C - Can I pass child class as parameter instead of base class-CodePudding

I have this:

class DATABASE_API MySQLConnection
{
}

And then a child class:

class DATABASE_API WorldDatabaseConnection : public MySQLConnection
{
}

Then I have this:

class GameDatabase {
public:
    GameDatabase(PreparedStatement<MySQLConnection>* preparedStatement, GameDatabaseFlags gdf)
    {
        _gameObjectDatabaseFlag = gdf;
        _preparedStatement = preparedStatement;
    };
    GameDatabaseFlags _gameObjectDatabaseFlag;
protected:
    uint32_t versionId;
    virtual void MapGameDatabase(GameDatabaseContainer gameDatabaseContainer) = 0;
    PreparedStatement<MySQLConnection>* _preparedStatement;
};

When I try to initialize GameDatabase like so:

PreparedStatement<WorldDatabaseConnection> * stmt = WorldDatabase.GetPreparedStatement(WORLD_SEL_GAMEOBJECTS_TEMPLATE);

auto gameDatabase = new GameDatabase(stmt,GAMEOBJECTS_DB);

I get the following error:

No matching constructor for initialization of 'GameDatabase'

Why is that? Can't I simple use the child class WorldDatabaseConnection in place of the base class MySQLConnection?

CodePudding user response：

Even though WorldDatabaseConnection is a child class of MySQLConnection, the template class PreparedStatement<WorldDatabaseConnection> is not a child class of PreparedStatement<MySQLConnection>.

CodePudding user response：

You are mixing up two distinct concepts: compile-time typing with a template vs. run-time typing with inheritance. As mentioned by Nielsen, PreparedStatement<WorldDatabaseConnection> is not a child class of PreparedStatement<MySQLConnection>.

However, you should be able to do this:

PreparedStatement<MySQLConnection> * stmt = WorldDatabase.GetPreparedStatement(WORLD_SEL_GAMEOBJECTS_TEMPLATE);

With appropriate casting, of course. You will certainly want to use dynamic_cast in your class to extract the proper object. Whether you are in the habit of doing so in a constructor is your own choice.