Could someone explain the pros of deleting (or keeping) unused code?

Question

I have heard many times that unused code must be deleted from the project. However it is not clear for me \"why?\".

My points for not deleting that are:

Answer 1

• Fear. This makes the team worry more and produce less. The amount of fear goes up exponentially when more dead code is introduced. "We don't know if that bit is used, so we don't dare remove it or touch it."

• Sweeping changes. If something that needs to be changed everywhere in the system also exists in the dead code, do you change it? It's very hard to know if it is definitely not used somewhere, so it's always a risk. And even if it wouldn't break anything, would the dead code work at all if it would be taken back to use after this change?

  When dealing with a sweeping change the developers will also have to check every place that contains the code and in the case of dead code this is redundant. And checking them takes longer when the code's dead since it's hard to verify that it isn't used anywhere.

• Mental load. Everytime you need to think about whether something is used or whether you should do something to the dead code, it takes some of your brain power.
• Wild goose chases. "I need an example on how to use Foobar. Oh it's in these places in the codebase. I'll check the first hit and find out where this is in the UI. Hmm... Can't find it anywhere."
• Inflated reports (e.g. how many lines of code, classes, routines, changes). Distorts visibility of the project and decisions on what parts of the codebase should be worked on and estimations of future projects.
• Weakened trust on the codebase. This can result in more time spent on redundant tasks and it breaks the flow of using the codebase. Developers might have to check extremely carefully that everything they use will work in the way they think it should.

It is extremely valuable if you know that a part of the codebase is not used because then you can remove it. If you let it stay then in the future it can be hard or almost impossible to be certain that it is actually not used. For example, some of the things that use code in surprising ways: reflection, dynamically calling routines concatenated from strings, eval, framework magic.

However, if there is a high probability that code will be used in the future, it is easier to add if it's right there along the other code instead of in the version control system. You might not remember any words that the code had after a while so it can be very hard to find the code from the bowels of the VCS. But I'd let dead code exist only rarely and even then I'd comment the code out.