Add front() methods SGQueue, SGLockedQueue, and SGBlockingQueue so that the

can be used more interchangably with a regular STL queue.

simgear/threads/SGQueue.hxx

@@ -47,6 +47,13 @@ public:
      */
     virtual void push( const T& item ) = 0;
 
+    /**
+     * View the item from the head of the queue.
+     *
+     * @return T next available object.
+     */
+    virtual T front() = 0;
+
     /**
      * Get an item from the head of the queue.
      *
@@ -99,6 +106,18 @@ public:
 	fifo.push( item );
     }
 
+    /**
+     * View the item from the head of the queue.
+     *
+     * @return T next available object.
+     */
+    virtual T front() {
+	SGGuard<SGLOCK> g(mutex);
+	assert( ! fifo.empty() );
+	T item = fifo.front();
+	return item;
+    }
+
     /**
      * Get an item from the head of the queue.
      *
@@ -167,6 +186,22 @@ public:
 	not_empty.signal();
     }
 
+    /**
+     * View the item from the head of the queue.
+     * Calling thread is not suspended
+     *
+     * @return T next available object.
+     */
+    virtual T front() {
+	SGGuard<SGMutex> g(mutex);
+
+	assert(fifo.empty() != true);
+	//if (fifo.empty()) throw ??
+
+	T item = fifo.front();
+	return item;
+    }
+
     /**
      * Get an item from the head of the queue.
      * If no items are available then the calling thread is suspended