As many have said here, it's about actually writing code in the end, personally I learned C++ by writing an electronic schematic editor as a front end to a SPIC

As many have said here, it's about actually writing code in the end, personally I learned C++ by writing an electronic schematic editor as a front end to a SPICE circuit simulator, so I learned C++ and the Windows API at the same time. Of course as well as writing code, reading other peoples is useful too, when i was learning there weren't the reasources there are today, you can go over to github or sourceforge and find lots of interesting code to learn from, and do code reviews on to see how you might have done things differently.