Tag: add class const_iterator and methods begin(), end()

Enables using range-based "for".

src/tag/Set.cxx

@@ -30,10 +30,10 @@ CopyTagItem(TagBuilder &dest, TagType dest_type,
 	    const Tag &src, TagType src_type)
 {
 	bool found = false;
-	const unsigned n = src.num_items;
-	for (unsigned i = 0; i < n; ++i) {
-		if (src.items[i]->type == src_type) {
-			dest.AddItem(dest_type, src.items[i]->value);
+
+	for (const auto &item : src) {
+		if (item.type == src_type) {
+			dest.AddItem(dest_type, item.value);
 			found = true;
 		}
 	}
@@ -87,11 +87,10 @@ TagSet::CheckUnique(TagType dest_type,
 		    uint32_t group_mask)
 {
 	bool found = false;
-	for (unsigned i = 0; i < tag.num_items; ++i) {
-		if (tag.items[i]->type == src_type) {
-			InsertUnique(tag, dest_type,
-					tag.items[i]->value,
-					group_mask);
+
+	for (const auto &item : tag) {
+		if (item.type == src_type) {
+			InsertUnique(tag, dest_type, item.value, group_mask);
 			found = true;
 		}
 	}

src/tag/Tag.cxx

@@ -118,9 +118,9 @@ Tag::GetValue(TagType type) const
 {
 	assert(type < TAG_NUM_OF_ITEM_TYPES);
 
-	for (unsigned i = 0; i < num_items; i++)
-		if (items[i]->type == type)
-			return items[i]->value;
+	for (const auto &item : *this)
+		if (item.type == type)
+			return item.value;
 
 	return nullptr;
 }

src/tag/Tag.hxx

@@ -25,6 +25,7 @@
 #include "Compiler.h"
 
 #include <algorithm>
+#include <iterator>
 
 #include <stddef.h>
 
@@ -136,6 +137,59 @@ struct Tag {
 	 */
 	gcc_pure
 	bool HasType(TagType type) const;
+
+	class const_iterator {
+		friend struct Tag;
+		const TagItem *const*cursor;
+
+		constexpr const_iterator(const TagItem *const*_cursor)
+			:cursor(_cursor) {}
+
+	public:
+		constexpr const TagItem &operator*() const {
+			return **cursor;
+		}
+
+		constexpr const TagItem *operator->() const {
+			return *cursor;
+		}
+
+		const_iterator &operator++() {
+			++cursor;
+			return *this;
+		}
+
+		const_iterator operator++(int) {
+			auto result = cursor++;
+			return const_iterator{result};
+		}
+
+		const_iterator &operator--() {
+			--cursor;
+			return *this;
+		}
+
+		const_iterator operator--(int) {
+			auto result = cursor--;
+			return const_iterator{result};
+		}
+
+		constexpr bool operator==(const_iterator other) const {
+			return cursor == other.cursor;
+		}
+
+		constexpr bool operator!=(const_iterator other) const {
+			return cursor != other.cursor;
+		}
+	};
+
+	const_iterator begin() const {
+		return const_iterator{items};
+	}
+
+	const_iterator end() const {
+		return const_iterator{items + num_items};
+	}
 };
 
 /**