Oh ok.
Would it be safe to say, then, that d(f, g) =max|f − g| does not define a metric on X for this particular case, because X is a set of functions that map [0,1] to R, and R is unbounded?
So one might be able to prove that "if d(f, g) =max|f − g| = 0, then f = g", but not the converse...