Add virtual destructor to Expression, just in case.

Doesn't hurt and can help.

libwccl/ops/expression.h

@@ -9,7 +9,8 @@ namespace Wccl {
 /**
  * Abstract base class for WCCL expressions
  */
-class Expression : public boost::noncopyable {
+class Expression : boost::noncopyable
+{
 public:
 	/**
 	 * @returns String representation of the expression.
@@ -21,6 +22,13 @@ public:
 	 * @note Might be incomplete and/or contain internal info.
 	 */
 	virtual std::string to_raw_string() const = 0;
+
+	/**
+	 * Virtual destructor, so if delete is ever called on a base-class-type
+	 * of pointer, the destruction is forwarded to proper destructor
+	 * in a derived class, if a special destructor was needed for that class.
+	 */
+	virtual ~Expression() {};
 };
 
 } /* end ns Wccl */