There are a few reasons you would do this in CPP, though I'm not sure whether they apply to this case. You might put function definitions in headers to (1) avo

There are a few reasons you would do this in CPP, though I'm not sure whether they apply to this case. You might put function definitions in headers to (1) avoid linking a library (2) explicitly declare a function as inline, maybe (3) it's simple enough that you don't lose readability by inlining it. The one about putting large classes entirely in CPP files: I wouldn't do it this way for readability reasons, but putting it in the CPP file directly does restrict anybody else from using what is supposed to be a one-off helper class. I think that's the intent here.