Re: [boost-ext] DI questions

From: Peter Dimov (pdimov_at_[hidden])
Date: 2021-02-20 23:10:26

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".