It should be (eliding namespaces):

    transform(begin(tmp), end(tmp), begin(tmp), [] (char c) { return toupper(c); });

...the only noisy parts are the calls to begin() and end(). On the other hand, it will work for any collection type with good iterators (vectors, lists, strings, deques, ropes, etc.).

It is also trivial to write a mylib::transform_in_place() that cuts down on the noise with a one-line function.