I doubt that SFML is particularly widely used in the games industry since most companies will either buy or develop an engine with cross-platform functionality that includes consoles/mobiles, etc. That's not a negative though, use SFML or SDL for your demos and experiments so you can get on with the interesting stuff instead of messing around with awkward platform functionality.

That said a familiarity with OpenGL and/or Direct3D will probably be considered a plus from a new hire. Looking briefly at SFML it seems you can choose to use their abstracted 2D graphics calls, or just use SFML to handle the windowing and use OpenGL calls direct. I'd recommend you do the latter.

What they mean is that you know C++ so well that you can tackle any problem with the language and understand what the performance impact of that solution is. So use reference passing instead of by value for large objects, but also what functions are called when you assign a new object to a variable.

Knowing additional API's is helpful as well, the ability to know how to program and use C++ however is far more appreciated then knowing one particular API, unless the job advert asks for it of course. What we want is a programmer that can be productive nearly immediately from the day he starts, of course you will have to learn how to use the codebase but we will not teach you how to use C++ at the same time though.

I would think core is referring to the most commonly used libraries and understanding the language itself. So that would be operator overloading, pointers, memory management techniques, by reference, value, pointer, const correctness, functors, stl, OpenGL, DirectX, multi-threading including semaphores and critical sections, IO (files and sockets), OO, etc. The list I am sure is much longer. Make your own game and then to have a good idea of the kinds of things needed to program a game you might want to look at some open source projects code such as irrlicht and make sure you pay attention to the details. For instance, pay attention the constructors. what does it mean to have MyClass(MyClass& aClass); as a constructor.

to have a good idea of the kinds of things needed to program a game you might want to look at some open source projects code such as irrlicht

Only if you are looking at it for the humour value.

There is very little code in IrrLicht (and the same goes for many similar projects) of a quality to exist in a production codebase.

the ability to know how to program and use C++ however is far more appreciated then knowing one particular API, unless the job advert asks for it of course. What we want is a programmer that can be productive nearly immediately from the day he starts, of course you will have to learn how to use the codebase but we will not teach you how to use C++ at the same time though.

to have a good idea of the kinds of things needed to program a game you might want to look at some open source projects code such as irrlicht

Only if you are looking at it for the humour value.

There is very little code in IrrLicht (and the same goes for many similar projects) of a quality to exist in a production codebase.

It doesn't have much to do with the size of the codebase - it's a matter of the experience and calibre of the developers involved.

Did they establish a coding style? Did they establish a testing methodology? Did they provide adequate documentation? Have they performed profile-guided optimisation? Did they produce a comprehensive threat model?

These may not be the fun parts of 'programming', but when a company advertises a position for a 'C++ developer', they are looking for all of the above.