added collection, collection proxy and collection resolver implementations

include/matador/object/collection.hpp

@@ -1,7 +1,7 @@
 #ifndef COLLECTION_HPP
 #define COLLECTION_HPP
 
-#include <vector>
+#include "matador/object/collection_proxy.hpp"
 
 namespace matador::object {
 
@@ -9,12 +9,43 @@ template < class Type >
 class collection {
 public:
   using value_type = Type;
+  using iterator = typename std::vector<value_type>::iterator;
+  using const_iterator = typename std::vector<value_type>::const_iterator;
 
-  void push_back(const Type& value) { data_.push_back(value); }
+  void push_back(const value_type& value) {
+    proxy_->items().push_back(value);
+  }
+
+  iterator begin() {
+    return proxy_->items().begin();
+  }
+
+  iterator end() {
+    return proxy_->items().end();
+  }
+
+  const_iterator begin() const {
+    return proxy_->items().begin();
+  }
+
+  const_iterator end() const {
+    return proxy_->items().end();
+  }
+
+  [[nodiscard]] size_t size() const {
+    return proxy_->items().size();
+  }
+
+  [[nodiscard]] bool empty() const {
+    return proxy_->items_.empty();
+  }
+
+  void reset(std::shared_ptr<collection_proxy<typename Type::value_type>> proxy) {
+    proxy_ = std::move(proxy);
+  }
 
-  [[nodiscard]] size_t size() const { return data_.size(); }
 private:
-  std::vector<Type> data_;
+  std::shared_ptr<collection_proxy<typename Type::value_type>> proxy_;
 };
 
 }