Andrzej Krzemienski wrote: ...
So, I want to apply the Dependency Injection philosophy. I get:
class State { Rect area; Point location; // invariant: location is inside area; public: State(Rect a, Point loc) : area{a}, location{loc} {} };
int main() { State s{Rect{{0, 0}, {2, 2}}, Point{1, 1}}; } ```
Now, I choose to use the DI-library (this is where I have troubles with understanding: why would I want to do that?).
You are thinking in C++ (value semantics) but you need to be thinking in Java (reference semantics, OOP, object DAGs.) Suppose you need to have a logger object shared_ptr<ILogger> pl; // new FileLogger( log_file_name fn ); and a database connector object (which uses a logger to log things) shared_ptr<IDbConn> pdb; // new MysqlDb( mysql_db_info mdi, shared_ptr<ILogger> pl ) and an abstract configuration query interface, that can use an .ini file, or a JSON file, or a database shared_ptr<IConfig> pc; // new DbConfig( shared_ptr<IDbConn> pdb, shared_ptr<ILogger> pl ); Now when you need to create the config query object, you need to give it a database connector and a logger, and you also need to give the same logger to the database connector. So with [NotYetBoost].DI you'll write something like auto injector = make_injector( bind<log_file_name>.to( "something.log" ), bind<ILogger>.to<FileLogger>(), bind<>.to( mysql_db_info{ ... } ), bind<IDbConn>.to<MysqlDb>(), bind<IConfig>.to<DbConfig>() ); auto cfg = injector.create<IConfig>(); although I don't really know if this will compile or work. :-) Now imagine that same thing but with 10 objects, or 20 objects. Also imagine that your day to day vocabulary includes "business logic", "enterprise", "UML" and "XML".