Added an abstract base class for the connection

include/Connection.hpp

@@ -0,0 +1,29 @@
+#ifndef SQLGEN_CONNECTION_HPP_
+#define SQLGEN_CONNECTION_HPP_
+
+#include <string>
+#include <vector>
+
+#include "Result.hpp"
+#include "dynamic/Insert.hpp"
+#include "dynamic/SelectFrom.hpp"
+
+namespace sqlgen {
+
+/// Abstract base class to be implemented by the different
+/// database connections.
+struct Connection {
+  /// Reads the results of a SelectFrom statement.
+  virtual Result<std::vector<std::vector<std::string>>> read(
+      const dynamic::SelectFrom& _query) = 0;
+
+  /// Writes data into a table. Each vector in data MUST have the same length as
+  /// _stmt.columns.
+  virtual Result<Nothing> write(
+      const dynamic::Insert& _stmt,
+      const std::vector<std::vector<std::string>>& _data) = 0;
+};
+
+}  // namespace sqlgen
+
+#endif