Sitecore 9 was release couple of weeks ago. For those you haven't already checked it out,
I urge you to do so. There are a whole set of features that make development very enjoyable with an out of the box flavour.   
Among those features, the already famous Sitecore Dependency Injection (DI) support out of the box since Sitecore 8.2 with its Inversion of Control (IoC).  

But What if this default container does not fit your need for whatever reason but you still want to override the container the way Sitecore described it in its documentation?