What's weird about it? In practice it just means writing classes which instead of newing things inside the constructor, allows them to be passed in by the caller.
This is a pretty fundamental aspect of polymorphism and composition.
This is a pretty fundamental aspect of polymorphism and composition.
What I meant about weird was that OP's proposed "solution" to DI is to "do it manually", while the exact reason while we want DI in the first place is not to do it manually (or, in the way that I heard it most in my life, "to be able to change it without rebuilding")
No, the main reason we want DI is to change the behaviors based on what's passed. That's dependency injection. Framework passing the dependencies is automatic dependency injection and what I'm advocating against.
but which behaviours are passed depend on something external to the system - config files, etc. - so you need something that gets the information from somewhere and instantiates the correct class - and trust me, you don't want to write
Protocol proto = null;
if(config == "protocolA")
proto = new ProtocolA;
else if(config == "protocolB")
proto = new ProtocolB;
else if(config == "protocolC")
proto = new ProtocolC;
else if(config == "protocolD")
proto = new ProtocolD;
else
throw whatever;
return new MyObjectWithDependencies(proto);