From: Mikko Rasa <tdb@tdb.fi>
Date: Sun, 19 Apr 2020 14:09:07 +0000 (+0300)
Subject: Allow non-loadable types in collections
X-Git-Url: http://git.tdb.fi/?p=libs%2Fdatafile.git;a=commitdiff_plain;h=465ac6c7ceda80ce7e2f6e3f7b6901ce0921406e

Allow non-loadable types in collections

Custom collection classes may wish to provide objects of such types
through creator functions.
---

diff --git a/source/collection.h b/source/collection.h
index 5521975..d0e8a1a 100644
--- a/source/collection.h
+++ b/source/collection.h
@@ -19,6 +19,20 @@ class CollectionItemTypeBase;
 template<typename T>
 class CollectionItemType;
 
+template<typename T>
+class LoadableCollectionItemType;
+
+template<typename T, bool = HasLoader<T>::value>
+struct CollectionItemTypeChooser;
+
+template<typename T>
+struct CollectionItemTypeChooser<T, true>
+{ typedef LoadableCollectionItemType<T> Type; };
+
+template<typename T>
+struct CollectionItemTypeChooser<T, false>
+{ typedef CollectionItemType<T> Type; };
+
 /**
 A collection of objects that can be loaded from a datafile.  Each object is
 identified by a name, which must be unique across the entire collection.
@@ -44,7 +58,7 @@ public:
 	*/
 	class Loader: public DataFile::Loader
 	{
-		template<typename T> friend class CollectionItemType;
+		template<typename T> friend class LoadableCollectionItemType;
 
 	private:
 		Collection &coll;
@@ -235,12 +249,12 @@ protected:
 	/** Adds a type to the collection.  The returned descriptor object reference
 	can be used to define how objects of that type can be loaded. */
 	template<typename T>
-	CollectionItemType<T> &add_type();
+	typename CollectionItemTypeChooser<T>::Type &add_type();
 
 	/** Returns a mutable reference to an existing type descriptor.  This can be
 	used to e.g. override the creator function of a type added by a base class. */
 	template<typename T>
-	CollectionItemType<T> &modify_type();
+	typename CollectionItemTypeChooser<T>::Type &modify_type();
 
 private:
 	/** Returns the descriptor for a type, or null if one isn't defined.  An
@@ -323,9 +337,11 @@ protected:
 public:
 	virtual ~CollectionItemTypeBase();
 
+protected:
 	void set_keyword(const std::string &);
-	const std::string &get_keyword() const { return kwd; }
 	void add_suffix(const std::string &);
+public:
+	const std::string &get_keyword() const { return kwd; }
 	bool match_name(const std::string &) const;
 	virtual bool is_same_type(const CollectionItemTypeBase &) const = 0;
 	virtual bool check_item_type(const Variant &) const = 0;
@@ -448,8 +464,8 @@ public:
 	virtual bool check_item_type(const Variant &var) const
 	{ return var.check_type<RefPtr<T> >(); }
 
-	virtual void add_to_loader(Collection::Loader &loader) const
-	{ loader.add(kwd, &Collection::Loader::item<T, T>); }
+	virtual void add_to_loader(Collection::Loader &) const
+	{ }
 
 	virtual bool can_create() const
 	{ return creat!=0; }
@@ -463,6 +479,20 @@ public:
 			coll.add(name, obj);
 	}
 
+	virtual void load_item(Collection &, Parser &, const std::string &) const
+	{
+		throw std::runtime_error("this type cannot be loaded");
+	}
+};
+
+
+template<typename T>
+class LoadableCollectionItemType: public CollectionItemType<T>
+{
+public:
+	virtual void add_to_loader(Collection::Loader &loader) const
+	{ loader.add(this->kwd, &Collection::Loader::item<T, T>); }
+
 	virtual void load_item(Collection &coll, Parser &parser, const std::string &name) const
 	{
 		RefPtr<T> obj = new T;
@@ -486,15 +516,15 @@ T &Collection::extract(const Variant &var) const
 }
 
 template<typename T>
-CollectionItemType<T> &Collection::add_type()
+typename CollectionItemTypeChooser<T>::Type &Collection::add_type()
 {
-	CollectionItemType<T> *type = new CollectionItemType<T>;
+	typename CollectionItemTypeChooser<T>::Type *type = new typename CollectionItemTypeChooser<T>::Type;
 	types.push_back(type);
 	return *type;
 }
 
 template<typename T>
-CollectionItemType<T> &Collection::modify_type()
+typename CollectionItemTypeChooser<T>::Type &Collection::modify_type()
 {
 	for(TypeList::const_iterator j=types.begin(); j!=types.end(); ++j)
 		if(CollectionItemType<T> *t = dynamic_cast<CollectionItemType<T> *>(*j))
diff --git a/source/meta.h b/source/meta.h
index ac6ec1b..51bc924 100644
--- a/source/meta.h
+++ b/source/meta.h
@@ -18,6 +18,18 @@ struct CheckCollectionType: Sfinae
 
 template<typename T> struct NeedsCollection: Sfinae::Evaluate<CheckCollectionType, T> { };
 
+/**
+Helper struct to determine whether a type has a Loader class.
+*/
+struct CheckLoader: Sfinae
+{
+	template<typename T>
+	static Yes f(typename T::Loader *);
+	using Sfinae::f;
+};
+
+template<typename T> struct HasLoader: Sfinae::Evaluate<CheckLoader, T> { };
+
 } // namespace DataFile
 } // namespace Msp