From 80b0732be1ff6a88a53d822635b3bd387c5e7908 Mon Sep 17 00:00:00 2001
From: Mike Gerwitz <mike@mikegerwitz.com>
Date: Wed, 29 Dec 2010 22:40:23 -0500
Subject: [PATCH] Interfaces cannot be instantiated

---
 lib/interface.js              | 86 +++++++++++++++++++++++------------
 test/test-interface-extend.js |  4 +-
 test/test-interface.js        |  6 +++
 3 files changed, 66 insertions(+), 30 deletions(-)

diff --git a/lib/interface.js b/lib/interface.js
index 0417483..7a41687 100644
--- a/lib/interface.js
+++ b/lib/interface.js
@@ -45,40 +45,70 @@ exports.extend = function()
 function Interface() {}
 
 
-function extend()
+var extend = ( function( extending )
 {
-    var args      = Array.prototype.slice.call( arguments ),
-        props     = args.pop() || {},
-        base      = args.pop() || Interface,
-        prototype = new base();
+    return function extend()
+    {
+        // ensure we'll be permitted to instantiate interfaces for the base
+        extending = true;
 
-    // sanity check
-    inheritCheck( prototype );
+        var args      = Array.prototype.slice.call( arguments ),
+            props     = args.pop() || {},
+            base      = args.pop() || Interface,
+            prototype = new base();
 
-    var new_interface = function() {};
+        // sanity check
+        inheritCheck( prototype );
 
-    util.propCopy( props, prototype, {
-        each: function( name, value )
-        {
-            if ( util.isAbstractMethod( value ) === false )
+        var new_interface = createInterface();
+
+        util.propCopy( props, prototype, {
+            each: function( name, value )
             {
-                throw TypeError(
-                    "Only abstract methods are permitted within Interface " +
-                        "definitons"
-                );
+                if ( util.isAbstractMethod( value ) === false )
+                {
+                    throw TypeError(
+                        "Only abstract methods are permitted within Interface " +
+                            "definitons"
+                    );
+                }
+            },
+        } );
+
+        attachExtend( new_interface );
+        new_interface.prototype   = prototype;
+        new_interface.constructor = new_interface;
+
+        // freeze the interface (preventing additions), if supported
+        util.freeze( new_interface );
+
+        // we're done; let's not allow interfaces to be instantiated anymore
+        extending = false;
+
+        return new_interface;
+    };
+
+
+    /**
+     * Creates a new interface constructor function
+     *
+     * @return  {function()}
+     */
+    function createInterface()
+    {
+        return function()
+        {
+            // allows us to extend the interface without throwing an exception
+            // (since the prototype requires an instance)
+            if ( !extending )
+            {
+                // only called if someone tries to create a new instance of an
+                // interface
+                throw Error( "Interfaces cannot be instantiated" );
             }
-        },
-    } );
-
-    attachExtend( new_interface );
-    new_interface.prototype   = prototype;
-    new_interface.constructor = new_interface;
-
-    // freeze the interface (preventing additions), if supported
-    util.freeze( new_interface );
-
-    return new_interface;
-}
+        };
+    }
+} )( false );
 
 
 /**
diff --git a/test/test-interface-extend.js b/test/test-interface-extend.js
index efa3236..7573536 100644
--- a/test/test-interface-extend.js
+++ b/test/test-interface-extend.js
@@ -75,7 +75,7 @@ var SubType = Interface.extend( BaseType,
 });
 
 assert.ok(
-    ( new SubType() instanceof BaseType ),
+    ( SubType.prototype instanceof BaseType ),
     "Generic interface extend method can extend from other interfaces"
 );
 
@@ -102,7 +102,7 @@ var SubType2 = BaseType.extend(
 });
 
 assert.ok(
-    ( new SubType2 instanceof BaseType ),
+    ( SubType2.prototype instanceof BaseType ),
     "Interface extend method can extend interfaces"
 );
 
diff --git a/test/test-interface.js b/test/test-interface.js
index 5cc8f2e..0236940 100644
--- a/test/test-interface.js
+++ b/test/test-interface.js
@@ -35,6 +35,12 @@ assert.ok(
     "Interface extend method creates a new interface object"
 );
 
+assert.throws( function()
+{
+    new FooType();
+}, Error, "Interfaces cannot be instantiated" );
+
+
 // only perform check if supported by the engine
 if ( Object.isFrozen )
 {