What happens if I stash changes in a branch and then delete that branch

What happens if I stash changes in a branch and then delete that branch?

(1) Do I lose the stashed changes?

Example:

git stash
git checkout other_branch
git branch -D previous_branch

(2) Also, are the above stash and delete steps equivalent to commiting the changes in a branch and then deleting that branch (in terms of the final state achieved) ? ie:

git commit -m "Dummy commit, I am terminating this branch"
git checkout other_branch
git branch -D previous_branch

Solution

(1) No. The changes saved by git stash are saved as a pair (or sometimes trio) of commits. Those commits are referenced by the name stash. If you think of branch names and tags as labels for commits (which is what they are), then you can draw a picture, e.g.:

          O1 - O2             <-- other_branch
        /
M1 - M2 - M3 - M4             <-- master
   \
     B1 - B2 - B3             <-- branch
      .
       .......................<-- tag

(this might work better if I could color the tag differently; I used dots to suggest that the tag tag points to commit B1, which is "on branch branch").

If you're on branch branch and have some unsaved changes and run git stash, this is what it does:

        /
M1 - M2 - M3 - M4             <-- master
   \
     B1 - B2 - B3             <-- HEAD=branch
                | \
                i - w ........<-- stash

(I left other_branch and tag out of the diagram but they're still there too. I wrote HEAD=branch to imply that HEAD is pointing to branch, and branch is pointing to commit B3.)

Here i and w are the "index" and "work-tree" states that you stashed. The name stash points directly at commit w, and w has two parents (i.e., is a "merge commit" even though it's not really a merge at all), with B3 as its first parent and i as its second.

When you run git branch -D, all you are doing is erasing the label. So let's say you git checkout other_branch and then erase the label branch. Now you have this:

          O1 - O2             <-- HEAD=other_branch
        /
M1 - M2 - M3 - M4             <-- master
   \
     B1 - B2 - B3
                | \
                i - w ........<-- stash

There is no label directly on B3 but the stash points to w, and w points back to B3. So everything is still there, and will remain there as long as the stash (or the reflog, or both) keep B3 on git's internal radar.

(2) No: as you can see, the stash still has a reference to the branch. However, once you drop that stash—i.e., erase the stash label—with git stash drop for instance—you get this:

          O1 - O2             <-- HEAD=other_branch
        /
M1 - M2 - M3 - M4             <-- master
   \
     B1 - B2 - B3
                | \
                i - w

Had you git add-ed and git commited all your changes, you would have gotten a (single, non-merge) commit, which we could call B4, on branch. The branch label would have been moved to point to B4, and then you would check out other_branch and delete the label, giving:

          O1 - O2             <-- HEAD=other_branch
        /
M1 - M2 - M3 - M4             <-- master
   \
     B1 - B2 - B3 - B4

which is almost (but not quite) the same.

(Note that if tag tag still points to B1, commit B1 will stick around until the tag is also erased. Commits B2 through B4, or B2 through w, will only stick around as long as they stay there-but-invisible in the reflog. After 30 days (or whatever you set for reflog expiry), the reflog entries expire, and those commits become eligible for garbage collection.)