Programmer
Hi
I need to know what is it that I can build with C++ so that I can demostrate to people who hire start-ups. I just want to show them that I am capable so that they'l hire me.
That being said, there certainly are startups that use it, but they're often working in the world of embedded devices, IOT, etc; contexts where development on a traditional server isn't practical or even possible. Much of finance and HPC also use languages like C and C++ (not to mention FORTRAN and others).
So if I were in your shoes, I'd look up some startups where you'd like to work, and find out exactly what they're doing with C++. (You can often glean that kind of information from hiring posts, Angellist, etc.) Once you have that, build something as relevant to their business as possible.
Sorry for the vague advice, but here are a few examples: If you want to work in embedded devices or IOT, grab yourself an Arduino, and use C or C++ to program it to do something cool. Any complex behavior will demonstrate at least a working-proficiency with the languages. If you want to work in finance, maybe building some black-box trading software would be a fun experiment. Get creative, I'm sure you'll think of something.
I would suggest to build a product that you could sell to customers with such financial success that you could buy out those start ups, and being majority share holder, you would force them to hire you.
(Obviously, you may have additional constraints, but you didn't mention any, so...)