Most of this documentation expects you to have the generator installed and available in $PATH
, unless specified otherwise. Therefore, you might want to have a look at the installation page first.
ObjectBox Generator uses FlatBuffer schema file (.fbs) as its primary input. The Generator also maintains some metadata around the data model in a JSON file (objectbox-model.json). Based on these two files, it generates code for the selected language (C or C++).
Let’s have a look at a sample schema and how Generator helps us.
tasklist.fbstable Task {id: ulong;text: string;date_created: ulong;date_finished: ulong;}
Launch the following command to generate the binding code from the FlatBuffers schema file:
objectbox-generator -cpp tasklist.fbs
The following files will be generated:
objectbox-model.h
objectbox-model.json
tasklist.obx.hpp
tasklist.obx.cpp
objectbox-generator -c tasklist.fbs
The following files will be generated:
objectbox-model.h
objectbox-model.json
tasklist.obx.h
You should add all these generated files to your source control (e.g. git), most importantly objectbox-model.json
which ensures compatibility with previous versions of your database after you make changes to the schema.
Bet you wondered where our name comes from :)
From ObjectBox you vend Box instances to manage your entities. While you can have multiple Box instances of the same type (for the same Entity) "open" at once, it's usually preferable to just use one instance and pass it around your code.
Now, you can include the generated headers in your application and start working with your database. Consider the following main file:
main.cpp#include "objectbox.hpp"#include "objectbox-model.h"#include "tasklist.obx.hpp"int main(int argc, char* args[]) {// create_obx_model() provided by objectbox-model.h// obx interface contents provided by objectbox.hppobx::Store store(create_obx_model());obx::Box<Task> box(store);obx_id id = box.put({.text = "Buy milk"}); // Createstd::unique_ptr<Task> task = box.get(id); // Readif (task) {task->text += " & some bread";box.put(*task); // Updatebox.remove(id); // Delete}return 0;}
main.c#include "objectbox.h"#include "objectbox-model.h"#include "tasklist.obx.h"obx_err print_last_error() {printf("Unexpected error: %d %s\n",obx_last_error_code(), obx_last_error_message());return obx_last_error_code();}int main(int argc, char* args[]) {int rc = 0;OBX_store* store = NULL;OBX_box* box = NULL;Task* task = NULL;// Firstly, we need to create a model for our data and the store{OBX_model* model = create_obx_model(); // generated in objectbox-model.hif (!model) goto handle_error;if (obx_model_error_code(model)) {printf("Model definition error: %d %s\n",obx_model_error_code(model), obx_model_error_message(model));obx_model_free(model);goto handle_error;}OBX_store_options* opt = obx_opt();obx_opt_model(opt, model);store = obx_store_open(opt);if (!store) goto handle_error;// obx_store_open() takes ownership of model and opt and frees them.}box = obx_box(store, Task_ENTITY_ID); // Note the generated "Task_ENTITY_ID"obx_id id = 0;{ // CreateTask new_task = {.text = "Buy milk"};id = Task_put(box, &new_task); // generated in tasklist.obx.hif (!id) goto handle_error;printf("New task inserted with ID %d\n", id);}{ // Readtask = Task_Get(store, box, id); // generated in tasklist.obx.hif (!task) goto handle_error;printf("Task %d read with text: %s\n", id, task->text);}{ // Updateconst char* appendix = " & some bread";// Updating a string property is a little more involved// because of C memory management.size_t old_text_len = task->text ? strlen(task->text) : 0;char* new_text =(char*) malloc((old_text_len + strlen(appendix) + 1) * sizeof(char));if (task->text) {memcpy(new_text, task->text, old_text_len);// free the memory allocated previously before overwritting belowfree(task->text);}memcpy(new_text + old_text_len, appendix, strlen(appendix) + 1);task->text = new_text;printf("Updated task %d with a new text: %s\n", id, task->text);}// Deleteif (obx_box_remove(box, id) != OBX_SUCCESS) goto handle_error;free_resources: // free any remaining allocated resourcesif (task) Task_free(task); // free allocs by Task_new_from_flatbuffer()if (store) obx_store_close(store); // and close the storereturn rc;handle_error: // print error and clean uprc = print_last_error();if (rc <= 0) rc = 1;goto free_resources;}
If you've followed the installation instructions, you should be able to compile the example by running:
g++ main.cpp tasklist.obx.cpp -I. -std=c++11 -lobjectbox
gcc main.c -I. -lobjectbox -lflatccrt
The command snippet assumes you have the libraries installed in a path recognized by your OS (e.g. /usr/local/lib/) and all the referenced headers are in the same folder alongside the main.c/.cpp file.
Wherever you have access to a Box, you can use it to persist objects and fetch objects from disk. Boxes are thread-safe. Here are some of the basic operations, have a look at the objectbox.h(pp) for more:
put: persist an object at the given ID: either creating a new one or overwriting an existing one.
get: read an object from the database. There's also a variant that takes a list of IDs as an argument and returns multiple objects.
remove: deletes a previously persisted object from its box.
count: the number of objects stored in this box.
Have a look at the following TaskList example apps, depending on your programming language and preference:
C, cursor, no generated code - plain C; using flatcc directly; without any generated code
C, with generated code - plain C, using code generated by objectbox-generator
C++, with generated code - C++, using code generated by objectbox-generator