You must use std::abs, which has overloads for the primitive types.
You are falling through to the C++ version in one case, and the old C version (which converts its parameters to integral types) in the other.
Things to avoid: (1) using namespace std; (here is why) and (2) not having the appropriate #includes to bring in the functionality you need. Don’t rely on C++ standard library implementations to include files for you implicitly.
Some compilers will warn you of a “lossy” conversion if you set the warning level appropriately.