Hi all,

I'm new in C++ who have an interest in the implementation of C++ STL.
Recently, I tried to implement my own vector template. However, when I
implemented my own iterator, I found that I had no idea how to
detect whether it is dereferencable. Therefore, I began to explore the
libstdc++, but I found that for some member functions, I can only find the
declaration but no definition. For example, the 'bool
__gnu_debug::_Safe_iterator_base::_M_singular(  ) const', 'void
__gnu_debug::_Safe_iterator_base::_M_detach(  )', etc. They are declared in
'libstdc++-v3/include/debug/safe_base.h'. I can guess that '_M_singular'
may just check if '_M_sequence' is NULL, but I still want to find their
definition. Can someone help me? Thank you so much.

Best,
Qiye Tan