MXS-1503: Add EqualPointees<T> template

The templated provides an unary predicate for (shared) pointers that
allows containers containing pointers to objects to be searched, not by
pointer comparisons but by comparisons of the values that they point
to. This allows std::find_if to be used with shared pointers in a similar
manner that std::find is used with non-pointer objects.

include/maxscale/session_command.hh

@@ -89,22 +89,6 @@ public:
      */
     bool eq(const SessionCommand& rhs) const;
 
-    class Equals: public std::unary_function<const SSessionCommand&, bool>
-    {
-    public:
-        Equals(const SSessionCommand& base):
-        m_base(base)
-        {}
-
-        bool operator ()(const SSessionCommand& rhs)
-        {
-            return m_base->eq(*rhs);
-        }
-
-    private:
-        const SSessionCommand& m_base;
-    };
-
 private:
     mxs::Buffer m_buffer;    /**< The buffer containing the command */
     uint8_t     m_command;   /**< The command being executed */

include/maxscale/utils.hh

@@ -268,4 +268,31 @@ private:
     ContainerType m_registry;
 };
 
+// binary compare of pointed-to objects
+template<typename Ptr>
+bool equal_pointees(const Ptr& lhs, const Ptr& rhs)
+{
+   return *lhs == *rhs;
+}
+
+// Unary predicate for equality of pointed-to objects
+template<typename T>
+class EqualPointees
+{
+public:
+   EqualPointees(const T& lhs) : m_ppLhs(&lhs) {}
+   bool operator()(const T& pRhs)
+   {
+       return **m_ppLhs == *pRhs;
+   }
+private:
+   const T* m_ppLhs;
+};
+
+template<typename T>
+EqualPointees<T> equal_pointees(const T& t)
+{
+   return EqualPointees<T>(t);
+}
+
 }