Why valarray does not have a swap function like vector in C++03?

As title, I am confused about this. swap should be extremely useful if we are facing an iterative problem. The old vector and the new vector can be swapped by exchanging pointer-to-memory instead of contents. However, this is valid in std::vector but not in std::valarray. I am wondering why. In C++0x it seems the function is added, but why not in C++03?