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<table width="100%" summary="page for bindenv {base}"><tr><td>bindenv {base}</td><td style="text-align: right;">R Documentation</td></tr></table>

<h2>Binding and Environment Locking, Active Bindings</h2>

<h3>Description</h3>

<p>These functions represent an interface for adjustments
to environments and bindings within environments.  They allow for
locking environments as well as individual bindings, and for linking
a variable to a function.
</p>

<h3>Usage</h3>

<pre>
lockEnvironment(env, bindings = FALSE)
environmentIsLocked(env)
lockBinding(sym, env)
unlockBinding(sym, env)
bindingIsLocked(sym, env)

makeActiveBinding(sym, fun, env)
bindingIsActive(sym, env)
</pre>

<h3>Arguments</h3>

<table summary="R argblock">
<tr valign="top"><td><code>env</code></td>
<td>
<p>an environment.</p>
</td></tr>
<tr valign="top"><td><code>bindings</code></td>
<td>
<p>logical specifying whether bindings should be locked.</p>
</td></tr>
<tr valign="top"><td><code>sym</code></td>
<td>
<p>a name object or character string.</p>
</td></tr>
<tr valign="top"><td><code>fun</code></td>
<td>
<p>a function taking zero or one arguments.</p>
</td></tr>
</table>

<h3>Details</h3>

<p>The function <code>lockEnvironment</code> locks its environment argument,
which must be a normal environment (not base).  (Locking the base
environment and namespace may be supported later.)  Locking the
environment prevents adding or removing variable bindings from the
environment.  Changing the value of a variable is still possible unless
the binding has been locked.  The namespace environments of packages
with namespaces are locked when loaded.
</p>
<p><code>lockBinding</code> locks individual bindings in the specified
environment.  The value of a locked binding cannot be changed.  Locked
bindings may be removed from an environment unless the environment is
locked.
</p>
<p><code>makeActiveBinding</code> installs <code>fun</code> in environment <code>env</code>
so that getting the value of <code>sym</code> calls <code>fun</code> with no
arguments, and assigning to <code>sym</code> calls <code>fun</code> with one
argument, the value to be assigned.  This allows the implementation of
things like C variables linked to <span style="font-family: Courier New, Courier; color: #666666;"><b>R</b></span> variables and variables linked to
databases, and is used to implement <code><a href="../../methods/html/refClass.html">setRefClass</a></code>.  It may
also be useful for making thread-safe versions of some system globals.
Currently active bindings are not preserved during package installation,
but they can be created in <code><a href="ns-hooks.html">.onLoad</a></code>.
</p>

<h3>Value</h3>

<p>The <code>bindingIsLocked</code> and <code>environmentIsLocked</code> return a
length-one logical vector.  The remaining functions return
<code>NULL</code>, invisibly.
</p>

<h3>Author(s)</h3>

<p>Luke Tierney</p>

<h3>Examples</h3>

<pre>
# locking environments
e &lt;- new.env()
assign("x", 1, envir = e)
get("x", envir = e)
lockEnvironment(e)
get("x", envir = e)
assign("x", 2, envir = e)
try(assign("y", 2, envir = e)) # error

# locking bindings
e &lt;- new.env()
assign("x", 1, envir = e)
get("x", envir = e)
lockBinding("x", e)
try(assign("x", 2, envir = e)) # error
unlockBinding("x", e)
assign("x", 2, envir = e)
get("x", envir = e)

# active bindings
f &lt;- local( {
    x &lt;- 1
    function(v) {
       if (missing(v))
           cat("get\n")
       else {
           cat("set\n")
           x &lt;&lt;- v
       }
       x
    }
})
makeActiveBinding("fred", f, .GlobalEnv)
bindingIsActive("fred", .GlobalEnv)
fred
fred &lt;- 2
fred
</pre>

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