Accessing SQL databases from C++ applications is very simple with Qt library. Here is some short example that presents how to do it. I have chosen SQLite engine because it’s the easiest engine to set up (it requires no server, no configuration…), still it’s suitable for the most of possible applications.
Before you begin - sql drivers
Qt requires drivers to deal with SQL databases. Depending on your distribution, you can have the drivers installed by default with your Qt or not. If you face problems like “QSqlDatabase: QSQLITE driver not loaded” you need to install drivers manually. Error should be followed by a console output that informs you which drivers you have installed: “QSqlDatabase: available drivers:”. Read more about sql drivers….
Design and create the database
Let’s assume, for simplicity, that we need to store people data – only name and corresponding id. With sqlite you can create such a simple database with two console commands. First line is creating database people. Second one is creating the table people. It cointains a key (integer) and name (text). To leave the sqlite console just type “.quit”.
Congrats, your database is ready. Now we can create C++ application to play with it.
Access database from Qt application
Create new Qt project. In .pro file you need to add:
QT += sql
It will link your project against QtSql module. If you don’t use QtCreator but another IDE, for example Visual Studio, your project may need setting linker dependency to QtSql.lib. You need to do it if your compilation fails with unresolved external symbol error. In VS linker settings is located in
Properties/Configuration properties/Linker/Input. In the Additional Dependencies add Qt5Sqld.lib for Debug and Qt5Sql.lib for release.
Now you can create a class responsible for database access. Let’s call it DbManager.
The constructor of DbManager sets up the database connection and opens it. The path argument value is the path to your database file.
Define needed operations
For managing people resources we need the possibility to:
- add new person,
- remove person,
- check if person exists in database,
- print all persons,
- delete all persons.
Actions can be implemented in following way: define QSqlQuery, then call prepare method. Prepare argument can be raw SQL query string, or it can contain placeholders for variables — placeholders must begin with colon. After prepare call bindValue to fill the placeholders with proper values. When the query is ready, execute it.
QSqlQuery::lastError() is very useful, you should not skip it in your implementation.
Rules of creating queries are simple, so I think you already know how removing person looks like:
Print all persons:
Check if person with specific name exists:
To remove all persons, just run:
You can find the whole source code on my github. All you need to do before running this example is to create your SQLite database and write the valid path to it in main.cpp.