In the Visual Studio debugger, what does {null=???} mean?

Question

I was debugging a C++ program in VS 2003, and a boost variable showed up as having the value {null=???}. What does that mean?

Answer 1

Typically when you see ??? in the C++ debugger, it means the underlying expression evaluator had problems accessing the memory for the particular expression. So it's likely the value points to invalid or inaccessible memory.

It's also possible that this session is using an autoexp.dat file and it points to a member that is not accessible / available in the underlying expression. I believe this will also lead to the ??? display.