Just a nitpick: Assigning a negative value to an unsigned integer is perfectly safe and well defined (i.e. integer overflow for unsigned integers is well defined in the C++ spec).

Using -1 for size_t is sometimes even prefered to assign it's maximum value, since there is no cross-platform #define for it (most use either SIZE_T_MAX or SIZE_MAX).

In fact llvm's libc++ uses it for it's std::numeric_limits implementation and for std::string's npos.