diff --git a/src/newgrf_spritegroup.h b/src/newgrf_spritegroup.h
--- a/src/newgrf_spritegroup.h
+++ b/src/newgrf_spritegroup.h
@@ -288,8 +288,8 @@ struct IndustryProductionSpriteGroup : S
 struct ScopeResolver {
 	ResolverObject &ro; ///< Surrounding resolver object.
 
-	ScopeResolver(ResolverObject &ro);
-	virtual ~ScopeResolver();
+	ScopeResolver(ResolverObject &ro) : ro(ro) {}
+	virtual ~ScopeResolver() {}
 
 	virtual uint32 GetRandomBits() const;
 	virtual uint32 GetTriggers() const;
@@ -305,8 +305,20 @@ struct ScopeResolver {
  * to get the results of callbacks, rerandomisations or normal sprite lookups.
  */
 struct ResolverObject {
-	ResolverObject(const GRFFile *grffile, CallbackID callback = CBID_NO_CALLBACK, uint32 callback_param1 = 0, uint32 callback_param2 = 0);
-	virtual ~ResolverObject();
+	/**
+	 * Resolver constructor.
+	 * @param grffile NewGRF file associated with the object (or \c NULL if none).
+	 * @param callback Callback code being resolved (default value is #CBID_NO_CALLBACK).
+	 * @param callback_param1 First parameter (var 10) of the callback (only used when \a callback is also set).
+	 * @param callback_param2 Second parameter (var 18) of the callback (only used when \a callback is also set).
+	 */
+	ResolverObject(const GRFFile *grffile, CallbackID callback = CBID_NO_CALLBACK, uint32 callback_param1 = 0, uint32 callback_param2 = 0)
+		: default_scope(*this), callback(callback), callback_param1(callback_param1), callback_param2(callback_param2), grffile(grffile), root_spritegroup(NULL)
+	{
+		this->ResetState();
+	}
+
+	virtual ~ResolverObject() {}
 
 	ScopeResolver default_scope; ///< Default implementation of the grf scope.