Why does uncommitted change in a branch affect the master branch?

Question

I am new to git and I am trying to experiment with it to understand the concepts particularly the branching and merging.

So here is my set up,

I have a master br

Answer 1

Git keeps your uncommitted changes when checking out another branch, which is very practical.

You can see this as uncommitted changes "belong" only to your working copy, and not to any branch or commit. They are independent. When you will commit the changes in a branch, they will of course change if the checkout has a different version for the file.

The only exception to this behaviour is if the branch change brings an uncommitted file to a different version, it which case the checkout is canceled:

A--B - feature
 \
  -C - master

Let's say commit B in the feature branch changes a line to foo.txt, and that you have the master branch checked out. You have made a different change to foo.txt.

1. You commit the change in master and checkout feature

   git add foo.txt
   git commit -m "changed foo.txt"
   git checkout feature
   

   Here no problem, the change is recorded in master and when you go to feature foo.txt is changed accordingly.

2. If you don't commit and try to checkout feature, then Git will print an appropriate message, and keep the master branch checked out:

   git checkout feature
   

   error: Your local changes to the following files would be overwritten by checkout:
   foo.txt
   Please, commit your changes or stash them before you can switch branches. Aborting