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spec: clarify the difference between &T{} and new(T)

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Add a small paragraph and example pointing out
the difference for the case where T is a slice
or map. This is a common error for Go novices.

Fixes #29425.

Change-Id: Icdb59f25361e9f6a09b190fbfcc9ae0c7d90077b
Reviewed-on: https://go-review.googlesource.com/c/go/+/176338
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
This commit is contained in:
Robert Griesemer 2019-05-09 17:35:29 -07:00
parent 451cf3e2cd
commit eebb9db0ef
1 changed files with 14 additions and 0 deletions
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doc/go_spec.html
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@ -2513,10 +2513,24 @@ For array and slice literals the following rules apply:
generates a pointer to a unique <a href="#Variables">variable</a> initialized
with the literal's value.
</p>
<pre>
var pointer *Point3D = &amp;Point3D{y: 1000}
</pre>
<p>
Note that the <a href="#The_zero_value">zero value</a> for a slice or map
type is not the same as an initialized but empty value of the same type.
Consequently, taking the address of an empty slice or map composite literal
does not have the same effect as allocating a new slice or map value with
<a href="#Allocation">new</a>.
</p>
<pre>
p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
</pre>
<p>
The length of an array literal is the length specified in the literal type.
If fewer elements than the length are provided in the literal, the missing