template <unsigned int b, unsigned int n>
struct log
{
  enum { value = log<b, n / b>::value + 1 };
};

template <unsigned int b>
struct log<b, 1>
{
  enum { value = 0 };
};

How's this?

template <unsigned int b, unsigned int n>
struct log
{
  enum { value = log<b, n / b>::value + 1 };
};
template <unsigned int b>
struct log<b, 1>
{
  enum { value = 0 };
};