https://en.wikipedia.org/wiki/Fractal_landscape

How do I program this in C or C++?

>>3
ackchyually, it's a quite straightforward thing to do in any language, at least if you do it the most basic way. diamond-square algorithm is a classic, here are some basic implementations:
http://stevelosh.com/blog/2016/06/diamond-square/ - lithp
https://medium.com/@nickobrien/diamond-square-algorithm-explanation-and-c-implementation-5efa891e486f - C/old-school sepples

also, while fractal is good for 3D, simplex noise is better for 2D maps:
https://www.redblobgames.com/maps/terrain-from-noise/