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spec: restrict when len(x) is constant

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R=gri, iant, ken2, r
CC=golang-dev
https://golang.org/cl/1687047
This commit is contained in:
Russ Cox 2010-07-01 17:49:47 -07:00
parent 971a04f1dc
commit f4429181df
1 changed files with 22 additions and 8 deletions
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doc/go_spec.html
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@ -527,9 +527,10 @@ A constant value is represented by an
<a href="#String_literals">string</a> literal, <a href="#String_literals">string</a> literal,
an identifier denoting a constant, an identifier denoting a constant,
a <a href="#Constant_expressions">constant expression</a>, or a <a href="#Constant_expressions">constant expression</a>, or
the result value of some built-in functions such as <code>unsafe.Sizeof</code> the result value of some built-in functions such as
and <code>cap</code> or <code>len</code> applied to an array, <code>unsafe.Sizeof</code> applied to any value,
<code>len</code> applied to a string constant, <code>cap</code> or <code>len</code> applied to
<a href="#Length_and_capacity">some expressions</a>,
<code>real</code> and <code>imag</code> applied to a complex constant <code>real</code> and <code>imag</code> applied to a complex constant
and <code>cmplx</code> applied to numeric constants. and <code>cmplx</code> applied to numeric constants.
The boolean truth values are represented by the predeclared constants The boolean truth values are represented by the predeclared constants
@ -754,8 +755,7 @@ ElementType = Type .
The length is part of the array's type and must be a The length is part of the array's type and must be a
<a href="#Constant_expressions">constant expression</a> that evaluates to a non-negative <a href="#Constant_expressions">constant expression</a> that evaluates to a non-negative
integer value. The length of array <code>a</code> can be discovered integer value. The length of array <code>a</code> can be discovered
using the built-in function <code>len(a)</code>, which is a using the built-in function <code>len(a)</code>. The elements can be indexed by integer
compile-time constant. The elements can be indexed by integer
indices 0 through the <code>len(a)-1</code> (§<a href="#Indexes">Indexes</a>). indices 0 through the <code>len(a)-1</code> (§<a href="#Indexes">Indexes</a>).
Array types are always one-dimensional but may be composed to form Array types are always one-dimensional but may be composed to form
multi-dimensional types. multi-dimensional types.
@ -805,7 +805,7 @@ a slice of length up to that capacity can be created by `slicing' a new
one from the original slice (§<a href="#Slices">Slices</a>). one from the original slice (§<a href="#Slices">Slices</a>).
The capacity of a slice <code>a</code> can be discovered using the The capacity of a slice <code>a</code> can be discovered using the
built-in function <code>cap(a)</code> and the relationship between built-in function <code>cap(a)</code> and the relationship between
<code>len()</code> and <code>cap()</code> is: <code>len(a)</code> and <code>cap(a)</code> is:
</p> </p>
<pre> <pre>
@ -4358,12 +4358,12 @@ The implementation guarantees that the result always fits into an <code>int</cod
Call Argument type Result Call Argument type Result
len(s) string type string length in bytes len(s) string type string length in bytes
[n]T, *[n]T array length (== constant n) [n]T, *[n]T array length (== n)
[]T slice length []T slice length
map[K]T map length (number of defined keys) map[K]T map length (number of defined keys)
chan T number of elements queued in channel buffer chan T number of elements queued in channel buffer
cap(s) [n]T, *[n]T array length (== constant n) cap(s) [n]T, *[n]T array length (== n)
[]T slice capacity []T slice capacity
chan T channel buffer capacity chan T channel buffer capacity
</pre> </pre>
@ -4378,6 +4378,20 @@ At any time the following relationship holds:
0 <= len(s) <= cap(s) 0 <= len(s) <= cap(s)
</pre> </pre>
<p>
The expression
<code>len(s)</code> is a
<a href="#Constants">constant</a> if <code>s</code> is a string constant.
The expressions
<code>len(s)</code> and
<code>cap(s)</code> are
constants if <code>s</code> is an (optionally parenthesized)
identifier or
<a href="#Qualified_identifiers">qualified identifier</a>
denoting an array or pointer to array.
Otherwise invocations of <code>len</code> and <code>cap</code> are not
constant.
</p>
<h3 id="Allocation">Allocation</h3> <h3 id="Allocation">Allocation</h3>