In addition to the other comments (which are the primary reasons), beware that you can actually get different behavior due to something called "argument-depende

In addition to the other comments (which are the primary reasons), beware that you can actually get different behavior due to something called "argument-dependent lookup". The most well-known example being std::swap(a, b). In general it is not equivalent to using namespace std; swap(a, b), though in this case, you actually want to say something else (using std::swap; swap(a, b)) to avoid importing the entire std namespace while also ensuring you call the ADL version of swap that may have been provided.