How does merging a branch actually work (under the hood)?

Question

this may be a naive question, but, as asked in the object, what is the actual way used by versioning softwares to merge a branch back into the main trunk without generating broken code?

Here's a quick example: I make a branch out of the main trunk for the program "Hello World Power edition". I add support for Klingon. This is a radical change that changes the way the function printHelloWorld() works.

Meanwhile, because of bug #749 that cause "Hello World" to be written "Helo World", the function printHelloWorld() in the main trunk has been changed.

Now, the problem that I see here is: when i merge by branch back to the main trunk i experiment a clash in the function printHelloWorld() within the file sayHello.py

How does a VCS program know how to add the Klingon support from my branch and keep the bug fix in the main trunk? What are the human-driven or software-driven strategies to avoid this?

Thanks in advance.

Answer 1

How does a VCS program know how to add the Klingon support from my branch and keep the bug fix in the main trunk?

VCS knows nothing about semantic of your source code it sees it as a bunch of text/binary files. VCS system uses diff / merge algorithms to detect conflicts between yours and current file version. It is your responsibility to resolve such conflicts because only you know semantic of these changes. Some VCSs like SVN would require you to update your working copy with latest changes from trunk before allowing you to commit changes to make sure that nothing is lost.

To make sure that you didn't break anything and all previous bug fixes were not broken you should use code reviews, unit tests and other practices. Continuous integration is a good way to keep software healthy.