When I was using a matlab, I was using the method filloutliers. I was wondering if there is something similar to that in C++.
In other words, I want to know if there is any sort of a built-in method in a certain library that detect outliers in a data set and replace them.
No, there's no built-in standard library facility which does that. Numerical analysis is not a focus or a strong point of C++, though of course there are numerical analysis libraries available out there (available via a Google search). Note that Matlab's method is a very particular one: there's no precise and universal definition of an "outlier" (some would say there's no such thing as an outlier). So expect to have to come up with your own opinion of how to classify a point as an outlier.