world\n")
</pre>
<p>
If however we pass the address of <code>w</code>, the program will not compile.
</p>
<pre>
fmt.Fprintf(&amp;w, "hello, world\n") // Compile-time error.
</pre>
<p>
The one exception is that any value, even a pointer to an interface, can be assigned to
a variable of empty interface type (<code>interface{}</code>).
Even so, it's almost certainly a mistake if the value is a pointer to an interface;
the result can be confusing.
</p>
<h3 id="methods_on_values_or_pointers">
Should I define methods on values or pointers?</h3>