make computationgraph constructor private to force use of factory method

swig/README.md

@@ -149,7 +149,7 @@ it also on CPU. If you just use the Scala `myInitialize()` helper you should be
 
 ## Differences between Scala and C++
 
-### Delete Your ComputationGraphs
+### `ComputationGraph.getNew`
 
 DyNet does not like it if you try to instantiate more than one ComputationGraph at a time.
 
@@ -162,27 +162,21 @@ for (int i = 0; i < NUM_TIMES; i++) {
 }
 ```
 
-This works because here `cg` gets destructed each time it goes out of scope. The same is not true in Scala
+This works because here `cg` gets destructed each time it goes out of scope. 
 
-```scala
-// BAD! DO NOT DO THIS!
-for (i <- 0 until NUM_TIMES) {
-  val cg = new ComputationGraph
-  // do some computations
-}
-```
-
-The underlying C++ ComputationGraph gets destructed at some point 
+If you were to write the analogous code in Scala 
+(generating a new ComputationGraph each iteration)
+the underlying C++ ComputationGraph would get destructed at some point 
 (presumably whenever the Java GC runs),
-but not immediately. As a result, your program will crash with the dreaded
+but not at the end of each loop.
+As a result, your program would crash with the dreaded
 
 ```
 [error] Memory allocator assumes only a single ComputationGraph at a time.
 ```
 
-To avoid this, we added a static `getNew` method that remembers the
-last requested ComputationGraph and deletes it every time you request 
-another.
+To prevent this, in Scala 
+you can only get new `ComputationGraph`s using the static `getNew` method:
 
 ```scala
 for (i <- 0 until NUM_TIMES) {
@@ -191,16 +185,8 @@ for (i <- 0 until NUM_TIMES) {
 }
 ```
 
-Alternatively, you can manually call `.delete()` on each 
-`ComputationGraph` instance when you are done with it:
-
-```scala
-for (i <- 0 until NUM_TIMES) {
-  val cg = new ComputationGraph
-  // do some computations
-  cg.delete()
-}
-```
+which keeps track of the previously allocated `ComputationGraph` and deletes it
+whenever you request a new one.
 
 ### `std::vector`s
 

swig/dynet_swig.i

@@ -631,11 +631,14 @@ Expression trace_of_product(const Expression& x, const Expression& y);
     if (singletonInstance != null) {
       singletonInstance.delete();
     }
+
     singletonInstance = new ComputationGraph();
     return singletonInstance;
   }
 %}
 
+%javamethodmodifiers ComputationGraph::ComputationGraph() "private";
+
 struct ComputationGraph {
   ComputationGraph();
   ~ComputationGraph();

swig/src/main/java/edu/cmu/dynet/examples/XorExample.java

@@ -24,7 +24,7 @@ public static void main(String[] args) {
     System.out.println("Dynet initialized!");
     Model m = new Model();
     SimpleSGDTrainer sgd = new SimpleSGDTrainer(m);
-    ComputationGraph cg = new ComputationGraph();
+    ComputationGraph cg = ComputationGraph.getNew();
 
     // Declare parameters
     Parameter p_W = m.add_parameters(makeDim(new int[]{HIDDEN_SIZE, 2}));