Declare static random devices in cpp

static std::random_device rd;
static std::mt19937 g(rd());

std::string generator::generateSomething1() 
{
...
}
std::string generator::generateSomething2()
{
...
}

This solution for generate random numbers is well or no? Maybe global variable break some rules in programming c++? What do you think? I don't want declare in namespace generator, because i would have #include <random> in header, but i don't want declare that in other headers.